Question
I just read about a three microphone hearing aid system. Please tell me about the three mic system, versus the two mic system, versus the one mic system? What are the differences between them and do they really work? What is the cost for directional features?
Answer
Directional microphone systems operate on the assumption that the listener will face the source of the sound they are most interested in hearing. Therefore, directional technology allows the listener to (typically) hear sounds from the front better than sounds from the rear, as directional microphone systems typically reduce signals coming from the rear.
For instance, if you're in a crowded restaurant, sitting across the table from someone you'd like to converse with, it would be great to have directional hearing aids which would reduce the sounds behind you a little, while making the sounds from the front a little louder. However, it might be better still, to have a three microphone system which can also detect and reduce the sounds from the sides.
Directional microphones can help accomplish these goals, with more or less success depending on the technology, the noises, the specific room factors and with respect to other variables, alternatives and options. So in essence, YES. Directional microphones are useful in helping to reduce the background noise, but all directional microphones are not the same.
Directional microphones come in a variety of configurations. And I admit, it can be confusing for the patients and the professionals too! When you see the word "directional" as it applies to hearing aids, there are four basic choices:
1- A hearing aid with a single microphone - but the single microphone has two ports.
2- A hearing aid with two separate microphones, one is directional and one is omnidirectional. In this situation, the patient selects a given microphone using a switch or button.
3- A hearing aid with two omnidirectional microphones. In this situation, the directional effect is accomplished electronically. The patient selects the specific microphone through the use of a switch or button.
4- A hearing aid with three omnidirectional microphones, where the directional effect is accomplished electronically. In this situation, all three microphones are used for the directional effect, and only one is used for the omnidirectional setting. The hearing aid may be programmed to "automatically" switch from directional to omnidirectional, or the patient can override this function using a button to control separate programs.
Directional microphone hearing aids are assessed in the laboratory. One aspect of directional performance is described by a measurement called the directivity index (DI). The DI is a ratio that compares the hearing aid output for sounds from the front of the hearing aid, to sounds from all other azimuths (directions). For omnidirectional hearing aids, the DI can be less than 0 dB, and for new directional technology such as the three-microphone design described above, the DI can be 6.0 dB or higher.
While there is not a one-to-one correlation, the DI is a reasonably good predictor of the benefit a patient might expect in understanding speech when there is a surrounding noise source. For example, we would expect that speech understanding in background noise would be superior with a directional hearing aid with a DI of 4.0 dB, versus a directional hearing aid with a DI of 2.0 dB.
For more information on this topic and to get a better appreciation of how directional microphones may specifically impact their ability to hear in noise, I recommend that the consumers and patients review this information and their personal and specific questions with their hearing healthcare provider.
Bio.
Pamela Burton, M.A. is the Director, Product Management at Siemens Hearing Solutions, where she has been employed for the past 16 years. Ms. Burton obtained her Masters degree in Audiology from Hahnemann University in Philadelphia in 1984 and is enrolled in the distance learning program at Central Michigan University to earn an AuD degree. Ms. Burton is currently involved in product development, interfacing with Research & Development as well as Marketing, and has lectured extensively on the subject of programmable and digital hearing instrument technology, and their applications.
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