Dynamic Aspects Of Compression Flashcards Preview

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Flashcards in Dynamic Aspects Of Compression Deck (29):


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


WDRC vs Expansion re output

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


Purpose of expansion

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


2 ways to reduce audibility of components

Use expansion or increase TK


Dynamic aspects of compression

Compression ratios and kneepoint are static and unchanging over time

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



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



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


Attack times

How quickly the aid goes into compression after input increases


Time dimension


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

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



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


Release times

How quickly the aid goes out of compression


Release times versus attack times

Always longer than attack times


What would happen if release times were too fast

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


What would happen if release times were too slow?

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


Attack and release time strategies

Peak detection
Adaptive compression
Average detection
Syllabic compression
Automatic volume control


What is peak detection?

Developed in 1980s
Tracks peaks of incoming signal and compresses if kneepoint is exceeded


What are attack times for peak detection

Fixed attack time, short, around 50 msec


Release time for peak detection

Fixed release time, longer, around 150 msec


Adaptive compression

Developed in 1980s


Adaptive compression attack time

Fixed attack time


Release times for adaptive compression

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


Average detection

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


Slow attack and release times for average detection

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


Fast attack and release times for average detection

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


Syllabic compression

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,


Automatic volume control

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


Advantage of automatic volume control

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


Which attack and release times are usually used

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


Fast attack and release times combined with a high compression ratio

Therefore syllabic compression is usually used only with WDRC

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