The difference between the quietest sound a person can detect and the loudest sound they can comfortably tolerate is known as the ‘dynamic range’. A mixed and sensorineural hearing loss typically leads to a reduced dynamic range. A reduced dynamic range is when quiet sounds cannot be heard by a person but loud sounds are still perceived at the same or similar level as somebody with normal hearing. This phenomenon known as ‘recruitment’.
If a hearing aid therefore provided the same level of amplification for loud sounds as it did for quiet sounds in somebody with a reduced dynamic range, the loud sounds would be perceived as being uncomfortably loud. For this reason, digital hearing aids apply compression. Compression is when greater amplification is provided for quiet sounds so that are audible whilst less amplification is provided for loud sounds so that are comfortable.