Forum Proceedings

Technology Area

Clinicians, researchers, consumers and manufacturers all identified microphone technology as an area that could benefit from the development and application of new and innovative technology. Improved microphone technology underlies the development of improved hearing aids and assistive listening systems. Advancements to these systems would meet significant end users needs and represent significant business opportunities.

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Market Need

Most people rely upon hearing for communication, social participation and personal safety in their work, recreational, and daily living environments. A person's ability to hear is often reduced by trauma, drugs, disease, or the cumulative effects of aging. Diminished and uncorrected hearing can lead to social isolation, difficulty functioning within the workplace, increased risk to personal safety and a general reduction of a person's perceived quality of life. Assistive technology for hearing including (but not limited to) hearing aids and assistive listening systems, are an effective and enabling intervention for many of these persons.

Hearing and understanding speech accompanied by noise and reverberation is the principle concern of persons with hearing impairments, along with hearing local sound sources in background noise and hearing remote sound sources generally. Hearing aids; wearable, hand-held and remote FM microphones; and assistive listening systems are the principal interventions by which the comprehension of speech is improved. Microphones are an essential component of all hearing aids and most assistive listening systems.

According to the 1990-91 National Health Survey study, 3.6 million people self-identified as having hearing problems use hearing aids. Another 847,000 people use assistive listening systems other than these hearing aids (National Center for Health Statistics, 1997). An additional 16,000,000+ people in the United States could benefit from the use of a hearing aid or assistive listening devices. Hearing aids range from $800 to $1000 for Behind the Ear (BTE) models to $1200 to $2000 for completely in the ear canal models. In recent interviews, consumers stated that the hearing aids they use need better directionality. They said that their hearing aids are unable to narrow their focus to the source of the desired sound. The technology today to help people hear is not sufficient, and many people who are hearing impaired refuse to even use the current devices on the market (Frost & Sullivan, 1994). The United States population is aging with more people living well beyond age 65, into their 70's and 80's. All of these persons are potential consumers of hearing technology now and into the future. The aged population is expected to grow until the year 2036 when this population is expected to reach its maximum level. The world population is also following a similar growth trend.

The Americans with Disabilities Act (ADA) has increased the availability of Assistive Listening Systems (ALS's) for employment, education, and access to public buildings and transportation. ALS's can be permanently installed or set up upon request, although timely access to the ALS is not assured in the latter case. Of course, the assistive listening system must also be properly maintained (e.g. receiver batteries charged) in order to serve the user's needs.

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Basis for Discussion

Significant progress is being made improving the consumer's listening experience in noisy environments through the use of directional hearing aids, hand­held and wearable microphones and assistive listening systems.

Consumers are significantly more satisfied with directional hearing aids when compared to high performance non-directional hearing aids in listening environments such as restaurants, cars, concerts, movies, and church. The directional microphones available on many of today's advanced instruments allows the hearing aid to attenuate peripheral sounds and focus on sounds directly in front of the listener. This permits better speech comprehension despite the noise. In one-on-one conversations, directional microphones result in better speech comprehension in noisy environments (Center for Hearing Aid Research & Technical Training, 1999).

Wearable and hand-held "directional beam-forming microphone arrays" demonstrate superior directional performance relative to hearing aid directional microphones. These microphones, used in conjunction with hearing aids, have great potential to improve the listening experience of hearing impaired individuals (Andrea Electronics, 2000; Starkey Laboratories, 1999).

Assistive Listening Systems bring a remote, essentially noise free, sound signal directly to the hearing impaired listener across the intervening reverberant and noise filled acoustic space. ALS's extend the hearing range of these individuals.

Microphone improvements in hearing aids, wearable and hand-held devices, and ALS's will provide important benefits to end-users and business opportunities to manufacturers.

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Current Technology

A microphone is a device that transforms sound into an electrical signal. Almost all microphones have a diaphragm that moves when struck by sound waves. Microphones differ primarily in the means by which this diaphragm movement is transformed into an electrical signal. Here are three of the many microphone technologies (Elsea, 2000b; Tan, 1996).

In dynamic microphones the diaphragm is connected to a coil of wire placed in a magnetic field. When the diaphragm is struck by sound waves, coil movement in the magnetic field generates an electric current. For condenser microphones, the diaphragm is separated from a back-plate forming a capacitor (a charge storage device). A static charge is placed upon either the diaphragm or back-plate. When struck by a sound wave, the separation between the diaphragm and back-plate changes the capacitor's ability to store charge and an electric current is generated. Condenser microphones require a battery or external power source to maintain the charge on the diaphragm or back plate. Condenser microphones produce a very small signal that must be amplified. Any electrical noise produced by or picked up by the microphone is also amplified.

Electret microphones, a common variant of the condenser microphone, use an 'electret' material for the diaphragm or back plane. This electret material (e.g. plastic, silicon dioxide, teflon) has a permanently embedded static charge that eliminates the need for an external battery or power supply (Elsea, 2000b).

An important characteristic of microphones is their directional (polar) response. Here are several of the most important response patterns and ways in which these responses might be obtained.

• Omnidirectional microphones respond equally to sounds coming from any direction. The back of the diaphragm for an omnidirectional microphone is enclosed. Sound from any direction striking the microphone creates a pressure gradient across the diaphragm. This pressure gradient then causes diaphragm displacement.
• Bidirectional microphones respond strongly to sounds from the front or rear of the microphone but do not respond to sound from the sides. The back and front of the diaphragm for a bi-directional microphone is open. Sound striking the front or rear of the microphone creates a pressure gradient across the diaphragm, displacing it. A sound striking from the front causes a positive displacement while sounds striking from the rear cause a negative displacement. Sound from the side of the microphone places equal pressure on the front and back of the diaphragm, no displacement takes place.
• Cardioid microphones respond most strongly to sounds from the front of the microphone. Sounds from the back of the microphone are reduced (not eliminated). The directional response varies sharply with the sound frequency. Cardioid microphones often respond strongly at low frequencies when the sound source is close to the microphone. This is known as the "proximity effect." In principle a cardioid microphone response can be obtained by adding responses from properly matched omnidirectional and bi-directional microphones.
• Super-cardioid and hyper-cardioid microphones have a tighter forward directional response than a cardioid microphone but are increasingly responsive to sound from the rear of the microphone.

In practice, a practical cardioid microphone response is commonly obtained by the clever use of acoustic delays. Roughly, consider a diaphragm placed at the front of a short tube (Elsea, 2000a). Sound from in front of the microphone strikes the front of the diaphragm but must go up the length of the tube and back before it strikes the rear of the diaphragm. This arrival delay produces a pressure gradient and diaphragm displacement. The extra time needed to go up the length of the tube and back is the acoustic delay. In contrast, a sound from the rear of the microphone travels down the inside and outside of tube simultaneously and strikes the front and rear of the diaphragm nearly simultaneously. Only a small pressure gradient is produced with little displacement. This simple approach will only work well for a narrow range of frequencies. In order to spread the directional effect across a wider frequency range, real microphones generally provide many delay paths (with differing acoustic delays).

A noise-canceling microphone generally refers to a combination of a directional and omnidirectional microphone. When the desired signal-in-noise strikes the noise-canceling microphone, the directional microphone captures the sound of interest strongly and the surrounding noise less strongly (assuming the microphone is pointed at the sound source) whereas the omnidirectional microphone captures the sound of interest and surrounding noise equally. Roughly, subtracting the omnidirectional response from the directional response enhances the signal strength relative to the noise strength (signal-to-noise ratio). Signal processing hardware and algorithms may be employed to enhance this effect.

Recently beam-forming microphone arrays have been employed to separate a sound signal from noise. Roughly, an array of microphones is placed in a plane, each microphone apart from the others. A sound wave strikes the array. If its direction of movement is perpendicular to the plane in which the array lies, it arrives at each microphone at about the same time (in phase). If its direction of movement is not perpendicular to the plane, it strikes each microphone at a slightly different time (out of phase). This effect varies with the sound frequency and the angle at which the sound wave strikes the array. In addition, the speech and noise around us is a complex mix of frequencies. By application of powerful signal processing hardware and algorithms, this phase information can be used to accept sound coming from one direction but reject (significantly attenuate) sound coming from other directions.

A wireless remote microphone is essentially a microphone connected to a wireless (typically) FM transmitter. The microphone transmitter can be in the microphone body or in a separate 'box' often carried in a pocket or clipped to a belt Lavalier microphones (interviewer's collar pin) are a common example. Each wireless microphone usually transmits on a single unique frequency. A 'true diversity' wireless system has two antennas on the receiver. When the signal strength at the two antennas differs, the receiver uses the stronger signal. True diversity systems are typically less sensitive to radio interference and blockage than single antenna systems.

Hearing Aid Microphones

Digital microphones and digital noise reduction algorithms are two approaches used in hearing aid technology to distinguish speech from noise. Directional microphones actually improve the user's ability to perceive speech in noise. Digital noise reduction improves the user's hearing comfort in noise but does not generally improve their ability to perceive speech.

Conventional directional hearing aid microphones use a single microphone with two ports and acoustic delays. They are only available on BTE (behind-the-ear) hearing aids and often cannot be switched between directional and omnidirectional response patterns.

Newer directional hearing aid microphones use two microphones (two omni-directional microphones plus beam‑forming electronics or a directional plus omnidirectional microphone). Each microphone has its own port (Gennum Corporation, 2000). These dual port microphones available for both BTE and ITE (in-the-ear) hearing aids, can support a wide range of directivity patterns (e.g. cardioid to hypercardioid) and can typically be switched between directional and omnidirectional response patterns (Etymotic Research, 2000b). Generally, directional microphones are less sensitive at low frequencies with further sensitivity reduction with small microphone separation. Some directional microphones provide more than four decibels improvement in AI-DI (Orientation-Index-Weighted-Directivity Index; Etymotic Research, 2000a).

Hand Held and Wearable Microphones

The directional sensitivity of wearable beam-forming microphone arrays is (typically) much better than that of directional hearing aid microphones. The signal processing hardware and algorithms used by these microphones currently cannot be implemented at ear level (within a hearing aid). Instead, hand-held or wearable (e.g. pendant) beam-forming microphone arrays are coupled to the user's hearing aid by an inductive or FM wireless link.

Hand-held, desktop and wearable beam-forming microphone arrays often have advanced features such as user adjustable beam patterns; automatically changing the directional response from speaker to speaker; or automatically changing directional sensitivity in response to the noise characteristics of the environment. Hearing Aids can be linked by wire or wireless means, and act as a beam forming array themselves (Kompis, 2000).

Assistive Listening Systems

The purpose of many ALS's is to bring a remote and essentially `noise free' sound signal into the direct-proximity of the user's ear. ALS's process and then transmit the remote signal via a wireless link typically infrared light, inductive (electromagnetic) fields, or FM radio waves. There are a number of ways that a user can receive these transmissions. The receiver can be part of the hearing aid (e.g. built-in FM receivers or telecoils that pick up inductive transmissions), or a hearing aid accessory (e.g. an accessory FM receiver). In some cases, users wear an FM or IR receiver that outputs sound through headphones or ear buds. Other systems use FM or IR receivers, which then process and retransmit the signal via an inductive neck loop to be picked up by the hearing aid T-coil (Bakke, Levitt, Ross, & Erickson, 1999).

Beam-forming microphone arrays have good applications in meetings and small group settings in which the conversation jumps from one distinct speaker to another. These microphones can be integrated into ALS's - eliminating the need to pass around a single microphone from speaker to speaker. Because of their superior directional performance and ability to eliminate extraneous noise, these microphones are components in high-performance, PC-based voice recognition systems.

Various other types of microphones are used in ALS's. The most appropriate microphone is determined by the communication needs, the assistive listening system technology and the environment of use. They include:

• Boom - head-worn microphone worn close to mouth. Because the microphone is mounted on a headband, movement of the head does not affect loudness of sound going into the microphone.
• Collar - microphone is held in front of the user's lips by a bendable snake-like wire that wraps around the user's neck. Often used in classroom teaching.
• Directional Lapel - picks up sound mostly from in front of the microphone. This microphone is useful in noisy listening environments.
• Omni-directional Lapel - picks up sounds all around the microphone (although the body is in the way of the microphone). Worn 4-10 inches from lips and used in classroom teaching.
• Omni-directionaVPZM - the microphone is placed on a hard, flat surface (table) and picks up sounds from a 360° azimuth. Good for situations where you have listeners seated around a table. To work best, the room must be quiet and speakers must avoid generating additional noise.
• Voice Activated Microphones - activated only when person speaks. May work well in noisy situations. Usually part of a boom headset.

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Statement of the Problem

The following issues were raised during interviews and panels involving researchers, manufacturers and end-users.

• High performance hearing aids with directional microphones need to available at a reasonable price for persons with mild to moderate hearing loss.
• Superior directional microphone technology exists in research literature and military but has not been transferred to the hearing aid and assistive listening device industry.
• Assistive listening systems for small group settings currently use a single microphone (e.g. passed around) or multiple microphones (e.g. one selected to be active at a time, voice activated). With an adaptive beam‑forming microphone array that automatically switches from speaker to speaker, these small group ALS's could become true multi‑speaker systems.
• The "tunnel of sensitivity" for directional hearing aid microphones changes as the user moves his head. To be most effective the sound source or speaker must be directly in front of the user and the user's head must be still. Highly directional hearing aids may not work well when the conversation is jumping rapidly from speaker to speaker (e.g., social gathering), or when the environment is providing important safety or navigation cues (e.g., crossing an intersection). Manually switching the hearing aid from directional to non-directional mode is the best solution currently available.
• A wireless link between hearing aids, or between the hearing aids and an external processor, would support advanced binaural processing. This approach has been explored with hearing aids and hard-wired connections. Improved directionality was demonstrated but the hard-wire connection is cumbersome and unacceptable to the end-users.
• Increasing the geometry and number of microphones in wearable or hand-held beam forming directional arrays improves directional performance. Increasing the size or weight of a wearable (e.g. necklace, headband, or eyeglass frame) or hand-held device is likely to be burdensome and unacceptable to end-users.
• Microphones often have unpredictable directional performance. Manufacturing tolerances need to be improved so that a uniform directional response is obtained from microphone to microphone.
• Dual directional microphones have great value but there is still a need for improved performance for single directional microphones.
• Microphone improvements may require an increase in cost, size or power consumption but those changes would prevent them from being incorporated into the hearing aid.
• How can microphones be reduced in size, yet demonstrate good sensitivity (especially at low frequencies)?

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Issues to Consider

The Need

1. What are the important, unmet (or poorly met) user needs related to microphones in hearing aids; wearable and hand-held devices; or assistive listening systems?
2. What populations or demographics (e.g. degree of hearing loss, characteristics of hearing loss, cause of hearing loss, age, etc.) are most affected by these needs/problems?
3. In which environments and for which activities is this need or problem most significant?
4. What accommodations (or behavioral changes) do hearing impaired persons make in order to function in these environments and accomplish these activities?

State-of-the-Practice

5. What hearing and assistive listening products are used by, or prescribed for, hearing impaired persons' in order to address these problems or needs?
6. What are the strengths (e.g. performance, cost, etc.) of these products?
7. What are the weaknesses (e.g. performance, cost, etc.) of these products?

Future Technology and Products

8. What significant technical improvements are needed?
9. What technical barriers (e.g. environmental factors, power consumption, size, etc.) must be overcome in order to achieve these technical improvements?
10. What breakthrough technologies (not present in current products) might better address the identified needs and problems?
11. What technical barriers (e.g. environmental factors, power consumption, size, etc.) must be overcome in order to achieve these technical breakthroughs?

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References

Andrea Electronics, Corp. (2000). DA-400 Directional Array. Available: www.andreaelectronics.com.dsddesk.htm [April 25, 2000].

Bakke, M, Levitt, H, Ross, M, & Erickson, F. (1999). Large Area Assistive Listening System: Review and Recommendations. RERC on Hearing Enhancement. Available: http://www.hearingresearch.org/LargeAreaALS.htm [December 1, 2000].

Center for Hearing Aid Research & Technical Training. (1999, October 21-22, 1999). CHARTting the Course: Compression, Digital & Directional Hearing Aid Technology.

Elsea, P. (April 25, 2000a). Making Directional Microphones. University of Santa Cruz. Available: http://atrs.ucsc.edu/ems/music/tech_background/TE-20/ Directional_Microphones.html [April 25, 2000].

Elsea, P. (April 25, 2000b). Microphones. University of Santa Cruz. Available: http://atrs.ucsc.edu/ems/music/tech_background/TE-20/teces_20html [April 25, 2000].

Etymotic Research. (2000a). OEM Products, Articulation-Index-Weighted Directivity Index Explained. Available: http://www.etymotic.com/html/main.cgi?sub=38 [April 25, 2000].

Etymotic Research. (2000b). OEM Products, ER-81 D-Mic Directional Microphone. Available: http://www.etymotic.com/html/main.cgi?sub=38 [April 25, 2000].

Frost & Sullivan. (1994). World Audiology Product Markets. Available: http://www.frost.com [March 29, 2000].

Gennum Corporation. (2000). Frontwave Multi-Microphone Hybrid, Directional Systems. Available: http://frontwave.gennum.com/hip/frontwave/features.htm [April 25, 2000].

Kompis, M. (2000). A Combined Fixed/Adaptive Beamforming Hoise-Reduction System for Hearing Aids. IEEE. Available: http://www.orlinsel.ch/research/htm [April 25, 2000].

National Center for Health Statistics. (1997). Advance Data: Vital Statistics of the Centers for Disease Control and Prevention (292). Hyattsville, MD: National Center for Health Statistics.

Starkey Laboratories, Inc. (1999). Radiant Beam Array Fitting Guide, Starkey Product Application. Eden Prairie, MN: Starkey Laboratories, Inc.

Tan, P. (1996). Multimedia Bluffers' Guide: Microphones. Philip Tan. Available: http://atrs.uscs.edu/ems/music/tech_background/TE-20/teces_20html [April 25, 2000].

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