Compression Flashcards
What are the types of SNHL?
Type I, II, III
List characteristics of Type I HA?
- Elevated thresholds
- Normal LDLs
- Mild - Mod HL
- Easier to fit
- Give ~20dB of gain for soft sounds
- Give 0-minimal gain for loud since LDLs are normal
List characteristics of Type II HA?
- Elevated thresholds
- Reduced LDLs
- Mod Severe HL
- Need more gain for soft
- Need a little gain for restoration, but not a lot
List characteristics of Type III HA?
- Elevated thresholds
- Reduced LDLs
- Decreased WRS
- Profound HL
- Need significant gain for soft
- Need significant gain for loud, just under LDLs
What should one consider for the Types of SNHL?
- Size of DR
- Threshold
- LDL - Speech intelligibility
- Audibility is key - Frequency dependent
- Impacts compression
How do we recreate normal hearing for those with HL?
Compression by squeezing their normal DR into a reduced DR
What is the goal of compression?
- Soft = Audible
- Average = Comfortable
- Loud = Tolerable
What are the compression schemes?
- Linear
2. Nonlinear
For linear compresion, what happens when we increase the output?
- Saturation occurs
- Can’t get any louder - Peak Clipping
- Occurs when sound tries to get louder
What are linear compression HAs good at?
Making soft speech audible and clear
What are limitations to Linear compression?
- Makes average speech too loud
- Loud is uncomfortable
- Distortion occurs
When would you consider Linear Compression?
- Long-term HA users with familiarity with Linear Compression
- Severe/Profound HL
- CHL/MHL
When would we consider Nonlinear Compression?
- Fitting goal
2. HA circuitry
What are the terms for Compression?
- Compression Threshold
- Compression Ratio
- Attack & Release Time
What is CT?
The slope of the I/O function changes and is 2dB lower than it would be
What is CR?
Determines how much the signal will be compressed
When is compression applied?
Only above the TK and when the CR is greater than 1:1
What are the characteristics of CT?
- Low CT
- Most common with WDRC - High CT
- Keeps HA linear longer - Modern HAs have multiple CTs
When would we use a Low CR?
- To improve audibility of soft speech
- Restore loudness perception
- Ranges 1.1-4.0
What is the downside to Low CR?
May overamplify soft sounds
When would we use a High CR?
- Keep Sounds tolerable
2. Ranges from >4.0
What is the downside to High CR?
Can affect sound quality and clarity
Define Attack & Release Time?
The length of time required for the compression circuit to respond to sudden changes in the input
What happens during attack time?
Is when compression begins and reduced gain occurs
What happens during release time?
When the HA comes out of compression and gain is restored
What does attack time allow?
Maintains DR
What does release time allow?
Avoid pumping
What happens if release time is affected?
Speech intelligibility deteriorates if too slow
What happens if attack time is affected?
Sound quality deteriorates if too fast
What affects compression?
Volume Controls
List characteristics of Output Compression (AGC-O).
- Level detector is located after VC
- Compression is affected if VC changes
- Compression is activated when the output exceeds CT
- MPO is not affected
- Ex of Compression Limiting
List characteristics of Input Compression (AGC-I).
- Level detector is before VC
- Compression is not affected
- Gain is reduced at pre-amplifier if input exceeds CT
- MPO shifts with VC
- Ex of WDRC
What are the different ways to maintain DR?
- Compression Limiting
- Wide Dynamic Range Compression
- Expansion
What is Compression Limiting and its characteristics?
- Keep loud sounds tolerable without causing distortion
- High CT
- High CR
- Avoids peak clipping & distortion
- Severe to Profound Hl (Type III)
What is WDRC and its characteristics?
- Reproduce a full dynamic range w/pts with reduced DR
- Low CT
- High CR
- Mild-Moderate HL (Type I)
What issue can WDRC cause?
If adding too much gain to soft sounds make it too audible
When would you use compression?
- Limit the output of the HA w/o distortion
- Minimize loudness and discomfort
- Prevent further damage to the auditory system
- Optimize residual DR
- Restore normal loudness perception
- Maintain listening comfort
- Maximize speech recognition ability
- Recuce the adverse effects of noise
