Introductory Concepts

Question 1

Describe the acoustic era

Answer 1

involved speaking tubes, horns, trumpets.. at most these can result in an increase of around 10-15dB in the mid-frequencies Reasonable BGN reduction Popular in the 1800s

Question 2

Describe the Electric-Carbon Era

Answer 2

1899 first started Diaphragm compressed carbon granules which changed voltage Suitable for mild-moderate hearing losses Poor quality sound, little control over frequency response

Question 3

Describe the Vacuum Tube Era

Answer 3

1920, first wearable model 1934 Three components A) Ear receiver B) Microphone,Amplifier C) Batteries

Question 4

Describe the Transistor Era

Answer 4

First in 1952 Hearing aids could now be much smaller than before and slightly less power hungry

Question 5

Describe the digital era

Answer 5

The incoming voltage or current from the mic is an analogue signal (variations in voltage are analogous to variations in the sound source), we can sample that incoming signal at discrete time intervals converting it into a set of binary (number) representations. We can then perform all sorts of mathematical operations to try to get the signal to come out the way we want it. Hearing aids are almost all digital these days

Question 6

Basic Terminology: Gain

Answer 6

Amplification at each frequency We can apply different gains in different channels or bands

Question 7

Basic Terminology: Linear Amplification

Answer 7

Apply the same amount of gain regardless of the input level

Question 8

Basic Terminology: Compression/Non-Linear Amplification

Answer 8

Different levels of gain for different input levels

Question 9

Basic Terminology: Input/Output Diagram

Answer 9

Typical way of describing how much amplification

Question 10

Basic Terminology: Average Gain

Answer 10

Gain averaged over several frequencies

Question 11

Basic Terminoloy: Frequency Response

Answer 11

Shape of gain curve (gain plotted against frequency)

Usually shown graphically
Sometimes called the gain curve or gain frequency response
Important to specify measurement conditions and input level
To fully describe the gain of a linear amplifier it is necessary to state its gain at every frequency within the freq range of interest - that is the gain freq response

Question 12

Basic Terminology: Saturation SPL

Answer 12

All amplifiers become non-linear when the input or output signals exceed a certain level.

The highest value of SPL that can be produced is called the Saturation Sound Pressure Level, or SSPL. As with gain, the SSPL varies with frequency, and a useful measure is the SSPL response curve.

Terms closely related to SSPL are Output Sound Pressure Level for a 90 dB SPL input level (OSPL90), and Maximum Power Output (MPO)

SSPL90 is the SSPL measured with a 90dBSPL input, we assume that an input of 90dB is enough to cause any aid to saturate

Question 13

Problems faced by hearing impaired people and how hearing aids can help

Answer 13

Inaudibility of sounds

• Speech is loud enough but not clear enough
• Hearing aids can:
  
  • amplify different frequencies by different amounts
  • there are some limits to this, for example feedback

Reduced Dynamic Range and Recruitment

• “Speak up” followed by “no need to yell”
• Hearing Aids can:
  
  • use compression amplifiers to amplify soft sounds and not loud sounds

Reduced Frequency Discrimination

• frequency smearing
• not a lot hearing aids can do
  
  • there are some aids that can take information from dead regions and present them to hearing regions

Reduced Temporal Discrimination

• People with sensorineural hearing loss can lose their ability to extract snippets of speech information during the weaker moments in the background noise effectively.
• Loud sounds that occur immediately before or after soft sounds can mask them (essentially blocking them out). This makes it difficult to understand speech, especially in competing background noise.
• This is a difficult problem to overcome with hearing aids
  
  • Fast acting compression hearing aids
    
    • can increase the loudness of speech signals that occur between the noise
    • however this has the disadvantage of also increasing the softer components of the noise, as the hearing aid cannot distinguish between the signals that we want to hear and the signal that we don’t.

Overlapping/Masking of Noise

• Difficulty in BGN
• People with normal hearing have an SNR ability of -10, this is when the average noise level above speech level is 10dB
• People with SNHL often need the SNR of +4 - +10 dB

All these problems means someone could have a perfectly fitted hearing aid and still have difficulty understanding speech

Question 14

Difference between bilateral and unilater vs. binaural and monaural

Answer 14

Monaural = sound presented to one ear
Unilateral = fitting is the fitting of only one aid
Binaural = sound presented to both ears
Bilateral = fitting is the fitting of two hearing aids

Question 15

Advantages of Binaural Hearing

Answer 15

Localisation

• Horizontal
  
  • Brain uses differences between intensity of a sound and the time it takes the sound to reach each ear to determine the location of the sound
  • Timing and Level
• Vertical
  
  • Pitch (spectral) intensity cues are used
  • Pinna and ear canal alter the spectral intensity of the frequency components of the sound (by reflecting them differently)
• Adding a hearing aid changes the physical properties of the ear and can effect vertical localisation
• We need good localisation for speech in noise, multiple sound sources can become confusing without it, when hearing aids are worn for the first time this can impair localisation ability, possibly leading to the sensation that wearing hearing aids makes it harder to hear in noise

Head Shadow Effect

• Brain can listen to the ear with the best SNR

Binaural Squelch

• Bgn is suppressed. Requires central processing
• Requires that the signal reaching each ear is different
• Brain is able to attend to each ear separately and to attend to some signals whilst suppressing others

Binaural Redundancy

• Brain receives the same information from both ears independently
• Appears to results in a small advantage over monaural listening
• Effects are smaller than for binaural squelch

Loudness summation

• around 5dB, or 3-10dB so less gain needed
• Sound that is heard binaurally rather than monaurally is perceived as being twice as loud
• Improves speech perception in quiet and noise
• When fitting hearing aids, less gain is needed when two hearing aids are fitted than with just one

Question 16

Advantages of Bilateral Fitting

Answer 16

• Binaural advantage (above)
• Avoiding late-onset auditory deprivation: a contentious issue
• Balance
• Suppression of tinnitus
• Practical reasons e.g. flexibility and spare aid
• Safety
• Ease of listening: listening with only one ear can be tiring

Question 17

Disadvantages of Bilateral Fitting and Advantages of Unilateral Hearing

Answer 17

Disadvantages of bilateral fittings

• Cost
• Self-image
• Increased wind noise
• Management difficulties
• Confusion

Advantages of unilateral fitting

• Central lesions/processing problems
• Management: less difficult to cope with one device
• Cosmetic
• Cost
• Various client preferences (your first CE couldn’t stand the tinnitus that she got from one hearing aid so she only wore the other one)

Question 18

If fitting unilaterally, which ear do you fit?

Answer 18

The audiogram

• If the better ear is close to normal do not fit the hearing aid to it
• If the client has a hearing loss in both ears Newall, 2002 recommends
  
  • If loss is over 30dB in the better ear fit the better ear
  • If loss is over 90dB in the worse ear, fit the better ear
  • In other situations fit the worse ear

Medical factors
Client preference
Speech discrimination ability