Compression

The compression ratio describes the amount of compression, or in simple terms, the squash effect. Compression ratio can be calculated by dividing the change in input by the change in output. If an input signal change of 10 dB results in an output change of 10 dB, the ratio is 1:1, or linear. If the change input of 10 dB results in an output change of 5 dB, the ratio would be 2:1. Input/output functions for various compression ratios are illustrated.

The effect of compression on Attack and Release time

• The attack time is essentially how quickly the compression comes on
• the release time is how quickly the system comes out of compression
• can have the ability to change the attack and release, most hearing aid companies will ask for age when programming the hearing aid
• there is research that attack and release is judged differently for how old you are
• the amount of time that this takes for the system to say that this is a really loud sound (attack time)
• the recognition that the sounds are soft (release time)

Output is what the hearing aid is putting out, if doing REAL ear measurements it includes the hearing aid output as well as ear canl. If looking at it on the test box, just what the hearing aid is putting out, the gain is how much ins input into the system, esspentially output minus input. To look at the linear line segment, it is 1:1 to the kneepoint. As the inputs get higher , really adding the same amount of gain until the compression knee point.