Static Aspects Of Compression Flashcards by Bryna Sheinis

Goals of compression

To amplify the normal range of sounds and fit them into a restricted dynamic range

To prevent exposure to very loud sounds without the distortion caused by peak clipping

To alter the short term intensity relationships between speech sounds in order to promote speech intelligibility

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What is kneepoint?

When of compression; at what input compression kicks in

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Hard kneepoint

Sharp kneepoint as hearing aid abruptly goes into the desired level of compression

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Soft kneepoint

Curvilinear compression

Hearing aid achieves the desired level of compression gradually as input intensity increases

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Compression ratio

How much of compression

Input:output

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What is AGC

Compression change the gain on the hearing aid based on intensity of input

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Two types of compression

AGC OUTPUT

AGC INPUT

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Output compression configuration of hearing aid

MIC➡️VC➡️Amp➡️Rec

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Volume control effects of AGCo

VC affects kneepoint and gain but not MPO

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How can you tell a hearing aid is AGCo by listening to it

When you talk loudly volume will not change once it reaches the MPO

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AGCo is associated with low power aids

False

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Input compression configuration of aid

Mic➡️amp➡️VC➡️REC

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AGCi is associated with

Lower power aids
TK control
WDRC

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Uses of AGCi

Mild to moderate hearing losses have a greater dynamic range to work with, where varying MPO will not result in intolerable or damaging loudness

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Effects of AGCi

Soft - volume goes up and down because gain is affected by the VC
Loudly - volume does not change because MPO is not affected by the VC

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Two types of controls for altering compression kneepoint

Conventional compression control
Threshold kneepoint control

Controlled by clinician

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Conventional compression control

Adjusts compression kneepoint Threshold by altering amount of voltage required before compression kicks in
When control is turned to max compressor requires a higher voltage to begin compressing, so kneepoint is raised along with MPO
At max kneepoint, aid is linear for a wider range of sounds
Used to set MPO on AGCo aids
Useful for clients with severe hearing losses and very restricted dynamic ranges

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Threshold kneepoint control

Affects gain for soft sounds
Affects KP of compression for soft sounds, usually between 45 and 50 dB
Linear gain below kneepoint
Acts as gain booster for soft sounds
MPO not affected
As control is increased, gain for low intensity sounds is decreased
Right most line - gain is lowest and TK is set at the highest kneepoint
Left-most line - where gain is the highest and TK is set in the lowest kneepoint position

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Compression ratio for output limiting compression

High kneepoint and a high compression ratio
Greater than 60 dB
5:1 or greater

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Output limiting compression

Linear for most inputs, up to kneepoint
Then strong compression kicks in to limit output or MPO
Uses a high compression ratio to limit output instead of peak clipping, resulting in less harmonic distortion
Associated with powerful aids

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WDRC

Wide dynamic range compression
AGCi
Low kneepoint and low compression ratios

60 dB or less
4:1 or less
Provides weak compress for a wide range of input intensities

What kind of hearing loss WDRC

Mild to moderate sensorineural hearing loss

Normal loudness growth is more easily achieved using WDRC than with output limiting compression

Frequency response graphs

Used to show gain provided to different intensity inputs across frequencies for WDRC
Does not work for output limiting compression as all inputs are given the same linear gain and until MPO is reached

Two types of WDRC

BILL and TILL

BILL

Bass increase at low levels | BILL compressed loud low frequency sounds more than quiet low frequency sounds but does not affect the highs

TILL

treble increase at low levels

Kneepoint for BILL

Lower for low than high frequencies Low frequency sounds do not need to be very intense to trigger compression The louder the low frequency the more it is compressed

What is the purpose of BILL

Designed to reduce the upward spread of masking while retaining the intensity of high frequencies for consonants Leads to better speech intelligibility in background noise

Kneepoint for TILL

Low kneepoint for high frequencies Takes less sound intensity for a high frequency input to go into compression than for low Hearing aid will not compress soft high pitched sounds and therefore provides maximum gain for soft high pitched sounds

What is TILL used for?

Clients who have a high frequency loss and hence a reduced range in high frequencies Emphasizes soft high frequency consonants for clients with high frequency hearing loss

Severe to profound heat loss

Output compression Output limiting compression Conventional compression control

Clinical application of output compression

VC will change kneepoint and affect gain, will not increase MPO Protects residual hearing and client comfort

Clinical application for output limiting compression

High kneepoint and high compression ratio so high intensity outputs are highly compressed whereas soft and medium inputs are maximally amplified Uses as much as possible of the client’s restricted dynamic range

Mild to moderate hearing loss

Input compression WDRC TK control

Clinical application for input compression

VC doesn’t affect kneepoint but adjusts both gain and MPO Allows client to adjust both gain and MPO since they have a large dynamic range to work with

Clinical application of WDRC

Low kneepoint, low compression ratio, linear gain provided for very soft sounds and less gain for more intense inputs. Slowly brakes input so output does exceed UCL Provides different amounts of compression at different frequencies, depending on the configuration of the hearing loss so deals with sloping losses well

Clinical application for TK control

Attempt to mimic OHC function. Changes kneepoint and hence gain for soft inputs

What is the benefit ofTK control

Lower kneepoint provides max gain for soft consonant sound May raise kneepoint to reduce gain for soft sounds if the client thresholds if 40 dB HL or less, so hearing aid component sounds are not audible to the client Can use expansion to reduce the audibility if the hissing if the components

Clinician control effects for AGCo

Conventional compression control

Adjusts compression kneepoint used to set MPO (dB SPL - 15 dB greater than UCL)

Conventional compression control affects ______________ and ______________ but not _____________

Kneepoint, output (MPO) but not gain

AGCo is associated with ____________ kneepoint and ____________ compression ratio

High - greater than 60 dB SPL | High - 5:1 or greater

Who is AGCo appropriate for?

Severe to profound HL

AGCi volume control effects

Gain and MPO but not kneepoint

How can you distinguish a AGCi by listening to it

When you talk loudly volume will change at the max due to changing MPO

Clinician control effects for AGCi

Threshold kneepoint compression

TK affects ____ and _____ but not _____

KP gain MPO

What does increasing TK control result in

More gain for very soft sounds under 40 dB so the client is less bothered by component sounds

What clients would use AGCi?

Mild to moderate hearing losses thus varying MPO will not result in intolerable loudness

With respect to AGCi and AGCo what is used today

Usually use AGCi for softer inputs and AGCo for louder inputs