Directional microphones were developed by the sport-broadcast industry, and they have been increasingly adopted by the hearing aid industry during the last decade. This stands in contrast with the very early use of hearing aids with directional microphones which was not well received in the 1970s . Today, the empirical evidence supporting directional microphones is mixed, and it depends upon the user's listening circumstances (Ricketts, 2005).
How Do Directional Microphones Work?
A directional microphone is actually a pair of microphones working together. Microphone arrays are virtually never used in commercial hearing aids. Sound arriving at the rear microphone is delayed (by the sound travel time) and then subtracted from the sound reaching the front microphone.
Although burying the aid deep in the ear canal has obvious cosmetic benefits, In-The-Canal (ITC) and Completely-In-Canal (CIC) aids lose their ability to extract directional information, since all sounds are funneled down the ear canal.
Directional microphones are incorporated in some In-The-Ear (ITE) aids; these instruments are flat with the outer surface of the ear. Modern Behind-The-Ear (BTE) devices are tiny and virtually invisible (see photo). These aids offer both the advantages of directional mics and the cosmetic appeal of virtual invisibility.
The Low-Frequency Penalty
Directional microphones penalize low-frequency amplification (Quintino, Mondelli, and Ferrari, 2010). Because of their long wave length, low-frequency sounds reach the front and the rear microphones at the same time. Consequently, these sounds appear to be coming from the side rather than the front, and the directional hearing aid fails to amplify them. The closer the two microphones are to each other, the greater number of low-frequencies are affected.
Adaptive Directional Microphones
Hearing aid manufacturers have attempted the make their instruments automatically adjust to the environment in which the listener finds himself. Some devices, with adaptive directional microphones switch to a directional mode when the total noise level is high, but revert to an omnidirectional mode with lower levels of noise.
Directional Microphones Work in the Laboratory
There is clear and substantial evidence that directional microphones provide a significant advantage for understanding speech in noise when tested under laboratory conditions. However, these laboratory studies are designed to maximize the benefits of the directional microphone. The speech comes from directly in front of the the hearing aid user. The noise comes from angles other than the front. The walls of the test chamber do not reverberate sound to any appreciable extent. How often do these conditions occur in real life?
Are scientists cheating in the laboratory? Well, the answer is both yes and no. If a sizable and reliable benefit could not be demonstrated under ideal conditions, there would be little reason to test under real-life conditions. However, laboratory results should not be used in isolation to predict the benefits from a directional microphone.
Directional Microphones in the Real World
Empirical evidence from the real-world is mixed. Wu (2010) compared the use of directional microphones with twelve “young” and twelve “old” hearing-impaired individuals. “Young” was defined as being under 65 years of age. Both groups had similar pure tone hearing loses, and directional microphones provided demonstrable benefits in the laboratory tests.
Wu then had his subjects pick three “ideal” noise locations that they encountered in their daily lives. The subjects were asked to compare directional and omnidirectional modes of their microphones in these situations. The 24 subjects made a grand total of 1,367 comparisons. Only five subjects preferred the directional microphone in more than half of the “ideal” noisy situations they encountered in their daily life.
The eight participants in Blamey, Fiket, and Steele's study (2006) were told that their new hearing aids had two programs, and these participants were asked to evaluate the programs in various real-life situations. The hearing aid users were not informed that one program had an adaptive directional and the other had an omnidirectional microphone.
Most of the participants found that an adaptive directional microphones were beneficial in the following situations:
- In a restaurant, sitting across the table from their spouse
- Talking around a dinner table
- At a social gathering
- Listening to a speaker at a meeting
On the other hand, an adaptive directional microphone either offered no advantage or was actually an encumbrance in the following situations:
- Conversation in a car
- Listening to the news on TV or radio
- On the telephone
- Talking to a soft spoken person
Author's Preference
I am a retired engineer, but I have never worked for the hearing aid industry. I purchased a pair of hearing aids to help understand speech in a very particular situation, and I had to return to the audiologist twice in order to get the best fitting for my particular needs.
I purchased the aids so that I could understanding speakers during almost daily small group meetings. The meetings were semi-formal, with one person talking at a time. There was always fan noise from a slide projector, and noise from the heating and air-conditioning system. It was critical for me to catch the details of what was said, and I could not afford to miss a word here and there.
The hearing aids that I purchased could be set by the audiologist into one of three modes: omnidirectional, fixed directional, and adaptive directional. After some experience with using the hearing aids in my work situation, I had the audiologist set them to the fixed directional mode. This mode proved to be the best in the small meetings and adequate in most other situations.
Why the Adaptive Directional Mode did not Work for Me
The adaptive direction mode did not work for my small group meetings because the room noise was not loud enough to trigger the directional mode. In the omnidirectional mode, the fan noise was being amplified, and since the fan from the projector was closer than the speaker, the omnidirectional hearing aid actually decreased speech intelligibility.
Why the Adaptive Directional Mode does not Work in the Car
Road noise triggers the directional mode for an adaptive microphone. However, the driver cannot look at his passenger; he must keep his eyes on the road. So a directional microphone will hinder conversation, although it can enhance the radio listening experience if front speakers are used.
Conclusion
The proper choice of a fixed directional, fixed omnidirectional, or adaptive directional mode for hearing aid microphones is critical for maximum benefit from these devices. Most modern BTE and ITE aids will be initially set to an adaptive directional mode, because this is best for most users. However, you should discuss your particular hearing challenges with your audiologist or hearing aid professional so that the correct choice can be made for you.
On rare occasions, hearing loss can be a symptom of a more serious condition. Therefore, every hearing impaired individual should discuss their hearing difficulties with their primary care physician or ENT specialist.
Sources
- Ricketts TA, Directional Hearing Aidsen and Now, Journal of Rehabilitation Research and Development July/August Supplement 2 133-144, 2005
- Quintino CA, Mondelli MFCG, and Ferrari DV, Directivity and noise reion in hearing aids: speech perception and benefit, Braz J Otorhinolaryngology, 76, 630-638. 2010.
- Wu YH, Effect of Age on Directi Microphone Hearing Aid Benefit and Preference, J. Am Acad Audiol 21: 78-89 (2010)
- Blamey PJ,''''[[ Fiket HJ, and Steele BR, Improving Speech Intellility in Background Noise with an Adaptive Directional Microphone. J. Am Acad Audiol 17: 519-530 (2006)
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
Richard Walloch - Don't believe me because of my education or professional experience. Believe me because of the evidence and arguments I advance.
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