Dynamic Aspects Of Compression

Question 1

Expansion

Answer 1

Opposite of compression
Reduces gain for very soft sounds. Gain increases rapidly as input increases up to first kneepoint of compression
Less than linear gain for very quiet sounds, increasingly gain at kneepoint
Does work of OHC gain is greatest at kneepoint, to give max gain to soft speech sounds

Question 2

WDRC vs Expansion re output

Answer 2

Above knee both are the same
Below knee WDRC is linear

Expansion provides same gain as linear at the knee but less for soft sounds

Question 3

Purpose of expansion

Answer 3

Reduce audibility of hearing aid components for clients with some normal-moderate thresholds

Question 4

2 ways to reduce audibility of components

Answer 4

Use expansion or increase TK

Question 5

Dynamic aspects of compression

Answer 5

Compression ratios and kneepoint are static and unchanging over time

Dynamic aspects deal with the constantly changing intensities of input in the environment

Question 6

Attack

Answer 6

When input SPL exceeds TK, aid attacks the sound and goes into compression, reducing gain

Question 7

Release

Answer 7

When input SPL drops below TK, hearing aid releases from compression, restoring gain

Question 8

Attack times

Answer 8

How quickly the aid goes into compression after input increases

Adjustable

Time dimension

Question 9

Why are attack times very short <50 ms to react to rapid changes in SPL

Answer 9

If the attack time is too slow, aid will not react quickly enough to ongoing changes in loudness in the environment

Question 10

Release

Answer 10

When input SPL drops below kneepoint, hearing aid releases from compression, restoring gain

Question 11

Release times

Answer 11

How quickly the aid goes out of compression

Adjustable

Question 12

Release times versus attack times

Answer 12

Always longer than attack times

Question 13

What would happen if release times were too fast

Answer 13

If release time is too fast, can result in client perception of pumping as aid tracks the amplitude of individual sound waves

Fast release times that are too fast can also cause distortion

Question 14

What would happen if release times were too slow?

Answer 14

Gain may not return to higher levels once the loud sound has stopped. May result in under amplification of soft consonants following loud vowels or environmental sounds such as pen click

Question 15

Attack and release time strategies

Answer 15

Peak detection 
Adaptive compression 
Average detection
Syllabic compression 
Automatic volume control

Question 16

What is peak detection?

Answer 16

Developed in 1980s

Tracks peaks of incoming signal and compresses if kneepoint is exceeded

Question 17

What are attack times for peak detection

Answer 17

Fixed attack time, short, around 50 msec

Question 18

Release time for peak detection

Answer 18

Fixed release time, longer, around 150 msec

Question 19

Adaptive compression

Answer 19

Developed in 1980s

Question 20

Adaptive compression attack time

Answer 20

Fixed attack time

Question 21

Release times for adaptive compression

Answer 21

Variable release times
Depends on duration of intense incoming sounds so reduced pumping heard by clients
Long sound, long release time - machine noise, background chatter are suppressed
Short sound, short release time - spoon dropping won’t affect speech audibility as much with a short release time

Question 22

Average detection

Answer 22

Developed in the early 1990s
Sounds very natural
Adjustable attack and release times
If average SPL of the incoming signal over time exceeds kneepoint, gain is reduced

Question 23

Slow attack and release times for average detection

Answer 23

Average over 220 msec. Interval
If intensity over kneepoint, compression kicked in and was altered slowly, not noticeable to client - used most of the time

Question 24

Fast attack and release times for average detection

Answer 24

Average - over 10 msec. Worked to reduce gain for sudden sounds

Question 25

Syllabic compression

Answer 25

Late 1990s
Often used with BILL. loud low frequency sounds are compressed quickly to prevent lows from masking highs
Somewhat controversial - does reduce peaks of sound signal, making it easier for noise to fill in valleys, which would then be amplified by the aid

Distortion due to fast attack and release times

Compressor is trying to respond to all quiet environmental sounds as well as soft consonants so noise often fills in the valleys between louder consonants,

Question 26

Automatic volume control

Answer 26

Designed to mimic the length of time it takes a client to put their hand up and reduce the volume on their aid
Usually found with WDRC

Question 27

Advantage of automatic volume control

Answer 27

Reduces the need for the client to adjust the volume control; responds to general overall changes in sound intensity

Question 28

Which attack and release times are usually used

Answer 28

Average detection or average detection in the highs and syllabic compression in the lows

Question 29

Fast attack and release times combined with a high compression ratio

Answer 29

Distortion
Therefore syllabic compression is usually used only with WDRC

Long attack and release times can be used with either high or low compression ratios