Hearing Aids - Directional Microphones
During a normal face-to-face conversation, sound is normally coming from one direction with noises from all directions. The noise can muffle the voice making comprehension difficult. Hearing aids with directional microphones can pick up sound from a specific direction and "tune out" noises from other directions. An improvement of 3+ dB signal-to-noise ratio is possible.
There are 4 types of directional microphones - fixed, automatic fixed, adaptive, and automatic adaptive. Fixed and adaptive refer to the microphone's directionality or polar pattern, which indicates how sensitive the microphone is to sound from different directions.
The fixed directional device uses a fixed polar pattern that is either activated all the time or manually engaged via a switch or remote control. The pattern may be cardioid, supercardioid, hypercardioid, or bi-directional (di-pole) in nature, but the pattern does not change with different listening environments.
The automatic fixed directional microphone is similar to the fixed directional except that the directional microphones can be engaged automatically when background noise is present. The polar pattern is typically configured when the hearing aid is programmed.
The automatic adaptive directional microphone can activate directional microphone automatically and can also change the polar patterns in response to spatially dynamic listening environment (e.g. moving noise source) by altering the internal time delay between microphones.
The adaptive directional microphone offers manual selection of different polar patterns depending on the environment.
One major limitation with this system is if the noise and voice come from the same direction, then nothing would help. Also the primary talker must be within a "critical distance" (usually a few meters) from the listener for directional microphones to be effective. Reverberation can also impact the microphones' performance.
For young children, this feature can be problems. Infants and toddlers learn a large amount of speech and language by listening to others when they are talking around them. Directional microphones might reduce the child’s opportunities to receive some of this incidental language exposure. In addition, safety may be a concern if children are not able to hear warning sounds in their environment, such as an oncoming car. When this type of option is used with a child, the directional microphone feature must have an on/off switch. Parents and caregivers will need to understand when to turn on or off the option.