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CSD9518 > 14_Level Dependent Amplification > Flashcards

14_Level Dependent Amplification Flashcards

1
Q

How do we transfer the audiogram to speechmap? What are the dB units of each?

A

Audiogram units are in dB HL, and speechmap is in dB SPL, so we use a HL to SPL transform

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2
Q

In Speechmap, what does the red line represent? What does the shaded grey area represent?

A

Red line = thresholds in SPL
Grey area = normal LTASS (30 dB dynamic range), which shows us which frequencies are audible and which aren’t, compared to a particular person’s thresholds

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3
Q

What is a person’s dynamic range?

A

The difference b/w UCL and threshold

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4
Q

Do hearing impaired individuals usually have a larger or smaller dynamic range compared to normal hearing individuals? Why is this?

A

Smaller dynamic range because the hearing loss raises the “floor”, but the UCL’s do not change

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5
Q

Linear amplification means that, at any point, ____ is fixed

A

Gain

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6
Q

What is the name of the plot that shows output on the y-axis and input on the x-axis? How do we calculate gain?

A

Input/Output Curve

Gain is calculated by determining the difference b/w the input and corresponding output at any given input level.

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7
Q

What is the main problem with fixed gain (linear amplification) in hearing aids?

A

It is good for soft sounds, but bad for loud sounds because the output will exceed the UCLs.

  • sounds will hit the saturation level at some point/input level, which will lead to clipping, resulting in distortion
  • if distortion is at one frequency = total harmonic distortion (THD)
  • if distortion is at multiple frequencies (complex input level) = intermodulation distortion (IMD)
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8
Q

On an input/output plot, what is a threshold kneepoint (TK)?

A

The compression threshold - the input level at which the I/O curve has a change in its slope

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9
Q

What is a compression ratio (CR)?

A

The slope of an I/O curve

  • change in input is divided by change in output (“run over rise”)
  • given as a ratio to 1 e.g. 10:1
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10
Q

What is the CR for linear gain?

A

1:1

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11
Q

Will the CR be greater or less than 1 when there is a greater range of inputs being squeezed into a reduced range of outputs?

A

Greater than 1

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12
Q

What will “fixed gain” show up as on an Input/Gain Plot?

What about compression?

A

A flat line.

Compression = gain is decreasing

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13
Q

What does WDRC refer to?

A

Wide Dynamic Range Compression
(AKA Very Low Threshold (VLT)? )
- compressor with a low knee point -> wider range of inputs experiencing compression
- compression threshold is typically less than 50 dB SPL

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14
Q

What is Compression Limiting?

A

A way to ensure output does not exceed the maximum levels (below UCL)

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15
Q

Can you have WDRC and compression limiting in the same system?

A

Yes

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16
Q

How does curvilinear compression differ from traditional compression?

A

It also describes the relationship b/w input and output, but in the form of a curve (not segmented straight lines)

17
Q

What is AGC-I and AGC-O? What is the difference between the two?

A

Automatic Gain Control Input and Output

  • both need to determine the level of input first
  • AGC-I: manual volume control is after the application of gain (acts as linear amplifier, moves curve up and down, cannot guarantee output stays below LDLs)
  • AGC-O: level detection done after the manual volume control, thereby taking volume gain into account (results in lower kneepoint)
  • both can be applied in the same HA
18
Q

What is the difference b/w compression and expansion?

A 
Compression = gain reducing
Expansion = opposite (delta I / delta O < 1)
19
Q

Expansion is always ___ (above/below) the threshold

A

Below

20
Q

What is the purpose of expansion?

A

To reduce the gain for the internal noise of the HA

- close to 0 = more attenuation for input below kneepoint (steeper)

21
Q

What are time constants?

A

Time constants are how quickly the HA reacts to a a change in the input level

The attack time is the time it takes for the AGC to react to an increase in the input level (and thereafter, reduce the gain)
-typically quick (~3 ms)

The release time is the amount of time needed to restore the gain after the input has dropped
- more variable (fast or slow) (e.g 45 ms)

22
Q

Why might fast release times not be desireable?

A

Rapid gain variations result in “pumping and breathing” effects
- background noises are amplified during very brief speaking pauses, leading to decreased perceived quality

23
Q

Attack time is the time taken from the change in input to when the level is within ___ dB of steady state value

A

3 dB

24
Q

Release time is the time taken from the change in input to when the level is within ___ dB of steady state value

A

4 dB

25
Q

Which will have a wider dynamic range: fast or slow compressors? Why?

A

Slow compressors because they don’t react as quickly to changes in signal

26
Q

What is the difference between bands and channels?

A

Bands - independent areas for gain adjustment only (affect soft, med, and loud)
Channels - allow separate adjustment for soft and intense levels

27
Q

How many compression channels are adequate for most hearing impaired listeners?

A

3 - 8
Additionally, uniform vs non-uniform spacing for channel bandwidth may not be as critical for vowel or consonant perception

28
Q

Name 3 applications of compression (what does it do for the listener?)

A
  • limit the output of the HA s/ distortion
  • minimize loudness discomfort
  • prevent further damage to the auditory system
  • optimize the use of the residual dynamic range
  • maintain listening comfort
  • maximize speech recognition ability
  • reduce the adverse effects of noise
29
Q

What is OCL?

A

Output Compression Limiting

  • an application of compression that uses AGC-O to prevent loud sounds from exceeding LDL
  • high kneepoint
  • fast attack time
  • single or multi-channel compression
30
Q

Which channel has more variability: single or multi?

A

Multichannel

- acoustic events in a discrete frequency region do not affect hearing aid response at all frequencies

CSD9518 (13 decks)

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