The frequency response of an electret microphone is typically flat, but due to design choices and some inherent factors, this flat response can change. In hearing aids, electret microphones are intentionally designed to introduce low-frequency noise reduction.
Low-Frequency Noise Reduction
The low-frequency cutoff makes hearing aids less sensitive to low-frequency sounds in our environment. While these sounds may be undetectable to people with normal hearing, without this low-frequency cutoff, they could overload the hearing aid. Achieving low-frequency noise reduction is relatively simple: small pathways between the front and back of the diaphragm allow low-frequency sounds to enter both sides of the diaphragm nearly simultaneously, thus reducing the diaphragm's movement when responding to these frequencies.
The larger the opening in these pathways, the more restoration occurs, and the broader the frequency range for the reduction. These channels also balance the static pressure between the front and back of the diaphragm, similar to the function of the Eustachian tube in the ear. Traditional hearing aids typically use microphones with varying amounts of low-frequency noise reduction to achieve the desired gain frequency response.
Acoustic Resonance and Its Effect
The second change to a flat response is caused by the internal acoustic resonance of the microphone. Resonance occurs between the air in the inlet region (which causes sound distortion) and the air volume in front of the diaphragm (which has elastic properties). The mechanical compliance of the diaphragm itself also relates to resonance, known as Helmholtz resonance. At the resonance frequency of Helmholtz resonance, the gaps in the sound tube and connected parts vibrate freely, much like how an elastic medium responds to vibrations at its resonance frequency.
This resonance creates a peak in the gain frequency response, usually around 4kHz or 5kHz, with a gain of about 5dB. As the frequency increases, this resonance effect decreases the microphone's sensitivity. Recent microphones are designed with a cylindrical shape and a wider inlet, shifting the resonance frequency to a higher range, which results in a completely flat response curve within the hearing aid's bandwidth.
Conclusion
Understanding the frequency response of hearing aid microphones is essential for optimizing their performance. The intentional introduction of low-frequency noise reduction and managing acoustic resonance ensures that the microphone provides the necessary gain response while minimizing unwanted sounds. Modern designs have improved these features to create a more balanced and natural hearing experience for users.
