Measurement of Hearing Aid Performance

Question 1

If all seals are not sealed when testing in a couple box what will happen?

Answer 1

excessive gain at ~500Hz

Question 2

Problems with measuring uncomplicated signal like pure tone or broad band noise

Answer 2

noise reduction kick in to reduce gain

feedback reduction kick in to reduce output

measures at different frequencies made sequentially may differ from those made simultaneously

program change may result in some aids with these types of signal (e.g. switch to music program)

Question 3

What is the ICRA signal?

Answer 3

Artificial speech signals have been developed for the International Collegium of Rehabilitative Audiology

Benefit is is that it is somewhat standardised unlike proprietary modulated speech noises

Question 4

What are the two main measurements made in a test box

Answer 4

The Gain Frequency Response
-This is the output measured with a 60dBSPL input signal (Or 50dB for a power aid). The gain at any frequency can then be calculated by subtracting the input SPL from the output SPL

The OSPL90

   - Both the IEC and ANSI standards specify that hearing aid maximum output should be measured using a 90 dB SPL input signal, and both standards now use the term OSPL90 to describe the measurement.
- When the hearing aid output has reached its limit for any input signal, it is said to be saturated.
- Hearing aids with a steeply rising response will often not be saturated at low frequencies. In such cases, the measurement will underestimate the true maximum output of the hearing aid at low frequencies.
- If you want to see saturation across the whole frequency range then you need to have everything on max gain, not PPL
- The vertical axis of the OSPL90-frequency response graph is always shown in dB SPL
- Do not take this in clients ear

Question 5

Two commonly chosen sets of levels are

Answer 5

50,60,70,80and90dBSPL or 50,65, and 80

Question 6

What is the difference between gain-frequency response and input-output functions

Answer 6

Gain-frequency response shows the gain (or output level) at all frequencies for one input level, Input-output function shows the output level at all input levels, for one frequency or for one broad-band test signal

Question 7

What is the difference between responses (e.g. REUR) vs. gains (e.g. REIG)?

Answer 7

For any of the “Responses” (REUR, REAR, etc.) the figures you record are simple measures of the SPL in the ear canal.

For any of the “Gain’s” (REAG, REIG, etc.) the figures you are shown will be “total SPL – input level”.

Question 8

Where should the probe tip be placed and why?

Answer 8

Within 6mm of the eardrum to eliminate error caused by standing waves

Question 9

Information gained from IG measurements

Answer 9

• Amplification characteristics of the aid.
  
  • Acoustic modifications to the aid.
  • Acoustic effects of coupling the aid to the ear.
  • Loss of normal ear canal resonance.
  • Head/pinna effects

It is the REAG provided by the hearing aid, minus the REUG provided by the unobstructed ear

Question 10

If the person’s ear canal and concha provide 12dB of gain at 2kHz, and the REAG (the SPL at the eardrum relative to the SPL outside the head) at this frequency is 30dB, what is the REIG?

Answer 10

18dB

Question 11

Positioning of client for REIG

Answer 11

0.5-0.75m from speaker

At angle of 0-45degrees

Question 12

What does a typical REUG response look like?

Answer 12

Fairly flat for the low frequencies and then a peak somewhere between 2.2kHz and 3.2kHz between 12 and 22dB above the low frequency plateau

Question 13

Functional Gain

Answer 13

Aided thresholds = Unaided thresholds

Question 14

Benefits of Functional Gain

Answer 14

Measures the clients aided hearing ability

Can be used in infants and children who might not sit still enough for REIG

Useful for profound losses - whether they are likely to get much benefit from acoustic cues in speech

Potentially better effectiveness because it is behavioural

Question 15

Disadvantages of Functional Gain

Answer 15

Ambient noise floor can effect results

Needs cooperation from the client

Measured in 5dB steps so not heaps accurate

Only measures a small number of selected frequencies

Tells us how they respond to soft sounds not speech sounds

May cause feedback cancellation

Question 16

Advantages of REIG over FG

Answer 16

Quick

Many finely spaced frequencies instead of just the audiometric frequencies

Can be measured at a range of input levels

Not affected by problems of masked aided thresholds

only requires aided wearer to sit still