Compression

Question 1

What is peak clipping?

Answer 1

PC clips the peaks and troughs of the wave. It restricts the height of the wave and the intensity of the sound.
It restricts the maximum output of the hearing aid as it reduces the level at which saturation occurs - therefore causing distortion

Question 2

What is the compression theory?

Answer 2

Compression ensures audibility for quiet sounds, maintains recognition for moderate sounds and maintains comfort for loud sounds. As sound gets louder the level of rain is reduced. This is a feature of the hearing aid which automatically changes the amount of gain added to the level of incoming signal

Question 3

What is the threshold kneepoint?

Answer 3

This is the point where gain changes from linear to non linear gain. Above this level the gain reduces as the input increases. Compression works in the same way as normal linear aids until the input reaches this point.

Question 4

What is the compression ratio?

Answer 4

The change in input over the change in output

Question 5

What is AGCI

Answer 5

Automatic gain control input: aid goes into compression before the amplifier - therefore patient has access to VC even after the aid has gone into compression. This works best for mild to moderate losses.

Question 6

What is AGCO

Answer 6

Automatic gain control output: aid goes into compression after the amplifier, patient has no access to VC after aid has gone into compression. Works best for severe to profound losses.

Question 7

What is wide dynamic range compression

Answer 7

This uses the whole dynamic range. Great gain for quiet sounds, mild gain for speech and little to no gain for loud noise. Kneepoint must be below speech and has a low compression ratio so the aid is nearly always in compression.

Question 8

Attack and release times

Answer 8

Onset = attack time and offset= release time. Attack time should ideally be under 10 milliseconds whereas release time is open to debate as speech intelligibility May be affected.

Question 9

Possible pitfalls to attack and release times.

Answer 9

Release time;
Too fast = pumping sound heard
Too slow = under amplifying soft consonants following loud vowels.

Attack time too slow = device won’t react quick enough to dynamic changes in environment

Question 10

Adaptive compression

Answer 10

Release time depends of duration of loud input.
Sudden loud sounds = fast release
Continuous loud sounds = release delayed until sound has gone

Question 11

Dual compression

Answer 11

Short release for high frequency sounds like speech
Longer release for low frequency sounds like background noise.

Question 12

Syllabic compression

Answer 12

Short attack and release times within the duration of a syllable. Ideal when listening to speech only.

Question 13

Expansion

Answer 13

Reduces the audibility of very soft sounds and keeps aid from amplifying very soft sounds that are not of interest such as a fan or fridge sound