Lecture 4.0 - Callibration

Question 1

Describe the process of calibrating audiological equipment*

Answer 1

There are three types of calibration procedures:

• Stage A checks
• Stage B checks
• Stage C checks

Question 2

What is the rationale behind calibration of audiometric signals?*

Answer 2

As our audiometers are mechanical equipment, they can slowly change their output over time.

Mainly due to temperature fluctuations and knocks/drops.

Question 3

Why do we need to manage ambient noise?*

Answer 3

If the environment is noisy, the noise will “mask” the test signal.

To ensure the testing environments do not have too much ambient noise, they are tested every five years.

The ambient noise levels are compared to standards (different for different transducers) that determine which amplitude you can test down to.

NZ have two different standards for ambient noise
- ACC (less stringent - whole octave bands)
- ISO (1/3 octave bands)

If the ambient noise is too high you can:
- Improve sound attenuation and address any sources of the noise
- Increase the “floor” of the hearing test

Question 4

What is a reference equivalent threshold SPL?

Answer 4

It is the average threshold of hearing for normal people.

It varies across the different types of transducers.

RETSLPs provide the SPL value which correspond to 0 dB HL:
dB HL = dB SPL - RETSPL

Question 5

Describe the process of completing a Stage A check.

Answer 5

• To be completed daily
• Subjective test for gross malfunction
• Check transducers are matched correctly
• Listen to the different frequencies (other noises, distortion, clicks)

Question 6

Describe the process of completing a Stage B check and the rational behind them.

Answer 6

• Completed annually (ACC only requires every two years)
• Completed to determine if the output is within tolerances
• Other checks include: harmonic distortion, x-talk, masking noises, attenuator linearity (turning up 10 dB actually means an increase in 10 dB)

Protocol:
- A system is set up with the audiometer connected to an artificial ear/mastoid to a sound level meter
- Use the following equation to determine what the correct output should be:

dB SPL = attenuator + RETSPL + mic sensitivity correction

• Check measured value against reference values
• Adjust levels to fall within tolerance ranges

Why?
- The audiometers are mechanical devices that can undergo changes across time (due to temperature fluctuations and knocks/drops)

Question 7

True/false: The calibrator does not also need calibration

Answer 7

False, the calibrator also needs calibration.

This results in a chain of calibration.

This concept is referred to a train of traceability.

Question 8

Why may we see an air-bone gap at 3 and 4 KHz?

Answer 8

Originally, people thought this was due to acoustic radiation (sound leaking into the ear) and recommended that clinicians plug the ear at these frequencies.

However, this does not help as it was proven that the issue was not sound radiation, but an issue with the artificial mastoid used in the RETFL causing it to be around 14 dB too high.

Question 9

What is a limitation of using dB HL in rehabilitation?

Question 10

How are complex sounds calibrated?