14_Level Dependent Amplification Flashcards
How do we transfer the audiogram to speechmap? What are the dB units of each?
Audiogram units are in dB HL, and speechmap is in dB SPL, so we use a HL to SPL transform
In Speechmap, what does the red line represent? What does the shaded grey area represent?
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
What is a person’s dynamic range?
The difference b/w UCL and threshold
Do hearing impaired individuals usually have a larger or smaller dynamic range compared to normal hearing individuals? Why is this?
Smaller dynamic range because the hearing loss raises the “floor”, but the UCL’s do not change
Linear amplification means that, at any point, ____ is fixed
Gain
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?
Input/Output Curve
Gain is calculated by determining the difference b/w the input and corresponding output at any given input level.
What is the main problem with fixed gain (linear amplification) in hearing aids?
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)
On an input/output plot, what is a threshold kneepoint (TK)?
The compression threshold - the input level at which the I/O curve has a change in its slope
What is a compression ratio (CR)?
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
What is the CR for linear gain?
1:1
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?
Greater than 1
What will “fixed gain” show up as on an Input/Gain Plot?
What about compression?
A flat line.
Compression = gain is decreasing
What does WDRC refer to?
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
What is Compression Limiting?
A way to ensure output does not exceed the maximum levels (below UCL)
Can you have WDRC and compression limiting in the same system?
Yes
How does curvilinear compression differ from traditional compression?
It also describes the relationship b/w input and output, but in the form of a curve (not segmented straight lines)
What is AGC-I and AGC-O? What is the difference between the two?
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
What is the difference b/w compression and expansion?
Compression = gain reducing Expansion = opposite (delta I / delta O < 1)
Expansion is always ___ (above/below) the threshold
Below
What is the purpose of expansion?
To reduce the gain for the internal noise of the HA
- close to 0 = more attenuation for input below kneepoint (steeper)
What are time constants?
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)
Why might fast release times not be desireable?
Rapid gain variations result in “pumping and breathing” effects
- background noises are amplified during very brief speaking pauses, leading to decreased perceived quality
Attack time is the time taken from the change in input to when the level is within ___ dB of steady state value
3 dB
Release time is the time taken from the change in input to when the level is within ___ dB of steady state value
4 dB
Which will have a wider dynamic range: fast or slow compressors? Why?
Slow compressors because they don’t react as quickly to changes in signal
What is the difference between bands and channels?
Bands - independent areas for gain adjustment only (affect soft, med, and loud)
Channels - allow separate adjustment for soft and intense levels
How many compression channels are adequate for most hearing impaired listeners?
3 - 8
Additionally, uniform vs non-uniform spacing for channel bandwidth may not be as critical for vowel or consonant perception
Name 3 applications of compression (what does it do for the listener?)
- 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
What is OCL?
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
Which channel has more variability: single or multi?
Multichannel
- acoustic events in a discrete frequency region do not affect hearing aid response at all frequencies
