Introduction to Compression

Question 1

what is linear frequency response and would we use it in modern hearing aids?

Answer 1

-a linear frequency response provides the same gain at each frequency amplified by the the hearing aid

• With today’s technology and modern fitting rules it is extremely rare that this happens.

Question 2

what is non linear frequency response

Answer 2

If you have variation in the hearing loss across the frequencies you need a variable frequency response to provide more gain at the frequencies where the loss is worse

Question 3

what is linear amplification?

Answer 3

At each frequency of sound the gain stays the same, no matter what the intensity of the input sound to the hearing aid.

• However, there is usually a limit to the output from a hearing aid, so the linearity of the aid tails off at high input levels

Question 4

what is Non- Linear Amplification (aka Compression)?

Answer 4

• Refers to the change in gain as a result of changes in input intensity level, at a given frequency
• The gain will be different for quiet sounds than for loud sounds

Question 5

what does the gain from a hearing aid depend on?

Answer 5

• The frequency of sound
• The intensity of the input (at each frequency)

Question 6

what is the difference between the frequency response and the input- output function of a hearing aid?

Answer 6

• the Frequency Response of the hearing aid, which shows the variation in gain across the frequencies amplified by the aid
• The input-output function of the hearing aid, which shows the variation in gain across different input intensities, at a specified frequency

Question 6

what’s the difference between linear and non linear amplification with digital hearing aids?

Answer 6

Linear amplification maintains a consistent gain across frequencies regardless of input intensity, while non-linear amplification (compression) adjusts gain at each frequency based on changes in input intensity or output intensity.

As a result, the dynamic range of the output is reduced compared to the dynamic range of the input.

Question 6

What is the fundamental problem associated with hearing impairment?

Answer 6

The fundamental problem is that the range of sounds the normal auditory system can handle, typically ranging from 0 to 100 dBHL (decibels hearing level), is significantly larger than what an impaired auditory system can manage.

This results in a reduced dynamic range for patients with hearing impairment.

Question 6

what is the problem with linear amplification?

Answer 6

OVERAMPLIFICATION:

• Many individuals with sensorineural hearing loss experience a reduced dynamic range.
• Linear hearing aids may amplify sounds beyond the wearer’s uncomfortable loudness levels (ULLs), leading to discomfort.

In severe cases, this discomfort can deter individuals from using hearing aids altogether.

*To address this:
- Limiting the output of the hearing aid can resolve this problem.

Question 6

What is the purpose of output limiting in hearing aids?

Answer 6

Output limiting aims to reduce the dynamic range of incoming signals to match the patient’s dynamic range of hearing.

Question 6

What methods can be employed for output limiting in hearing aids?

Answer 6

• Peak Clipping (PC)
• Compression; also known as Automatic Gain Control (AGC) or Automatic Volume Control (AVC)

Question 7

what is peak clipping?

Answer 7

Peak clipping is a basic method of output limiting that involves clipping the peaks of the waveform when it exceeds a certain loudness threshold.

Question 8

How does peak clipping affect the signal in hearing aids?

Answer 8

• Peak clipping reduces the loudness of the signal to ensure it falls within the wearer’s dynamic range.
• However, it introduces distortion into the waveform, making it less ideal for output limiting.

Question 9

What is compression in the context of hearing aids?

Answer 9

Compression is a method of output limiting where the range of input sound intensities is compressed into a smaller range of output intensities.

Question 10

How does compression affect the dynamic range of sound in hearing aids?

Answer 10

Compression squashes the range of input intensities, resulting in a smaller dynamic range of output compared to the input.

Question 11

What typically affects the gain in compression?

Answer 11

The gain in compression is usually affected by the input level to the hearing aid. However, occasionally it can also be affected by the output level from the hearing aid.