HAT 4 Compression & Output Limiting

Question 1

How do you calculate gain?

Answer 1

Output (dBSPL) - Input (dBSPL) = Gain (dB)

Question 2

What limits gain?

Answer 2

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)

Question 3

How do you convert dBHL to dBSPL?

Answer 3

Add 10 to the dBHL figure
This is correct across speech range 500-8k

Question 4

What is the problem with linear gain?

Answer 4

Loud sounds become too loud

Question 5

What is peak clipping?

Answer 5

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

Question 6

Disadvantages of peak clipping?

Answer 6

Takes away clarity and causes distortion

Reduces dynamic range

Doesn’t suit busy lifestyles

Question 7

Peak clipping facts

Answer 7

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

Question 8

Peak clipping works well for?

Answer 8

Sudden loud sounds or level of protection

Mild sensorineural losses
Conductive hearing loss
Mixed hearing loss
Simple lifestyles
Sudden loud sounds

Question 9

Why use compression?

Answer 9

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

Question 10

What is a knee point?

Answer 10

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

Question 11

What is compression?

Answer 11

Compression is a feature of a hearing aid that automatically changes the amount of gain added to the level of the incoming signal

Question 12

What are the types of compression?

Answer 12

Automatic Gain Control (AGC)

Automatic Volume Control (AVC)

‘Non-Linear’ Amplification

Question 13

How do Automatic Gain Control (AGC) systems work?

Answer 13

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

Question 14

What is a compression ratio?

Answer 14

Low compression 2:1 / 3:1
Moderate compression 3:1 / 7:1
High compression 7:1 / 10:1

Question 15

How do you calculate a compression ratio?

Answer 15

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

Question 16

How to calculate ULL from input output graph?

Answer 16

Saturation point (dBSPL) - 10dB = ULL (dBHL)

Question 17

Hi do you calculate gain from input output graph?

Answer 17

Output (dBSPL) - input (dBSPL) = gain (dBSPL) take 10dB to convert to dBHL

Question 18

How to calculate patients loss from input output graph?

Answer 18

Reverse lybergers rule

Gain - 10dB (reserve) = half AC dBHL

Assuming no AB gap

Question 19

What are the 2 methods of compression?

Answer 19

AGCI - Automatic Gain Control Input
AGCO - Automatic Gain Control Output

Question 20

How does Automatic Gain Control Input (AGCI) work? Benefits?

Answer 20

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

Question 21

What is the effect of input on compression (AGCI)?

Answer 21

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

Question 22

What are the facts about Automatic Gain Control Output (AGCO)?

Answer 22

• 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

Question 23

What is WDRC?
Benefits?

Answer 23

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

Question 24

What is an attack time?
Detail?

Answer 24

Time taken for compression
- should be as short as possible 10ms

Question 25

What is a release time?

Answer 25

Time taken for offset of compression

Question 26

Name the 3 types of compression used and benefits?

Answer 26

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

Question 27

Attack / Release time pitfalls?

Answer 27

• 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

Question 28

What is expansion?

Answer 28

Reduces audibility of very soft sounds eg fan or internal sounds