People with hearing loss often have more difficulty understanding speech in noise than people with normal hearing. Therefore the ability of the hearing aids to reduce noise is of great importance to people with hearing loss.
Noise reduction can be achieved by means of noise reduction algorithms that reduce the noise in the sound signal while it is being processed by the hearing aid.
A second successful strategy involves reducing the amount of noise that is allowed to enter the hearing aid in the first place. This is what directional microphones are good at.
The directional microphone system found in Widex hearing aids, the HD Locator, consists of two omnidirectional microphones. The microphone system is adaptive, meaning that it will assume the polar pattern that produces the best signal-to-noise ratio in the current listening environment.
Suppressing noise via directional patterns
In a quiet environment with no noise, the microphone system will assume the omnidirectional pattern where it picks up sound evenly from all directions.
However, if noise is present, the system will assume the directional pattern which leads to the least amount of noise being picked up.
If the noise source is located behind the hearing aid user, for instance, the microphone system will assume a cardioid pattern which picks up sound from the front and eliminates most sound from the sides and rear.
The advantage of a dual microphone system
Another advantage of having a dual microphone system is that it makes it possible to split the signal from the two microphones into a number of narrow frequency areas which can then be processed separately.
This means that the directional pattern assumed to suppress the noise can be limited very narrowly to the frequency areas where the noise is actually present.
A system without this fine division of the signal would have to make the best compromise between all the frequency regions when deciding which directional pattern to assume.
If a low frequency noise source (e.g. a dishwasher) is located in one direction and a high frequency noise source (e.g. an espresso machine) in another, a dual microphone system can reduce the sensitivity to both sources of noise independently, effectively reducing the total amount of noise