06 Conventional hearing aids

Question 1

What are the typical components of a hearing aid?

Answer 1

A hearing aid captures incoming sound with a microphone, there is signal processing before being amplified and sent to the tympanic membrane via a loudspeaker.

The greater the damage to the hair cells the more severe the hearing loss and the greater the amplification needs to be (affecting the geometry and size of the hearing aid

Question 2

What type of hearing loss are conventional hearing aids supposed to be used for?

Answer 2

Sensorineural hearing loss.
They are useful from mild to profound hearing loss

Question 3

What are the two typed of conventional hearing aids?

Answer 3

For mild healing loos, the entire sytem can be placed in the external auditory canal, In the ear (ITE) hearing aids

For more sever hearing loss, behind the ear (BTE) systems are needed

Question 4

What are the basic amplification goals of hearing aids?

Answer 4

Hearing aids amplify sound with the goal of improving hearing of hearing disabled persons. The target is to amplify sound up to the lowest level of the speech banana.

Question 5

What is the speech banana?

Answer 5

Specific sounds in human speech have a “normal” frequency level and sound level.The speech banana illustrates the range of frequencies and sound levels where most human speech occurs.

If hearing loss occurs within the region of the speech banana, a child or an adult would have difficulty communicating because they cannot discern/hear the sounds that are made

Question 6

What is the Half Gain Rule in relation to hearing aids?

Answer 6

The Half Gain Rule states that approximately half of the hearing loss is compensated when calculating the amplification target of a hearing aid.

The Gain (G) or amplification should be approx. half of the hearing loss level at that frequency.

Question 7

What is the formula for calculating gain in hearing aids?

Answer 7

Gain is calculated as gain = P_out / P_in.

Question 8

What are the differences between insertion gain and open ear response in hearing aids?

Answer 8

Insertion gain is the theoretical gain which compensates the hearing loss (Half gain rule)

Open ear response is the natural amplification of the ear canal, which is lost with a hearing aid insertion and must be compensated for by the device.

Question 9

How is in situ gain determined?

Answer 9

In situ gain = insertion gain + open ear response

It is the practical (actual) amplification present in the patient’s ear canal with the hearing aid in place, accounting for individual ear acoustics and the hearing aid’s settings

Question 10

What is a linear hearing instrument?

Answer 10

Provides a constant amplification to all incoming sounds.

Low sounds are amplified but already loud sounds become too loud

Question 11

What does recruitment refer to in the context of hearing aids?

Answer 11

Recruitment refers to the reduced tolerance to loudness in hearing impaired persons, where soft sounds are perceived at a significantly higher sound pressure level.

This reduction of dynamic range is what is called recruitment

Question 12

What is broadband compression in hearing aids?

Answer 12

Broadband compression provides less amplification for loud sounds, meaning the louder the input sound, the less gain provided.

Question 13

What is the purpose of two-channel wide dynamic range compression?

Answer 13

Two-channel wide dynamic range compression separates frequency bands using a cross filter to manage sound amplification according to frequency levels.

Question 14

How do directional microphones enhance hearing aid functionality?

Answer 14

Directional microphones enhance the signal-to-noise ratio by reducing the intensity of rear sound while improving the clarity of sounds coming from the front.

(But the directional microphone has little effect if the speech and noise are coming from the same direction)

Question 15

What are the different directional microphone patterns?

Answer 15

1. Bidirectional pattern: sensitive to sound from 0 (in-front) and 180 degrees (behind).
2. Hypercardioid pattern: most sensitive to sound from 0 degrees.
3. Cardioid pattern: sensitive to sound from 0 degrees, suppressing sound from 180 degrees.

Question 16

What is static directionality in hearing aids?

Answer 16

The front and rear microphones have a set distance between them. This distance will create a constant delay between them. This delay is always subtracted from the front microphone signal (T in the diagram). This means that sound coming from behind will be received from the rear microphone and be delayed. The sound will also reach the front microphone after the delayed period.

The sounds will be the same but with opposing polarities which will cause the sound to be cancelled out

Question 17

What is adaptive directionality in hearing aids?

Answer 17

Adaptive directionality allows microphones to automatically switch between omnidirectional and directional modes to enhance speech clarity in different environments.

Question 18

What is the basic idea behind noise reduction in hearing aids?

Answer 18

The basic idea is to ensure that the level of the speech signal exceeds the noise level by reducing amplification in bands where noise dominates.

Question 19

What is the process of feedback cancellation in hearing aids?

Answer 19

Adaptive feedback cancellation suppresses unwanted acoustic signals from the receiver to the microphone, allowing the hearing aid to function effectively without feedback.

Question 20

What is the significance of the speech banana in audiograms?

Answer 20

The speech banana represents the range of frequencies and sound levels that are crucial for understanding speech, serving as a target for amplification in hearing aids.

Question 21

What are the characteristics of the dynamic range in hearing for normal and hearing-impaired individuals?

Answer 21

Normal hearing persons have a dynamic range of 10 dB to 90 dB, while hearing-impaired individuals have a reduced dynamic range, often from 50 dB to 90 dB.

Question 22

What are the key components of wide dynamic range compression?

Answer 22

Wide dynamic range compression involves processing signals to provide different gain for soft and loud sounds, typically using high-pass and low-pass filters.

Question 23

What are the advantages of using Discrete Fourier Transform (DFT) in hearing aids?

Answer 23

The DFT allows for individual gain application to each frequency, providing a perfect frequency match, though it introduces a time delay.

Question 24

What is the role of filter banks in hearing aids?

Answer 24

Filter banks split the input signal into different frequency bands for processing, allowing for less precise but faster calculations than DFT.

Question 25

What is a controllable filter in hearing aids?

Answer 25

A controllable filter produces continuous gain functions and processes the signal without splitting it, allowing for real-time adjustments to amplification.

Question 26

What should be done if b is more than a and signals are out of phase?

Answer 26

If b is less than a and the signals collected last time and this time are out of phase, then b has to be decreased.

Question 27

What should be done if b is less than a and signals are in phase?

Answer 27

If b is less than a and the signals collected last time and this time are in phase, then b has to be increased to improve feedback cancellation.

Question 28

What is the goal of the classification of sound?

Answer 28

The goal is the identification of different acoustic environments.

Question 29

What does the Acoustic Directionality Diagram illustrate?

Answer 29

The Acoustic Directionality Diagram illustrates how sound directionality is processed in hearing aids.

Question 30

What does the Noise Reduction Diagram depict?

Answer 30

The Noise Reduction Diagram depicts the mechanism by which noise is reduced in hearing aids.

Question 31

What is shown in the Wide Dynamic Range Compression (WDRC) Diagram?

Answer 31

The Wide Dynamic Range Compression (WDRC) Diagram shows how dynamic range compression is applied in hearing aids.

Question 32

What does the feedback loop element influence in a hearing aid?

Answer 32

The feedback loop element influences feedback cancellation.

Question 33

What factors are considered in the adjustment of hearing aids?

Answer 33

The adjustment of hearing aids is based on identifying the classification of the signal such as Acoustic Directionality, Speech, Noise Reduction, WDRC, and Feedback Cancellation.

Question 34

What are the conditions under which speech is important for hearing aids?

Answer 34

1. If speech is in front,
2. When speech is in noise,
3. If high amplification is needed.

Question 35

What considerations are important for music in hearing aids?

Answer 35

Room acoustics are important for music and ideally no compression.

Noise reduction is not suitable for music because it will affect the frequency of the music (music always inculdes low frequency signals).

Question 36

What does ‘Phantom signal’ refer to in the context of hearing aids?

Answer 36

‘Phantom signal’ refers to a signal that may be perceived in certain conditions, possibly due to feedback or processing in the hearing aid.