Forum Proceedings

Summary

Manufacturers, researchers, and clinicians have identified beam-forming microphone arrays for body worn, tabletop, binaural hearing aid and directional hearing aid microphones as the critical technology. These technologies address important, unmet needs of persons with hearing impairments and represent good business opportunities for manufacturers.

[ Top of Page ]

Market

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. However, it is the microphone(s) contained within these hearing systems that translate sound into an electrical signal that is re-translated back to sound for the wearer to hear.

It is estimated that more than 20 million people in the United States experience some form of hearing loss. However, according to the 1990-91 National Health Survey, only 18% of those who identified themselves as having hearing problems use hearing aids (over the age of three and non-institutionalized). The reasons people who experience hearing loss but chose not to use the available technologies include: "hearing aids do not perform in noisy situations" (7.1 million), "provide too much whistle or feedback" (6.4 million), "do not work well" (4.8 million) or "work only in limited situations" (4.3 million), "have poor sound quality" (3.9 million), "break down too much" (3.4 million), "can not be used on the telephone" (3.1 million), and "negative experiences of friends" (3.9 million) (Kochkin, 1997).

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 (such as differentiating voices in a crowded room in order to participate in a conversation), and hearing remote sound sources in general (for example, hearing a doorbell or telephone ring, or the sirens on an emergency vehicle). Hearing aids; body worn, hand-held, and remote FM microphones; and assistive listening systems are the principal interventions by which the comprehension of sound is improved. Microphones are an essential component of all hearing aids and most assistive listening systems.

According to the Advance Data Report (National Center for Health Statistics, 1997) there are approximately 874,000 people in the US who are hard-of-hearing and who use assistive hearing devices; this is less than 5% of the potential hard-of-hearing market segment. This leaves more than 16 million people who have substantially correctable hearing loss but whose needs are not being met by the current technologies available to them.

[ Top of Page ]

Current Technology

Recently, beam forming microphone arrays have been employed in hearing aids, hand held batons, body worn, eyeglass frames (NIH sponsored research which resulted in a design with four microphones on the frame), and table top systems. The performance of beam-forming microphone array depends upon the placement and spacing of these microphones on a hearing aid, body worn or other device. Some of these microphones can adaptively change their directional response (narrow the directional response in the presence of noise, track a moving speaker, switch to new speaker). For persons with intact hearing, the brain quickly orients the person's eyes to the sound source. For adaptive microphones that orient to new speakers the brain can't (directly) fulfill this function.

In one-on-one conversations, hearing aids with directional microphones result in better speech comprehension in noisy environments (Trine, 1999).  But, the user must face the speaker (sound source) and this may decrease the ability to locate new speakers. Conventional directional hearing aids 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 omni-directional response patterns.

Newer directional hearing aids use two microphones (two omni-directional or a directional plus omni-directional microphone) each with its own port plus digital signal processing (Bakke, Levitt, Ross, & Erickson, 1999). These dual port microphones are available for both BTE and ITE (in-the-ear) hearing aids. They support a wide range of directivity patterns (e.g. cardioid to hyper-cardioid) and generally can be switched (manually or automatically) between directional and omni-directional modes (Etymotic Research, 2000b). The small physical separation between microphones on hearing aids reduces their performance at low frequencies. Directional microphones on hearing aids can provide 4 to 5 decibels improvement (AI-DI, Orientation-Index-Weighted-Directivity Index) and have an effective hearing range of six feet or less (Etymotic Research, 2000a).

Hearing Aids can be linked by wire or wireless means and can act as a beam-forming microphone array themselves. Binaural hearing aids use the increased separation between microphones, head shadow effects and time and phase delays to mimic the capabilities of an intact hearing system. Binaural processing requires a bi-directional communication link between the two hearing aids.

The directional performance of hand-held and wearable microphone arrays is (typically) much better than that of directional hearing aid microphones. Some of these devices achieve a directivity index of about 12dB. The signal processing hardware used by these microphones cannot currently 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.  Used in conjunction with hearing aids, wearable and hand-held beam-forming microphone arrays have great potential to improve the listening experience of hearing impaired individuals (Andrea Electronics, 2000; Trine, 1999). Assistive Listening Systems (ALS) bring a remote (essentially 'noise free') sound into the direct-proximity of the user's ear in order to amplify a selected sound source, overcome background noise, enhance listening in large public venues, and improve one-to-one conversations. Used in combination with hearing aids an ALS can provide optimal sound clarity and speech comprehension. ALS are categorized by the wireless communication protocol used to link the remote sound source and the body-worn receiver. Common ALS include frequency modulated (FM), infrared (IR), and inductive loop (IL) systems. The receiver can be directly associated with the hearing aid (inbuilt FM receiver, FM-boot, telecoil). Alternatively, some IR and FM receivers retransmit the signal via an inductive neck loop to be picked up by the hearing aid telecoil (Bakke et al., 1999).

Systems for small group communication require that the microphone be passed around (single-microphone system) OR that everyone has their own microphone (multi-microphone system). For single-microphone systems it is often difficult or impossible to pass the microphone. Listeners often miss the start of the conversation as the microphone is passed to the next speaker. Multi-microphone systems are difficult to set up properly; they are cumbersome or impossible to place a microphone in front of every speaker. People are often intimidated (unfamiliar, uncomfortable) speaking into the microphone. As a result they speak too loudly or softly, or position themselves too close to or far from the microphone. It is often difficult for the listener to identify and orient toward the current speaker.

Some desktop microphone arrays have advanced features such as the ability to track a moving speaker or locate and orient toward a new speaker. These microphones are sometimes used in conjunction with high-performance, PC-based voice recognition systems. Desktop microphone arrays have good potential for small group communication in which the conversation jumps from one speaker to another. These microphones could be integrated into assistive listening systems - eliminating the need to pass around a single microphone from speaker to speaker and much easier to set up than a multi-microphone system.

[ Top of Page ]

Technology Requirements

Persons with hearing impairments need an improved ability to hear speech in noise and non-speech sounds in noise (e.g. flute in an orchestral piece). Technology solutions must work in complex acoustic environments with reverberation, multiple sound sources that may be moving about and complex noise sources. To be accepted by end-users, a market product must be aesthetically pleasing (e.g. look like jewelry, part of clothing or eyeglasses - does not look unnatural, look "cool", etc.). Manufacturers and researchers have identified beam forming microphone arrays as the critical technology for body worn, table top, binaural hearing aid and directional hearing aid microphones. Many of the capabilities identified for beam forming microphone arrays apply across these applications.

Beam Forming Microphone Arrays

• Should work in difficult listening environments (e.g. many sound sources, noise from all directions). Should sense noise from all sources and directions and adaptively change directional performance in response.
• Should adjust sensitivity in response to the environment (high sensitivity for quiet conversations in quiet environments, lower sensitivity for loud conversations in noisy environments).
• Should identify and eliminate common noise (adaptive filtering for wind, self-noise, outdoor, fan hum, etc.).
• Should pick up orienting cues (microphone anticipates who you want to hear, not who you are currently "pointing at.").
• Should orient to new speakers or track moving speakers.
• Should provide hearing-at-a-distance (analogous to depth-of-focus for a light microscope) especially in difficult acoustic environments (e.g. listening to a person some distance away is a problem even in a quiet environment).
• Should work in real time (fast response, no processing delay).
• Should be highly directional across all speech frequencies (200 Hz to 7 kHz).
• Should have wide dynamic range.
• Should have a linear response to sound signals over entire auditory frequency range.
• Should not generate more noise than a single microphone (24 dB - 27 dB with a filter band up to 10 kHz).
• Should (with necessary processing hardware) be small and inexpensive.

Binaural Hearing Aids

• Should do beam forming with the microphones from both hearing aids.
• Should have a wireless bi-directional communication link.
• Should employ binaural processing hardware/algorithms that take advantage of head-shadow effects, phase and time delays, etc.).

Table Top Microphones

• Should provide orientation cues (e.g. N S E W lights indicating direction of speaker).
• Should be adaptive (e.g. track moving speaker, pick out and orient toward new speaker, change directional performance in response to environmental cues).

Body Worn Microphones

• Should have a wireless link to the hearing aids.
• Should be adaptive (e.g. change directional performance in response to environment).
• Should have user selectable beam steering modes (user needs to know which modes will work in each environment).
• Should have acceptable size, weight and appearance (e.g. microphones spread over surface of necklace or eyeglasses)

Directional Hearing Aids

• Should employ adaptable beam steering that fits the acoustic environment (e.g. kids sitting to the driver's right or in the back seat of a car)
• Should employ advanced beam forming that does not increase power consumption (work with standard hearing aid batteries).
• Should have higher gain settings without feedback (through hearing aid microphones).
• Should have microphone directionality that is user and/or automatically controlled.
• Should have an improved user interface to control microphone directionality.

[ Top of Page ]

References

Andrea Electronics, C. (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].

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].

Kochkin, S. (1997). Customer Satisfaction & Subjective Benefit with High Performance Hearing Aids. Knowles Electronics. Available: www.knowlesinc.com [April 19, 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.

Trine, T. (1999). Use and Evaluation of Directional Technologies in Hearing Aids. In I Starkey Laboratories (Ed.). CHARTT Conference, October 21-21, 1999: Department of Speech and Hearing Sciences, Indiana University.

[ Top of Page ]