Category Archives: electronics

air filter

A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mould, and bacteria from the air. Filters containing an absorbent or catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants such as volatile organic compounds or ozone.[1] Air filters are used in applications where air quality is important, notably in building ventilation systems and in engines.

Some buildings, as well as aircraft and other human-made environments (e.g., satellites and space shuttles) use foam, pleated paper, or spun fiberglass filter elements. Another method, air ionisers, use fibers or elements with a static electric charge, which attract dust particles. The air intakes of internal combustion engines and air compressors tend to use either paper, foam, or cotton filters. Oil bath filters have fallen out of favor. The technology of air intake filters of gas turbines has improved significantly in recent years, due to improvements in the aerodynamics and fluid dynamics of the air-compressor part of the gas turbines.

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Automotive cabin air filters[edit]

The cabin air filter is typically a pleated-paper filter that is placed in the outside-air intake for the vehicle’s passenger compartment. Some of these filters are rectangular and similar in shape to the combustion air filter. Others are uniquely shaped to fit the available space of particular vehicles’ outside-air intakes.

The first automaker to include a disposable filter to clean the ventilation system was the Nash MotorsWeather Eye“, introduced in 1940.[2]

Being a relatively recent addition to automobile equipment, this filter is often overlooked, and can greatly reduce the effectiveness of the vehicle’s air conditioning and heating performance. Clogged or dirty cabin air filters can significantly reduce airflow from the cabin vents, as well as introduce allergens into the cabin air stream. The poor performance of these filters is obscured by manufacturers by not using the MERV rating system. Some people mistakenly believe that some of these are HEPA filters.

Internal combustion engine air filters[edit]

The combustion air filter prevents abrasive particulate matter from entering the engine’s cylinders, where it would cause mechanical wear and oil contamination.

Most fuel injected vehicles use a pleated paper filter element in the form of a flat panel. This filter is usually placed inside a plastic box connected to the throttle body with ductwork. Older vehicles that use carburetors or throttle body fuel injection typically use a cylindrical air filter, usually a few inches high and between 6 inches (150 mm) and 16 inches (410 mm) in diameter. This is positioned above the carburetor or throttle body, usually in a metal or plastic container which may incorporate ducting to provide cool and/or warm inlet air, and secured with a metal or plastic lid. The overall unit (filter and housing together) is called the air cleaner.


Main article: Filter paper

Pleated paper filter elements are the nearly exclusive choice for automobile engine air cleaners, because they are efficient, easy to service, and cost-effective. The “paper” term is somewhat misleading, as the filter media are considerably different from papers used for writing or packaging, etc. There is a persistent belief amongst tuners, fomented by advertising for aftermarket non-paper replacement filters, that paper filters flow poorly and thus restrict engine performance. In fact, as long as a pleated-paper filter is sized appropriately for the airflow volumes encountered in a particular application, such filters present only trivial restriction to flow until the filter has become significantly clogged with dirt. Construction equipment engines also use this.


Oil-wetted polyurethane foam elements are used in some aftermarket replacement automobile air filters. Foam was in the past widely used in air cleaners on small engines on lawnmowers and other power equipment, but automotive-type paper filter elements have largely supplanted oil-wetted foam in these applications. Foam filters are still commonly used on air compressors for air tools up to 5Hp. Depending on the grade and thickness of foam employed, an oil-wetted foam filter element can offer minimal airflow restriction or very high dirt capacity, the latter property making foam filters a popular choice in off-road rallying and other motorsport applications where high levels of dust will be encountered. Due to the way dust is captured on foam filters, large amounts may be trapped without measurable change in airflow restriction.


Oiled cotton gauze is employed in a growing number of aftermarket automotive air filters marketed as high-performance items. In the past, cotton gauze saw limited use in original-equipment automotive air filters. However, since the introduction of the Abarth SS versions, the Fiat subsidiary supplies cotton gauze air filters as OE filters.

Stainless steel[edit]

Stainless steel mesh is another example of medium which allow more air to pass through. Stainless steel mesh comes with different mesh counts, offering different filtration standards. In an extreme modified engine lacking in space for a cone based air filter, some will opt to install a simple stainless steel mesh over the turbo to ensure no particles enter the engine via the turbo.

Oil bath[edit]

An oil bath air cleaner consists of a sump containing a pool of oil, and an insert which is filled with fibre, mesh, foam, or another coarse filter media. When the cleaner is assembled, the media-containing body of the insert sits a short distance above the surface of the oil pool. The rim of the insert overlaps the rim of the sump. This arrangement forms a labyrinthine path through which the air must travel in a series of U-turns: up through the gap between the rims of the insert and the sump, down through the gap between the outer wall of the insert and the inner wall of the sump, and up through the filter media in the body of the insert. This U-turn takes the air at high velocity across the surface of the oil pool. Larger and heavier dust and dirt particles in the air cannot make the turn due to their inertia, so they fall into the oil and settle to the bottom of the base bowl. Lighter and smaller particles are trapped by the filtration media in the insert, which is wetted by oil droplets aspirated there into by normal airflow.

Oil bath air cleaners were very widely used in automotive and small engine applications until the widespread industry adoption of the paper filter in the early 1960s. Such cleaners are still used in off-road equipment where very high levels of dust are encountered, for oil bath air cleaners can sequester a great deal of dirt relative to their overall size without loss of filtration efficiency or airflow. However, the liquid oil makes cleaning and servicing such air cleaners messy and inconvenient, they must be relatively large to avoid excessive restriction at high airflow rates, and they tend to increase exhaust emissions of unburned hydrocarbons due to oil aspiration when used on spark-ignition engines.[citation needed]

Water bath[edit]

In the early 20th century (about 1900 to 1930), water bath air cleaners were used in some applications (cars, trucks, tractors, and portable and stationary engines). They worked on roughly the same principles as oil bath air cleaners. For example, the original Fordson tractor had a water bath air cleaner. By the 1940s, oil bath designs had displaced water bath designs because of better filtering performance.

HVAC Air Filters[edit]

Filter classes[edit]

European Normalisation standards recognise the following filter classes:

Usage Class Performance Performance test Particulate size
approaching 100% retention
Test Standard
Coarse filters(used as


G1 65% Average value >5 µm BS EN779
G2 65–80% Average value >5 µm BS EN779
G3 80–90% Average value >5 µm BS EN779
G4 90%– Average value >5 µm BS EN779
Fine filters(used as


M5 40–60% Average value >5 µm BS EN779
M6 60–80% Average value >2 µm BS EN779
F7 80–90% Average value >2 µm BS EN779
F8 90–95% Average value >1 µm BS EN779
F9 95%– Average value >1 µm BS EN779
Semi HEPA E10 85% Minimum value >1 µm BS EN1822
E11 95% Minimum value >0.5 µm BS EN1822
E12 99.5% Minimum value >0.5 µm BS EN1822
HEPA H13 99.95% Minimum value >0.3 µm BS EN1822
H14 99.995% Minimum value >0.3 µm BS EN1822
ULPA U15 99.9995% Minimum value >0.3 µm BS EN1822
U16 99.99995% Minimum value >0.3 µm BS EN1822
U17 99.999995% Minimum value >0.3 µm BS EN1822

from wikipedia

air purifier

An air purifier is a device which removes contaminants from the air in a room. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke. The commercially graded air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit (AHU) or to an HVAC unit found in the medical, industrial, and commercial industries. Air purifiers may also be used in industry to remove impurities such as CO2 from air before processing. Pressure swing adsorbers or other adsorption techniques are typically used for this.

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Use and benefits of purifiers[edit]

Dust, pollen, pet dander, mold spores, and dust mite feces can act as allergens, triggering allergies in sensitive people. Smoke particles and volatile organic compounds (VOCs) can pose a risk to health. Exposure to various components such as VOCs increases the likelihood of experiencing symptoms of sick building syndrome.[1] Air purifiers are becoming increasingly capable of capturing a greater number of bacterial, virus, and DNA damaging particulates.[citation needed]

Purifying techniques[edit]

Several different processes of varying effectiveness can be used to purify air.

  • Thermodynamic sterilization (TSS) – This technology uses heat sterilization via a ceramic core with micro capillaries, which are heated to 200 °C (392 °F). It is claimed that 99.9% of microbiological particles – bacteria, viruses, dust mite allergens, mold and fungus spores – are incinerated.[2] The air passes through the ceramic core by the natural process of air convection, and is then cooled using heat transfer plates and released. TSS is not a filtering technology, as it does not trap or remove particles.[3] TSS is claimed not to emit harmful by-products (although the byproducts of partial thermal decomposition are not addressed) and also reduces the concentration of ozone in the atmosphere.[4]
  • Ultraviolet germicidal irradiation – UVGI can be used to sterilize air that passes UV lamps via forced air. Air purification UVGI systems can be freestanding units with shielded UV lamps that use a fan to force air past the UV light. Other systems are installed in forced air systems so that the circulation for the premises moves micro-organisms past the lamps. Key to this form of sterilization is placement of the UV lamps and a good filtration system to remove the dead micro-organisms. For example, forced air systems by design impede line-of-sight, thus creating areas of the environment that will be shaded from the UV light. However, a UV lamp placed at the coils and drainpan of cooling system will keep micro-organisms from forming in these naturally damp places. The most effective method for treating the air rather than the coils is in-line duct systems, these systems are placed in the center of the duct and parallel to the air flow.
  • Filter – based purification traps airborne particles by size exclusion. Air is forced through a filter and particles are physically captured by the filter.
High-efficiency particulate arrestance (HEPA) filters remove at most 99.97% of 0.3-micrometer particles and are usually more effective at removing larger particles. HEPA purifiers, which filter all the air going into a clean room, must be arranged so that no air bypasses the HEPA filter. In dusty environments, a HEPA filter may follow an easily cleaned conventional filter (prefilter) which removes coarser impurities so that the HEPA filter needs cleaning or replacing less frequently. HEPA filters do not generate ozone or harmful byproducts in course of operation.
Filter HVAC at MERV 14 or above are rated to remove airborne particles of 0.3 micrometers or larger. A high efficiency MERV 14 filter has a capture rate of at least 75% for particles between 0.3 to 1.0 micrometers. Although the capture rate of a MERV filter is lower than that of a HEPA filter, a central air system can move significantly more air in the same period of time. Using a high-grade MERV filter can be more effective than using a high-powered HEPA machine at a fraction of the initial capital expenditure. Unfortunately, most furnace filters are slid in place without an airtight seal, which allows air to pass around the filters. This problem is worse for the higher-efficiency MERV filters because of the increase in air resistance. Higher-efficiency MERV filters are usually denser and increase air resistance in the central system, requiring a greater air pressure drop and consequently increasing energy costs.
  • Activated carbon is a porous material that can adsorb volatile chemicals on a molecular basis, but does not remove larger particles. The adsorption process when using activated carbon must reach equilibrium thus it may be difficult to completely remove contaminants.[5] Activated carbon is merely a process of changing contaminants from a gaseous phase to a solid phase, when aggravated or disturbed contaminants can be regenerated in indoor air sources.[6] Activated carbon can be used at room temperature and has a long history of commercial use. It is normally used in conjunction with other filter technology, especially with HEPA. Other materials can also absorb chemicals, but at higher cost.
  • Polarized-media electronic air cleaners use active electronically enhanced media to combine elements of both electronic air cleaners and passive mechanical filters. Most polarized-media electronic air cleaners convert 24-volt current to safe DC voltage to establish the polarized electric field. Airborne particles become polarized as they pass through the electric field and adhere to a disposable fiber media pad. Ultra-fine particles (UFPs) that are not collected on their initial pass through the media pad are polarized and agglomerate to other particles, odor and VOC molecules and are collected on subsequent passes. The efficiency of polarized-media electronic air cleaners increases as they load, providing high-efficiency filtration, with air resistance typically equal to or less than passive filters. Polarized-media technology is non-ionizing, which means no ozone is produced.
  • Photocatalytic oxidation (PCO) is an emerging technology in the HVAC industry.[7] In addition to the prospect of Indoor Air Quality (IAQ) benefits, it has the added potential for limiting the introduction of unconditioned air to the building space, thereby presenting an opportunity to achieve energy savings over previous prescriptive designs. As of May 2009[8][9] there was no more disputable concern raised by the Lawrence Berkeley National Laboratory data that PCO may significantly increase the amount of formaldehyde in real indoor environments.[10] As with other advanced technologies, sound engineering principles and practices should be employed by the HVAC designer to ensure proper application of the technology. Photocatalytic oxidation systems are able to completely oxidize and degrade organic contaminants. For example, Volatile Organic Compounds found low concentrations within a few hundred ppmv or less are the most likely to be completely oxidized.[5](PCO) uses short-wave ultraviolet light (UVC), commonly used for sterilization, to energize the catalyst (usually titanium dioxide (TiO2)[11]) and oxidize bacteria and viruses.[12] PCO in-duct units can be mounted to an existing forced-air HVAC system. PCO is not a filtering technology, as it does not trap or remove particles. It is sometimes coupled with other filtering technologies for air purification. UV sterilization bulbs must be replaced about once a year; manufacturers may require periodic replacement as a condition of warranty. Photocatalytic Oxidation systems often have high commercial costs.[5]
A related technology relevant to air purification is photoelectrochemical oxidation (PECO) Photoelectrochemical oxidation. While technically a type of PCO, PECO involves electrochemical interactions among the catalyst material and reactive species (e.g., through emplacement of cathodic materials) to improve quantum efficiency; in this way, it is possible to use lower energy UVA radiation as the light source and yet achieve improved effectiveness.[13]
  • Ionizer purifiers use charged electrical surfaces or needles to generate electrically charged air or gas ions. These ions attach to airborne particles which are then electrostatically attracted to a charged collector plate. This mechanism produces trace amounts of ozone and other oxidants as by-products.[1] Most ionizers produce less than 0.05 ppm of ozone, an industrial safety standard. There are two major subdivisions: the fanless ionizer and fan-based ionizer. Fanless ionizers are noiseless and use little power, but are less efficient at air purification. Fan-based ionizers clean and distribute air much faster. Permanently mounted home and industrial ionizer purifiers are called electrostatic precipitators.
  • Immobilized cell technology removes microfine particulate matter from the air by attracting charged particulates to a bio-reactive mass, or bioreactor, which enzymatically renders them inert.
  • Ozone generators are designed to produce ozone, and are sometimes sold as whole house air cleaners. Unlike ionizers, ozone generators are intended to produce significant amounts of ozone, a strong oxidant gas which can oxidize many other chemicals. The only safe use of ozone generators is in unoccupied rooms, utilising “shock treatment” commercial ozone generators that produce over 3000 mg of ozone per hour. Restoration contractors use these types of ozone generators to remove smoke odors after fire damage, musty smells after flooding, mold (including toxic molds), and the stench caused by decaying flesh which cannot be removed by bleach or anything else except for ozone. However, it is not healthy to breathe ozone gas, and one should use extreme caution when buying a room air purifier that also produces ozone.[14]
  • Titanium dioxide (TiO2) technology – nanoparticles of TiO2, together with calcium carbonate to neutralize any acidic gasses that may be adsorbed, is mixed into slightly porous paint. Photocatalysis initiates the decomposition of airborne contaminants at the surface.[15]

Consumer concerns[edit]

Other aspects of air cleaners are hazardous gaseous by-products, noise level, frequency of filter replacement, electrical consumption, and visual appeal. Ozone production is typical for air ionizing purifiers. Although high concentration of ozone is dangerous, most air ionizers produce low amounts (< 0.05 ppm). The noise level of a purifier can be obtained through a customer service department and is usually reported in decibels (dB). The noise levels for most purifiers are low compared to many other home appliances.[citation needed] Frequency of filter replacement and electrical consumption are the major operation costs for any purifier. There are many types of filters; some can be cleaned by water, by hand or by vacuum cleaner, while others need to be replaced every few months or years. In the United States, some purifiers are certified as Energy Star and are energy efficient.

HEPA technology is used in portable air purifiers as it removes common airborne allergens. The US Department of Energy has requirements manufacturers must pass to meet HEPA requirements. The HEPA specification requires removal of at least 99.97% of 0.3 micrometers airborne pollutants. Products that claim to be “HEPA-type”, “HEPA-like”, or “99% HEPA” do not satisfy these requirements and may not have been tested in independent laboratories.

Air purifiers may be rated on: CADR(Clean Air Delivery Rate); efficient area coverage; air changes per hour; the clean air delivery rate, which determines how well air has been purified; energy usage; and the cost of the replacement filters. Two other important factors to consider are the length that the filters are expected to last (measured in months or years) and the noise produced (measured in decibels) by the various settings that the purifier runs on. This information is available from most manufacturers.

Potential ozone hazards[edit]

As with other health-related appliances, there is controversy surrounding the claims of certain companies, especially involving ionic air purifiers. Many air purifiers generate some ozone, an energetic allotrope of three oxygen atoms, and in the presence of humidity, small amounts of NOx. Because of the nature of the ionization process, ionic air purifiers tend to generate the most ozone.[citation needed] This is a serious concern, because ozone is a criteria air pollutant regulated by health-related US federal and state standards. In a controlled experiment, in many cases, ozone concentrations were well in excess of public and/or industrial safety levels established by US Environmental Protection Agency, particularly in poorly ventilated rooms.[16]

Ozone can damage the lungs, causing chest pain, coughing, shortness of breath and throat irritation. It can also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections—even in healthy people. People who have asthma and allergy are most prone to the adverse effects of high levels of ozone.[17] For example, increasing ozone concentrations to unsafe levels can increase the risk of asthma attacks.

Due to the below average performance and potential health risks, Consumer Reports has advised against using ozone producing air purifiers.[18] IQAir, the educational partner of the American Lung Association, has been a leading industry voice against ozone-producing air cleaning technology.[19]

Ozone generators used for shock treatments (unoccupied rooms) which are needed by smoke, mold, and odor remediation contractors as well as crime scene cleanup companies to oxidize and permanently remove smoke, mold, and odor damage are considered a valuable and effective tool when used correctly for commercial and industrial purposes. However, there is a growing body of evidence that these machines can produce undesirable by-products.[20]

In September 2007, the California Air Resources Board announced a ban of indoor air cleaning devices which produce ozone above a legal limit. This law, which took effect in 2010, requires testing and certification of all types of indoor air cleaning devices to verify that they do not emit excessive ozone.[21][22]

from wikipedia

Bone conduction

Bone conduction is the conduction of sound to the inner ear through the bones of the skull. Bone conduction transmission can be used with individuals with normal or impaired hearing.

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Bone conduction is one reason why a person’s voice sounds different to them when it is recorded and played back. Because the skull conducts lower frequencies better than air, people perceive their own voices to be lower and fuller than others do, and a recording of one’s own voice frequently sounds higher than one expects it to sound.[1][2]

Musicians may use bone conduction while tuning stringed instruments to a tuning fork. After the fork starts vibrating placing it in the mouth with the stem between the back teeth ensures that one continues to hear the note via bone conduction, and both hands are free to do the tuning.[3]:6

Hearing aids[edit]

Some hearing aids employ bone conduction, achieving an effect equivalent to hearing directly by means of the ears. A headset is ergonomically positioned on the temple and cheek and the electromechanical transducer, which converts electric signals into mechanical vibrations, sends sound to the internal ear through the cranial bones. Likewise, a microphone can be used to record spoken sounds via bone conduction. The first description, in 1923, of a bone conduction hearing aid was Hugo Gernsback‘s “Osophone”,[4] which he later elaborated on with his “Phonosone”.[5]

After the discovery of Osseointegration around 1950 and its application to dentistry around 1965, it was noticed that implanted teeth conducted vibrations to the ear. As a result, bone anchored hearing aids were developed and implanted from 1977 on.


Bone conduction products are usually categorized into three groups:

One example of a specialized communication product is a bone conduction speaker that is used by scuba divers. The device is a rubber over-moulded, piezoelectric flexing disc that is approximately 40 millimetres (1.6 in) across and 6 millimetres (0.24 in) thick. A connecting cable is moulded into the disc, resulting in a tough, waterproof assembly. In use, the speaker is strapped against one of the dome-shaped bone protrusions behind the ear and the sound, which can be surprisingly clear and crisp, seems to come from inside the user’s head.[6]

Use in the 21st century[edit]

The Google Glass device employs bone conduction technology for the relay of information to the user through a transducer that sits beside the user’s ear. The use of bone conduction means that any vocal content that is received by the Glass user is nearly inaudible to outsiders.[7]

German broadcaster Sky Deutschland and advertising agency BBDO Germany collaborated on an advertising campaign that uses bone conduction that was premiered in Cannes, France at the International Festival of Creativity in June 2013. The “Talking Window” advertising concept uses bone conduction to transmit advertising to public transport passengers who lean their heads against glass train windows. Academics from Australia’s Macquarie University suggested that, apart from not touching the window, passengers would need to use a dampening device that is made of material that would not transmit the vibration from the window.[8][9]

from wikipedia


Headphones (or head-phones in the early days of telephony and radio) are a pair of small listening devices that are designed to be worn on or around the head over a user’s ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound in the user’s ear. Headphones are designed to allow a single user to listen to an audio source privately, in contrast to a loudspeaker, which emits sound into the open air, for anyone nearby to hear. Headphones are also known as earspeakers, earphones[1] or, colloquially, cans.[2] Circumaural and supra-aural headphones use a band over the top of the head to hold the speakers in place. The other type, known as earbuds or earphones[1] consist of individual units that plug into the user’s ear canal. In the context of telecommunication, a headset is a combination of headphone and microphone. Headphones connect to a signal source such as an audio amplifier, radio, CD player, portable media player, mobile phone, video game console, or electronic musical instrument, either directly using a cord, or using wireless technology such as bluetooth or FM radio. The first headphones were developed in the late 1800s from telephone receivers for use by telephone operators, to keep their hands free. Initially the audio quality was mediocre and a step forward was the invention of high fidelity headphones.[3]

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Headphones are made in a range of different audio reproduction quality capabilities. Headsets designed for telephone use typically cannot reproduce sound with the high fidelity of expensive units designed for music listening by audiophiles. Headphones that use cables typically have either a 1/4 inch (6.35mm) or 1/8 inch (3.5mm) phone jack for plugging the headphones into the audio source. Some stereo earbuds are wireless, using Bluetooth connectivity to transmit the audio signal by radio waves from source devices like cellphones and digital players.[4] Due to the spread of wireless devices in recent years headphones are increasingly used by people in public places such as sidewalks, grocery stores, and public transit. Headphones are also used by people in various professional contexts, such as audio engineers mixing sound for live concerts or sound recordings and DJs, who use headphones to cue up the next song they will play without the audience hearing, aircraft pilots and call center employees. The latter two types of employees use headphones with an integrated microphone.


Brandes radio headphones, circa 1920

Headphones originated from the earpiece, and were the only way to listen to electrical audio signals before amplifiers were developed. The first truly successful set was developed in 1910 by Nathaniel Baldwin, who made them by hand in his kitchen and sold them to the United States Navy.[5][6]

Some very sensitive headphones, such as those manufactured by Brandes around 1919, were commonly used for early radio work. These early headphones used moving iron drivers, with either single ended or balanced armatures. The requirement for high sensitivity meant that no damping was used, thus the sound quality was crude. These early models lacked padding, and often produced excessive clamping forces on the wearer’s head. Their impedance varied; headphones used in telegraph and telephone work had an impedance of 75 ohms. Those used with early wireless radio had to be more sensitive and were made with more turns of finer wire. Impedance of 1000 to 2000 ohms was common, which suited both crystal sets and triode receivers.

In early powered radios, the headphone was part of the vacuum tube‘s plate circuit and carried dangerous voltages. It was normally connected directly to the positive high voltage battery terminal, and the other battery terminal was securely grounded. The use of bare electrical connections meant that users could be shocked if they touched the bare headphone connections while adjusting an uncomfortable headset.

In 1958, John C. Koss, an audiophile and jazz musician from Milwaukee, produced the first stereo headphones. Previously, headphones were used only by telephone and radio operators, and individuals in similar industries.

The 3.5 mm radio and phone connector, which is the most commonly used in portable application today, has been used at least since the Sony EFM-117J radio which was released in 1964.[7][8] It became very popular with its application on the Walkman in 1979.


Headphones may be used with stationary CD and DVD players, home theater, personal computers, or portable devices (e.g., digital audio player/mp3 player, mobile phone). Cordless headphones are not connected to their source by a cable. Instead, they receive a radio or infrared signal encoded using a radio or infrared transmission link, such as FM, Bluetooth or Wi-Fi. These are powered receiver systems, of which the headphone is only a component. Cordless headphones are used with events such as a Silent disco or Silent Gig.

Sennheiser HD 555 headphones, used in audio production environments (2007)

In the professional audio sector, headphones are used in live situations by disc jockeys with a DJ mixer, and sound engineers for monitoring signal sources. In radio studios, DJs use a pair of headphones when talking to the microphone while the speakers are turned off to eliminate acoustic feedback while monitoring their own voice. In studio recordings, musicians and singers use headphones to play or sing along to a backing track or band. In military applications, audio signals of many varieties are monitored using headphones.

Wired headphones are attached to an audio source by a cable. The most common connectors are 6.35 mm (¼″) and 3.5 mm phone connectors. The larger 6.35 mm connector is more common on fixed location home or professional equipment. The 3.5 mm connector remains the most widely used connector for portable application today. Adapters are available for converting between 6.35 mm and 3.5 mm devices.

Electrical characteristics[edit]

Electrical characteristics of dynamic loudspeakers may be readily applied to headphones, because most headphones are small dynamic loudspeakers.


Headphones are available with low or high impedance (typically measured at 1 kHz). Low-impedance headphones are in the range 16 to 32 ohms and high-impedance headphones are about 100-600 ohms. As the impedance of a pair of headphones increases, more voltage (at a given current) is required to drive it, and the loudness of the headphones for a given voltage decreases. In recent years, impedance of newer headphones has generally decreased to accommodate lower voltages available on battery powered CMOS-based portable electronics. This has resulted in headphones that can be more efficiently driven by battery-powered electronics. Consequently, newer amplifiers are based on designs with relatively low output impedance.

The impedance of headphones is of concern because of the output limitations of amplifiers. A modern pair of headphones is driven by an amplifier, with lower impedance headphones presenting a larger load. Amplifiers are not ideal; they also have some output impedance that limits the amount of power they can provide. In order to ensure an even frequency response, adequate damping factor, and undistorted sound, an amplifier should have an output impedance less than 1/8 that of the headphones it is driving (and ideally, as low as possible). If output impedance is large compared to the impedance of the headphones, significantly higher distortion will be present.[9] Therefore, lower impedance headphones will tend to be louder and more efficient, but will also demand a more capable amplifier. Higher impedance headphones will be more tolerant of amplifier limitations, but will produce less volume for a given output level.

Historically, many headphones had relatively high impedance, often over 500 ohms in order to operate well with high-impedance tube amplifiers. In contrast, modern transistor amplifiers can have very low output impedance, enabling lower-impedance headphones. Unfortunately, this means that older audio amplifiers or stereos often produce poor-quality output on some modern, low-impedance headphones. In this case, an external headphone amplifier may be beneficial.


Sensitivity is a measure of how effectively an earpiece converts an incoming electrical signal into an audible sound. It thus indicates how loud the headphones will be for a given electrical drive level. It can be measured in decibels of sound pressure level per milliwatt (dB (SPL)/mW) or decibels of sound pressure level per volt (dB (SPL) / V).[10] Unfortunately, both definitions are widely used, often interchangeably. As the output voltage (but not power) of a headphone amplifier is essentially constant for most common headphones, dB/mW is often more useful if converted into dB/V using Ohm’s Law:

{\displaystyle \mathrm {dB(SPL)} /\mathrm {V} =\mathrm {dB(SPL)} /\mathrm {mW} -10\cdot \log 10{\frac {\mathrm {Impedance} }{1000}}}{\displaystyle \mathrm {dB(SPL)} /\mathrm {V} =\mathrm {dB(SPL)} /\mathrm {mW} -10\cdot \log 10{\frac {\mathrm {Impedance} }{1000}}}

Alternatively, online calculators can be used.[11][12] Once the sensitivity per volt is known, the maximum volume for a pair of headphones can be easily calculated from the maximum amplifier output voltage. For example, for a headphone with a sensitivity of 100 dB (SPL)/V, an amplifier with an output of 1 root-mean-square (RMS) voltage will produce a maximum volume of 100 dB.

Pairing high sensitivity headphones with power amplifiers can produce dangerously high volumes and damage headphones. The maximum sound pressure level is a matter of preference, with some sources recommending no higher than 110 to 120 dB. In contrast, the American Occupational Safety and Health Administration recommends an average SPL of no more than 85 dB(A) to avoid long-term hearing loss, while the European Union standard EN 50332-1:2013 recommends that volumes above 85 dB(A) include a warning, with an absolute maximum volume (defined using 40–4000 Hz noise) of no more than 100 dB to avoid accidental hearing damage.[13] Using this standard, headphones with sensitivities of 90, 100 and 110 dB (SPL)/V should be driven by an amplifier capable of no more than 3.162, 1.0 and 0.3162 RMS volts at maximum volume setting, respectively to reduce the risk of hearing damage.

The sensitivity of headphones is usually between about 80 and 125 dB/mW and usually measured at 1 kHz.[14]


Headphone size can affect the balance between fidelity and portability. Generally, headphone form factors can be divided into four separate categories: circumaural (over-ear), supra-aural (on-ear), earbud and in-ear.


Circumaural headphones have large pads that surround the outer ear.

Circumaural headphones (sometimes called full size headphones) have circular or ellipsoid earpads that encompass the ears. Because these headphones completely surround the ear, circumaural headphones can be designed to fully seal against the head to attenuate external noise. Because of their size, circumaural headphones can be heavy and there are some sets that weigh over 500 grams (1 lb). Ergonomic headband and earpad design is required to reduce discomfort resulting from weight. These are commonly used by drummers in recording.


A pair of supra-aural headphones

Supra-aural headphones have pads that press against the ears, rather than around them. They were commonly bundled with personal stereos during the 1980s. This type of headphone generally tends to be smaller and lighter than circumaural headphones, resulting in less attenuation of outside noise. Supra-aural headphones can also lead to discomfort due to the pressure on the ear as compared to circumaural headphones that sit around the ear. Comfort may vary due to the earcup material.

Open or closed back[edit]

Both circumaural and supra-aural headphones can be further differentiated by the type of earcups:

Open-back headphones have the back of the earcups open. This leaks more sound out of the headphone and also lets more ambient sounds into the headphone, but gives a more natural or speaker-like sound, due to including sounds from the environment.

Closed-back (or sealed) styles have the back of the earcups closed. They usually block some of the ambient noise. Closed-back headphones tend to be able to produce stronger low frequencies than open-back headphones.

Semi-open headphones, have a design that can be considered as a compromise between open-back headphones and closed-back headphones. Some[who?] believe the term “semi-open” is purely there for marketing purposes. There is no exact definition for the term semi-open headphone. Where the open-back approach has hardly any measure to block sound at the outer side of the diaphragm and the closed-back approach really has a closed chamber at the outer side of the diaphragm, a semi-open headphone can have a chamber to partially block sound while letting some sound through via openings or vents.

Ear-fitting headphones[edit]

Earbuds/earphones sit in the outer ear


Earphones are very small headphones that are fitted directly in the outer ear, facing but not inserted in the ear canal. Earphones are portable and convenient, but many people consider them to be uncomfortable.[15] They provide hardly any acoustic isolation and leave room for ambient noise to seep in; users may turn up the volume dangerously high to compensate, at the risk of causing hearing loss.[15][16] On the other hand, they let the user be better aware of their surroundings. Since the early days of the transistor radio, earphones have commonly been bundled with personal music devices. They are sold at times with foam pads for comfort. (The use of the term “earbuds”, which has been around since at least 1984, did not hit it’s peak until after 2001, with the success of Apple’s MP3 player.[17])

In-ear headphones[edit]

Main article: In-ear monitor

In-ear monitors extend into the ear canal, providing isolation from outside noise.

In-ear headphones, also known as in-ear monitors (IEMs) or canalphones,[18] are small headphones with similar portability to earbuds that are inserted in the ear canal itself. IEMs are higher-quality in-ear headphones and are used by audio engineers and musicians as well as audiophiles.

Because in-ear headphones engage the ear canal, they can be prone to sliding out, and they block out much environmental noise. Lack of sound from the environment can be a problem when sound is a necessary cue for safety or other reasons, as when walking, driving, or riding near or in vehicular traffic.

Generic or custom-fitting ear canal plugs are made from silicone rubber, elastomer, or foam. Custom in-ear headphones use castings of the ear canal to create custom-molded plugs that provide added comfort and noise isolation.[15]


Main article: Headset (audio)

A typical example of a headset used for voice chats

A headset is a headphone combined with a microphone. Headsets provide the equivalent functionality of a telephone handset with hands-free operation. Among applications for headsets, besides telephone use, are aviation, theatre or television studio intercom systems, and console or PC gaming. Headsets are made with either a single-earpiece (mono) or a double-earpiece (mono to both ears or stereo). The microphone arm of headsets is either an external microphone type where the microphone is held in front of the user’s mouth, or a voicetube type where the microphone is housed in the earpiece and speech reaches it by means of a hollow tube.

Telephone headsets[edit]

Sony Ericsson Cordless bluetooth headset

Telephone headsets connect to a fixed-line telephone system. A telephone headset functions by replacing the handset of a telephone. Headsets for standard corded telephones are fitted with a standard 4P4C commonly called an RJ-9 connector. Headsets are also available with 2.5 mm jack sockets for many DECT phones and other applications. Cordless bluetooth headsets are available, and often used with mobile telephones. Headsets are widely used for telephone-intensive jobs, in particular by call centre workers. They are also used by anyone wishing to hold telephone conversations with both hands free.

For older models of telephones, the headset microphone impedance is different from that of the original handset, requiring a telephone amplifier for the telephone headset. A telephone amplifier provides basic pin-alignment similar to a telephone headset adaptor, but it also offers sound amplification for the microphone as well as the loudspeakers. Most models of telephone amplifiers offer volume control for loudspeaker as well as microphone, mute function and switching between headset and handset. Telephone amplifiers are powered by batteries or AC adaptors.

Ambient noise reduction[edit]

Among different types of headphones, in-ears are good for noise isolation.

Unwanted sound from the environment can be reduced by excluding sound from the ear by passive noise isolation, or, often in conjunction with isolation, by active noise cancellation.

Passive noise isolation is essentially using the body of the earphone, either over or in the ear, as a passive earplug that simply blocks out sound. The headphone types that provide most attenuation are in-ear canal headphones and closed-back headphones, both circumaural and supra aural. Open-back and earbud headphones provide some passive noise isolation, but much less than the others. Typical closed-back headphones block 8 to 12 dB, and in-ears anywhere from 10 to 15 dB. Some models have been specifically designed for drummers, with the aim to be able to monitor the recorded sound while shutting out the sound coming directly from the drums at the same time as much as possible. Such headphones claim to reduce ambient noise by around 25 dB.

Active noise-cancelling headphones use a microphone, amplifier, and speaker to pick up, amplify, and play ambient noise in phase-reversed form; this to some extent cancels out unwanted noise from the environment without affecting the desired sound source, which is not picked up and reversed by the microphone. They require a power source, usually a battery, to drive their circuitry. Active noise cancelling headphones can attenuate ambient noise by 20 dB or more, but the active circuitry is mainly effective on constant sounds and at lower frequencies, rather than sharp sounds and voices. Some noise cancelling headphones are designed mainly to reduce low-frequency engine and travel noise in aircraft, trains, and automobiles, and are less effective in environments with other types of noise.

Transducer technology[edit]

Various types of transducer are used to convert electrical signals to sound in headphones.


A typical moving-coil headphone transducer

The moving coil driver, more commonly referred to as a “dynamic” driver is the most common type used in headphones. It consists of a stationary magnet element affixed to the frame of the headphone which sets up a static magnetic field. The magnet in headphones is typically composed of ferrite or neodymium. A voice coil, a light coil of wire, is suspended in the magnetic field of the magnet, attached to a diaphragm, typically fabricated from lightweight, high stiffness to mass ratio cellulose, polymer, carbon material, or the like. When the varying current of an audio signal is passed through the coil, it creates a varying magnetic field which reacts against the static magnetic field, exerting a varying force on the coil causing it and the attached diaphragm to vibrate back and forth. The vibrating diaphragm pushes on the air producing sound waves.


Electrostatic loudspeaker diagram

Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes). The electrical sound signal is applied to the electrodes creating an electrical field; depending on the polarity of this field, the diaphragm is drawn towards one of the plates. Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated. Electrostatic headphones are usually more expensive than moving-coil ones, and are comparatively uncommon. In addition, a special amplifier is required to amplify the signal to deflect the membrane, which often requires electrical potentials in the range of 100 to 1000 volts.

Due to the extremely thin and light diaphragm membrane, often only a few micrometers thick, and the complete absence of moving metalwork, the frequency response of electrostatic headphones usually extends well above the audible limit of approximately 20 kHz. The high frequency response means that the low midband distortion level is maintained to the top of the audible frequency band, which is generally not the case with moving coil drivers. Also, the frequency response peakiness regularly seen in the high frequency region with moving coil drivers is absent. Well-designed electrostatic headphones can produce significantly better sound quality than other types.[citation needed]

Electrostatic headphones require a voltage source generating 100 V to over 1 kV, and are on the user’s head. They do not need to deliver significant electric current, which limits the electrical hazard to the wearer in case of fault.


An electret driver functions along the same electromechanical means as an electrostatic driver. However the electret driver has a permanent charge built into it, where electrostatics have the charge applied to the driver by an external generator. Electret and electrostatic headphones are relatively uncommon. Original electrets were also typically cheaper and lower in technical capability and fidelity than electrostatics. Patent applications from 2009-2013 have been approved that show by using different materials,i.e. a “Fluorinated cyclic olefin electret film”, Frequency response chart readings can reach 50 kHz at 100db.When these new improved electrets are combined with a traditional dome headphone driver, Headphones can be produced that are recognised by the Japan Audio Society as worthy of joining the Hi Res Audio program.US patents 8,559,660 B2. 7,732,547 B2.7,879,446 B2.7,498,699 B2 .


Orthodynamic (also known as Planar Magnetic) headphones use similar technology to electrostatic headphones, with some fundamental differences. They operate similarly to Planar Magnetic Loudspeakers.

An orthodynamic driver consists of a relatively large membrane which contains an embedded wire pattern. This membrane is suspended between two sets of permanent, oppositely aligned, magnets. When current is passed through the wires which are embedded in the membrane, the magnetic field produced by the current reacts with the field caused by the permanent magnets and induces movement in the membrane, producing sound.

Balanced armature[edit]

Balanced armature transducer with armature balanced and exerting no force on diaphragm

A balanced armature is a sound transducer design primarily intended to increase the electrical efficiency of the element by eliminating the stress on the diaphragm characteristic of many other magnetic transducer systems. As shown schematically in the first diagram, it consists of a moving magnetic armature that is pivoted so it can move in the field of the permanent magnet. When precisely centered in the magnetic field there is no net force on the armature, hence the term ‘balanced.’ As illustrated in the second diagram, when there is electric current through the coil, it magnetizes the armature one way or the other, causing it to rotate slightly one way or the other about the pivot thus moving the diaphragm to make sound.

The JH Audio JH16 custom in-ear monitor utilizes 8 balanced armatures in a triple crossover configuration (4 low/2 mid/2 high). Multiple balanced armatures are often used to provide a higher fidelity sound.

The design is not mechanically stable; a slight imbalance makes the armature stick to one pole of the magnet. A fairly stiff restoring force is required to hold the armature in the ‘balance’ position. Although this reduces its efficiency, this design can still produce more sound from less power than any other[clarification needed]. Popularized in the 1920s as Baldwin Mica Diaphragm radio headphones, balanced armature transducers were refined during World War II for use in military sound powered telephones. Some of these achieved astonishing electro-acoustic conversion efficiencies, in the range of 20% to 40%, for narrow bandwidth voice signals.

Today they are typically used only in in-ear headphones and hearing aids, where their diminutive size is a major advantage. They generally are limited at the extremes of the hearing spectrum (e.g. below 20 Hz and above 16 kHz) and require a better seal than other types of drivers to deliver their full potential. Higher-end models may employ multiple armature drivers, dividing the frequency ranges between them using a passive crossover network. A few combine an armature driver with a small moving-coil driver for increased bass output.

The earliest loudspeakers for radio receivers used balanced armature drivers for their cones.

Thermoacoustic technology[edit]

The thermoacoustic effect generates sound from the audio frequency Joule heating of the conductor, an effect which is not magnetic and does not vibrate the speaker. In 2013 a carbon nanotube thin-yarn earphone based on the thermoacoustic mechanism was demonstrated by a research group in Tsinghua University.[19] The as-produced CNT thin yarn earphone has a working element called CNT thin yarn thermoacoustic chip. Such a chip is composed of a layer of CNT thin yarn array supported by the silicon wafer, and periodic grooves with certain depth are made on the wafer by micro-fabrication methods to suppress the heat leakage from the CNT yarn to the substrate.[citation needed]

Other transducer technologies[edit]

Transducer technologies employed much less commonly for headphones include the Heil Air Motion Transformer (AMT); Piezoelectric film; Ribbon planar magnetic; Magnetostriction and Plasma-ionisation. The first Heil AMT headphone was marketed by ESS Laboratories and was essentially an ESS AMT tweeter from one of the company’s speakers being driven at full range. Since the turn of the century, only Precide of Switzerland have manufactured an AMT headphone. Piezoelectric film headphones were first developed by Pioneer, their two models both used a flat sheet of film which limited the maximum volume of air that could be moved. Currently TakeT produce a piezoelectric film headphone which is shaped not unlike an AMT transducer but which like the driver Precide uses for their headphones, has a variation in the size of transducer folds over the diaphragm. It additionally incorporates a two way design by its inclusion of a dedicated tweeter/supertweeter panel. The folded shape of a diaphragm allows a transducer with a larger surface area to fit within smaller space constraints. This increases the total volume of air that can be moved on each excursion of the transducer given that radiating area.

Magnetostriction headphones, sometimes sold under the label of “Bonephones”, are headphones that work via the transmission of vibrations against the side of head, transmitting the sound via bone conduction. This is particularly helpful in situations where the ears must be left unobstructed or when used by those who are deaf for reasons which do not affect the nervous apparatus of hearing. Magnetostriction headphones though, have greater limitations to their fidelity than conventional headphones which work via the normal workings of the ear. Additionally, there was also one attempt to market a plasma-ionisation headphone in the early 1990s by a French company called Plasmasonics. It is believed that there are no functioning examples left.

Benefits and limitations[edit]

Two Sony MDR-V6 headphones; one folded for travel

Headphones may be used to prevent other people from hearing the sound either for privacy or to prevent disturbance, as in listening in a public library. They can also provide a level of sound fidelity greater than loudspeakers of similar cost. Part of their ability to do so comes from the lack of any need to perform room correction treatments with headphones. High quality headphones can have an extremely flat low-frequency response down to 20 Hz within 3 dB. Marketed claims such as ‘frequency response 4 Hz to 20 kHz’ are usually overstatements; the product’s response at frequencies lower than 20 Hz is typically very small.[20]

Headphones are also useful for video games that use 3D positional audio processing algorithms, as they allow players to better judge the position of an off-screen sound source (such as the footsteps of an opponent or their gun fire).

Although modern headphones have been particularly widely sold and used for listening to stereo recordings since the release of the Walkman, there is subjective debate regarding the nature of their reproduction of stereo sound. Stereo recordings represent the position of horizontal depth cues (stereo separation) via volume and phase differences of the sound in question between the two channels. When the sounds from two speakers mix, they create the phase difference the brain uses to locate direction. Through most headphones, because the right and left channels do not combine in this manner, the illusion of the phantom center can be perceived as lost. Hard panned sounds will also only be heard only in one ear rather than from one side.

Binaural recordings use a different microphone technique to encode direction directly as phase, with very little amplitude difference below 2 kHz, often using a dummy head, and can produce a surprisingly lifelike spatial impression through headphones. Commercial recordings almost always use stereo recording, rather than binaural, because loudspeaker listening has been more popular than headphone listening.

It is possible to change the spatial effects of stereo sound on headphones, to better approximate the presentation of speaker reproduction, by using frequency-dependent cross-feed between the channels.

Headsets can have ergonomic benefits over traditional telephone handsets. They allow call center agents to maintain better posture without needing to hand-hold a handset or tilt their head sideways to cradle it.[21]

Dangers and volume solutions[edit]

Product testing – headphones in an anechoic chamber

Using headphones at a sufficiently high volume level may cause temporary or permanent hearing impairment or deafness. The headphone volume often has to compete with the background noise, especially in loud places such as subway stations, aircraft, and large crowds. Extended periods of exposure to high sound pressure levels created by headphones at high volume settings may be damaging;[22][23] however, one hearing expert found that “fewer than 5% of users select volume levels and listen frequently enough to risk hearing loss.”[24] Some manufacturers of portable music devices have attempted to introduce safety circuitry that limited output volume or warned the user when dangerous volume was being used, but the concept has been rejected by most of the buying public, which favors the personal choice of high volume. Koss introduced the “Safelite” line of cassette players in 1983 with such a warning light. The line was discontinued two years later for lack of interest.

The government of France has imposed[25] a limit on all music players sold in the country:[25] they must not be capable of producing more than 100dBA (the threshold of hearing damage during extended listening is 80 dB, and the threshold of pain, or theoretically of immediate hearing loss, is 130 dB).[26] Motorcycle and other power-sport riders benefit by wearing foam earplugs when legal to do so to avoid excessive road, engine, and wind noise, but their ability to hear music and intercom speech is enhanced when doing so. The ear can normally detect 1-billionth of an atmosphere of sound pressure level,[27] hence it is incredibly sensitive. At very high sound pressure levels, muscles in the ear tighten the tympanic membrane and this leads to a small change in the geometry of the ossicles and stirrup that results in lower transfer of force to the oval window of the inner ear (the acoustic reflex).[28]

Some studies have found that people are more likely to raise volumes to unsafe levels while performing strenuous exercise.[29] A Finnish study[30] recommended that exercisers should set their headphone volumes to half of their normal loudness and only use them for half an hour.

Passive noise canceling headphones can be considered dangerous because of a lack of awareness the listener may have with their environment. Noise cancelling headphones are so effective that a person may not be able to hear oncoming traffic or pay attention to people around them. There is also a general danger that music in headphones can distract the listener and lead to dangerous situations.[31]

The usual way of limiting sound volume on devices driving headphones is by limiting output power. This has the additional undesirable effect of being dependent of the efficiency of the headphones; a device producing the maximum allowed power may not produce adequate volume in low-efficiency, while possibly reaching dangerous levels in very efficient ones.[citation needed]

from wikipedia