Hearing aid Compression Flashcards by Julia Moran

Method uses in HA to manage sounds that are too loud or too soft by adjusting the amplification automatically

compression

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Acoustic signal entering the HA as per ANSI-SPL at the mic of a HA

input

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The amplified signal that is delivered to the ear

output

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The amount of amplification applied to the input signal (? = output - input)

gain

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A graphical representation of the output of a HA at various input levels. According to ANSI the I/O graph has the output SPL on the ordinate (y-axis) with the input on the abscissa (x-axis)

input/output function (graph rises)

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A graphical representation of the gain of a HA at various input levels

input/gain function (graph sloping)

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A graphical representation of the HA output as a function of freq. The input level and overall gain of the HA are fixed. Freq on x-axis

freq response curve

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A graph showing the gain of a HA as a function of freq under specific test conditions. Freq on x-axis

freq gain curve

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When the input level and gain exceed the MPO, the HA is said to be in

saturation

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It is one method of controlling or limiting the MPO of a HA. Produces an output signal that is distorted, often described as sound scratchy

peak clipping

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occurs when a single frequency is presented to the input of a hearing aid and the output contains the original frequency plus additional undesired frequencies that are harmonically related to the original frequency.

Harmonic distortion

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the summed power of all the harmonic distortion produces relative to the power of the original input signal. Expressed as %

total harmonic distortion (THD)

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Occurs when 2 freqs are presented simultaneously to a HA and the output contains one or more freqs that are related to the sum and differences of the 2 input freqs

intermodulation distortion

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Goal of compression

-restore audibility of soft sounds
-maintain listening comfort for moderate sounds
-prevent loudness discomfort
-improve speech intelligibility

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Key compression features

compression threshold
compression ratio
attack time
release time

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The point on the I/O function where compression starts to reduce gain (the output level is 2dB lower than it would be if no compression had occurred). AKA threshold knee point (TK)

Compression threshold (CT)

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True or false. A compression system may have high or low TKs

true

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is used to limit the output of a hearing aid not to exceed the user’s loudness discomfort levels and to maximize listening comfort.

high TK (85dBSPL or higher)

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is used to improve audibility of softer components of speech.

low TK (typically below 50dB SPL)

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Ratio of input change to output change after threshold. Determines how much the signal with be compressed

compression ratio (CR)

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WHat is the CR formula

CR= input/output

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CR expressed in terms of number of dB which the input must change to cause

a 1 dB change in the output

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is used to limit the output of a HA to not exceed the individual’s loudness discomfort.

a higher CR (5.0 or greater)

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may be used to improve audibility of softer components of speech and/or to restore loudness perception.

a low CR (typically b/w 1.0 and 5.0)

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The time it takes for the hearing aid to respond to a sudden increase in sound level(e.g., a loud noise). Typically short (a few milliseconds)to protect the listener from sudden loud sounds.

attack time

The time it takes for the hearing aid to return to its normal gain settingafter the loud sound has passed. Can be fast or slow, depending on the desired listening experience.

release time

better access to soft sounds that follow loud sounds

fast release

smoother, more natural sound, less distortion in fluctuating environments

slow release

temporary spike in output before gain reduces

overshoot happens during attack time

temporary dip in output before gain increases

undershoot occurs during release time

minimizes overshoot, useful for limiting sudden loud sounds.

fast AT

can cause pumping(audible changes in background noise) if not carefully set.

fast RT

true or false. Slow AT/RT respond to brief events rather than overall level changes.

false to overall level changes rather than brief events (e.g., speech pauses).

A fast AT and slow RT combo might cause audibility issues for speech after sudden loud sounds. Attack/release times <100 ms are typically imperceptible; longer times (>2s) impact background ambiance more than speech clarity.

trade-offs

dynamically adjust based on the rate and magnitude of input level changes. Offers a more adaptive HA response, especially in dynamic environments. Can improve comfort and clarity without requiring constant manual adjustments

variable attack and release times

Faster variable attack and release times respond to

sudden, large changes in inputs (e.g. slammed door or burst of laughter)

Slower variable attack and release times respond to

gradual or small change, help maintain a more natural sound

what are the benefits of both fast and slow compression:

1. protects listeners from sudden loud sounds 2. avoid excessive gain fluctuations (pumping) in background noise 3. preserves speech cues by minimizing distortion during continuous speech

A general term for compression systems in hearing aids. Involves a level detector that adjusts gain based on input sound level

automatic gain control (AGC)

The level detector is placed before the volume control. Compression acts on the input signal to the hearing aid. When the input exceeds the threshold knee (TK), gain is reduced at the pre-amplifier. Volume control adjustments affect overall gain but not the TK or compression ratio (CR).

input-controlled compression (AGC-I)

user's volume control changes do not influence how the compression system behaves (i.e., the CR and TK remain constant regardless of volume setting).

clinical implications of ACG

the level detector is placed after the volume control. Compression acts on the output of the hearing aid, rather than the input.

output-controlled compression (AGC-O)

True or false. The input-controlled compressor (AGC-I) is activated when the output signal exceeds the threshold knee (TK).

false output-controlled compression (AGC-O)

Adjusting the volume control changes both

overall gain and TK (higher volume=higher TK)

In contrast to AGC-I systems, AGC-O circuits shift the TK when volume is adjusted. This means user volume changes can alter how compression behaves, which may affect sound quality and comfort.

clinical implication of AGC-O

Each compression system required difference settings for

compression ratio, attack time, and release time

Different types of compression exist because they serve distinct purposes in hearing aid processing, such as

limiting loud sounds, enhancing soft speech, or minimizing background noise

how does output limiting compression work

uses high compression ratios (often >10:1) and a high threshold to clamp the output once a certain loudness is reached

what does output limiting compression replace to avoid distortion

traditional peak clipping

Main purpose of output limiting compression

prevent loud sounds form becoming uncomfortable or damaging

how does WDRC work

Uses low compression thresholds and low compression ratios (e.g., 2:1) to compress a wide range of inputs into the listener’s reduced dynamic range.

Main purpose of WDRC

restore audibility of soft sounds while keeping loud sounds comfortable

How is WDRC clinically used

ideal for speech intelligibility especially for those with SNHL

How does expansion work

applies inverse compression (gain decrease as input decrease) below a certain threshold

what is the purpose of expansion

reduce amplification of very soft environmental or internal noise (mic noise)

how is expansion used clinically

to prevent soft unwanted sounds from being amplified when no speech is present

frequency bands within a hearing aid that can be processed independently. They allow different gain and compression settings for different parts of the frequency spectrum.

channels

Since HL varies across freqs this helps tailor amplification where it’s needed most. * Enables more precise control of loudness, comfort, and sound quality. * Each channel applies its own CT, CR, AT, and RT. * For example, higher CR might be used in high frequencies to prevent loudness discomfort.

multiple channel

People with hearing loss experience greater loudness deficits for soft sounds, but may still perceive loud sounds as loud, due to

recruitment

differ between normal hearing and SNHL

loudness growth functions

shifts the entire curve leftwards- may leave soft sounds inaudible and loud sounds uncomfortably loud

linear amplificiation

better mimics normal loudness growth: soft- audible, moderate- comfortable, loud-tolerable loud.

WDRC (wide dynamic range compression)

true or false. All users will love WDRC better than linear amplifications right away

false. Users accustomed to linear amplification may initially resist WDRC due to its more balanced (less exaggerated) sound.

true or false. WDRC allows for comfort without sacrificing audibility of speech

true

How are user's about to tolerate loud with WDRC even when the HA output is below the listener's loudness discomfort level (LDL)

gradually reducing gain as input increases, some users prefer output reduced further for comfort

(e.g., fast attack, slow release) offer a balance between clarity and comfort.

hybrid approaches

is fine-tuning compression settings crucial

yes

what do successful fitting rely on

verifying patient preferences and using real-ear measures

Factors to consider when fitting a compression

audiometric data, case histroy, lifestyle factors, physcial and congitive capabilities

What degress of HL may benefit significantly from WDRC, espcially combined with output limiting compression (OCL)

mild to moderately severe losses

What degree of HL my traditionally fit with linear amplification, but advances now make WDRC viable with powerful enough aids

severe to profound

Which type of HL * Acts like a volume knob turned down across all input levels—soft, moderate, and loud sounds are all attenuated equally. * Loudness growth remains normal(no recruitment). * Linear amplificationis often appropriate because it evenly restores volume. * However, compressioncan still be used to prevent hearing aid output from reaching its max or causing distortion.

conductive HL

Which type of HL: * Results in abnormal loudness growth(recruitment)—soft sounds may be inaudible, while loud sounds can still feel loud or even uncomfortably loud. * WDRC is typically recommended: provides more gain for soft inputs, reduces gain for loud inputs, helps restore more natural loudness perception.

SNHL

Which type of HL: * Fitting strategy typically uses enough gain to overcome conductive attenuation, compression to manage recruitment from the SNHL component. * Often requires greater customization and fine-tuning of settings.

Mixed HL

impacts speech understanding in noise, working memory, and auditory processing. More slower compression or conservative processing setting's may be better for some older adults with this

cognitive decline

Difference between linear gain and WDRC

LG= may over-amp loud sounds WDRC= provides more comfortable listening across sound levels

Fitting Considerations by Hearing Loss Configuration: More straightforward; WDRC across all channels.

flat loss

Fitting Considerations by Hearing Loss Configuration: May require multichannel compression to preserve high-frequency cues while maintaining comfort.

sloping loss

Fitting Considerations by Hearing Loss Configuration: Must consider whether to fit both ears or one and adapt compression accordingly.

asymmetric loss

approaches are used to generate an initial fitting target for hearing aids, based on the audiological characteristics of the hearing loss.

prescribing amplification/ prescriptive approach

Early formulas (e.g., NAL-RP, POGO, Berger, Libby 1/3 Gain) were designed for linear amplification: * Target gain is constant regardless of input level. * Focus is on making average conversational speech (around 65 dB SPL)audible and comfortable.

Linear Prescriptive Formulas

Do not account for the varying gain needed across input levels (e.g., soft vs. loud sounds). Additionally, not well-suited for today’s non-linear, compression-based hearing aids.

Limitations of Linear Prescriptions

Designed to restore normal loudness perception across a wide range of input levels using compression. Additionally, these methods prescribe different amounts of gain for soft, moderate, and loud sounds.

Non-Linear Prescriptive Approaches

NAL-NL2 (National Acoustic Laboratories –Non-Linear Version 2): - Developed to optimize speech intelligibility while maintaining comfort. - Based on loudness equalization: aims to equalize loudness across frequency bands. - Suitable for adults and some pediatric fittings.

non-linear prescriptive approaches

A non-linear prescriptive approach designed primarily for pediatric patients. - Focuses on loudness normalization, ensuring all sounds, including soft ones, are audible and within the child’s residual dynamic range. - Prescribes more gain for soft inputs than NAL.

DSL v5.0 (Desired Sensation Level):

Hearing aid Compression Flashcards

(85 cards)