HAT 4 Compression & Output Limiting Flashcards
How do you calculate gain?
Output (dBSPL) - Input (dBSPL) = Gain (dB)
What limits gain?
Will depend on the power of the aid and the volume setting
There will be a maximum output beyond which the aid cannot go
Saturation Sound Pressure Level (SSPL)
How do you convert dBHL to dBSPL?
Add 10 to the dBHL figure
This is correct across speech range 500-8k
What is the problem with linear gain?
Loud sounds become too loud
What is peak clipping?
Clips the peak and troughs of the waves
Restricts the height of the wave
Restricts the intensity of the sound
Restricts the max output of the HA
Only happens to loud sounds
Disadvantages of peak clipping?
Takes away clarity and causes distortion
Reduces dynamic range
Doesn’t suit busy lifestyles
Peak clipping facts
Used to coincide with ULL’s
At saturation level the sound can’t go any louder
Peak clipping trimmer reduces the max output of the aid by 5dB
Only the intensity that exceeds the saturation point will be clipped
Can be set at different frequencies
When PC is activated, saturation will occur
Peak clipping works well for?
Sudden loud sounds or level of protection
Mild sensorineural losses
Conductive hearing loss
Mixed hearing loss
Simple lifestyles
Sudden loud sounds
Why use compression?
Ensure audibility of sounds
- higher level of gain
Maintains recognition of moderate sounds
- reduced level of gain
Maintain comfort of loud sound
- little or no gain
Compression is especially valuable when the hearing loss results in a reduction in dynamic range
What is a knee point?
Where sound changes from linear gain to a different type of gain it is called a ‘knee point’ no compression until after the knee point
What is compression?
Compression is a feature of a hearing aid that automatically changes the amount of gain added to the level of the incoming signal
What are the types of compression?
Automatic Gain Control (AGC)
Automatic Volume Control (AVC)
‘Non-Linear’ Amplification
How do Automatic Gain Control (AGC) systems work?
Work in the same way as linear gain
Until the input reaches a ‘certain level’
Above the ‘certain’ level the gain automatically reduces as the input intensity increases
The point at which the auto system activated is
- knee point
- threshold of compression
- threshold of knee point
- in some aids this can be altered using TK adjustment
Beyond kneepoint
- as input gets louder the amount of gain applied decreases
Less gain for loud inputs
More gain for soft inputs
What is a compression ratio?
Low compression 2:1 / 3:1
Moderate compression 3:1 / 7:1
High compression 7:1 / 10:1
How do you calculate a compression ratio?
From an input output graph
Input level @ saturation point - input level @ knee point = A
Output level a saturation point - output level @ knee point = B
Compression ratio A:B
How to calculate ULL from input output graph?
Saturation point (dBSPL) - 10dB = ULL (dBHL)
Hi do you calculate gain from input output graph?
Output (dBSPL) - input (dBSPL) = gain (dBSPL) take 10dB to convert to dBHL
How to calculate patients loss from input output graph?
Reverse lybergers rule
Gain - 10dB (reserve) = half AC dBHL
Assuming no AB gap
What are the 2 methods of compression?
AGCI - Automatic Gain Control Input
AGCO - Automatic Gain Control Output
How does Automatic Gain Control Input (AGCI) work? Benefits?
Compression happens before the amplifier; this allows for volume control even after compression
- really good for mild to moderate losses
- once in compression in a louder environment, the client can still have significant effect on output with the VC
- Compression is based on input level
What is the effect of input on compression (AGCI)?
Compression only applies to input above knee point.
Apply compression ratio to input above knee point
Add gain
Eg
Input 100
Gain 40
Kneepoint 60
60 pass through (no compression)
40 compression is applied (2:1 ratio for ex) = 20
60+20= 80dBSPL
Add gain of 40 = output 120dBSPL
What are the facts about Automatic Gain Control Output (AGCO)?
- access to VC before the sound goes into a state of compression
- suitable for a more profound HL
- HA needs to have access to as much dynamic range as possible
- compression is based on output level
What is WDRC?
Benefits?
Wide Dynamic Range Compression
Use whole dynamic range
- greater gain for quiet sounds, mild gain for speech and little or no gain for loud noise
- bring the kneepoint below speech and have a low compression ratio, means sounds nearly always in compression
- low kneepoint and low compression ratio
What is an attack time?
Detail?
Time taken for compression
- should be as short as possible 10ms
What is a release time?
Time taken for offset of compression
Name the 3 types of compression used and benefits?
Adaptive compression
- depends on the duration of the loud input
- sudden loud sounds - fast release
- continuous loud sounds - release delayed until the sound has gone
Dual Compression
- short release for higher frequency ‘speech sounds’
- longer release on lower frequency ‘background sounds’
Syllabic Compression
- short attack / short release
- very useful when listening to speech only
Attack / Release time pitfalls?
- release too fast - pumping sound in head
- attack too slow - device not react quickly enough to dynamic changes
- release time to slow - under amplification of sound
What is expansion?
Reduces audibility of very soft sounds eg fan or internal sounds
