Before each day’s use
A functional check: test a person with known, stable hearing thresholds and listen for distorted or unwanted sounds. A deviation of 10 dB or more requires an acoustic calibration.
Guide
How to calibrate an audiometer
Calibrating an audiometer means measuring what it actually puts out, through each earphone and bone oscillator at each frequency and level, and comparing that with the standard. Here is what you need, the steps in order, and what OSHA requires for occupational hearing testing.
Before you start
What you need to calibrate an audiometer
- A precision measurement microphone with a sound level meter or measurement system
- An acoustic calibrator or pistonphone, to check the microphone before measuring
- Couplers that match the transducers: NBS-9A/IEC-303 or IEC-318 for headphones, IEC-126/HA-2 or IEC-711 for insert earphones, IEC 60318-2 for high frequency phones
- An artificial mastoid for bone oscillators
- The reference levels for each transducer (ISO 389) and the tolerances of the standard you calibrate to (ANSI S3.6 or IEC 60645)
- The audiometer’s service manual, for making adjustments
- A quiet room, so background noise doesn’t affect the measurements
Step by step
The steps of an audiometer calibration
An exhaustive calibration covers every step below for each transducer. An annual acoustic calibration under OSHA can be limited to the output levels in step 3.
| 1. Prepare | Switch on the audiometer and the measuring equipment and let them warm up. Check the microphone with the calibrator. Record the make, model and serial number of the audiometer and of each headphone, insert earphone and bone oscillator. |
|---|---|
| 2. Functional check | Listen through each transducer for distortion, hum, crackling cords or noise from the attenuator, and check that the patient response switch works. |
| 3. Air conduction output levels | Place each earphone on its coupler with the specified force. At each test frequency, present a tone at a fixed hearing level and measure the sound pressure. Compare it with the reference level for that earphone plus the dial setting; it must be within the tolerance of the standard. |
| 4. Bone conduction output levels | Place the bone oscillator on the artificial mastoid and measure the force level at each frequency against the bone conduction reference levels. |
| 5. Frequency and distortion | Measure the frequency of each test tone, including warble tones, and its harmonic distortion. |
| 6. Attenuator linearity | Step the hearing level dial through its range and check that each step changes the output by the stated amount. |
| 7. Tone switching and crosstalk | Check rise time, fall time and overshoot when a tone is presented, that nothing is heard with the tone off, and that sound does not leak into the other earphone. |
| 8. Masking noise and speech | Measure the levels of the narrowband, white and speech noise and of the speech channel. |
| 9. Adjust and re-measure | Adjust anything out of tolerance following the service manual, then measure again. Keep the values found before adjustment: they show how far the audiometer drifted. |
| 10. Document | Record every result, the equipment used and the standard on a calibration certificate. What a calibration certificate should include. |
With the ACS
Let the ACS run the calibration
Done by hand, those steps take hours per audiometer. The ACS has the reference levels and tolerances for ANSI S3.6, IEC 60645 and ISO 389 built in. On 73 audiometer models it controls the audiometer through its serial or USB port, sets every test point, measures it, adjusts it and records the pre-adjustment values, so an exhaustive calibration takes under 10 minutes. On other models it prompts the technician at each step.
When it finishes, it prints a certificate with every result, the transducer serial numbers, the microphone, coupler and mastoid used, and the standards applied.
See the ACS
OSHA requirements
How often an audiometer must be calibrated
For occupational hearing conservation programs in the United States, OSHA’s noise standard (29 CFR 1910.95, Appendix E) requires three levels of check. States with their own OSHA plan, such as Michigan and Indiana, have equivalent rules.
At least once a year
An acoustic calibration of the output levels, at least from 500 to 6000 Hz. A deviation of 15 dB or more requires an exhaustive calibration.
At least every two years
An exhaustive calibration to the sections of ANSI S3.6-1969 listed in Appendix E: output levels, frequency, linearity, tone switching and the rest of the specification.
Audiometric test records must include the date of the audiometer’s last acoustic or exhaustive calibration, and the employer’s most recent background noise measurements for the test room (29 CFR 1910.95(m)).
Test rooms
Audiometric test room requirements
OSHA Appendix D sets the highest background noise allowed in a room used for occupational audiometric testing, measured in octave bands.
| Octave band center frequency | Maximum sound pressure level |
|---|---|
| 500 Hz | 40 dB |
| 1000 Hz | 40 dB |
| 2000 Hz | 47 dB |
| 4000 Hz | 57 dB |
| 8000 Hz | 62 dB |
The ACS measures room background noise in 1/3 octave bands as part of a calibration, with a start delay so the technician can leave the room.
Ready to speed up your calibrations?
Get pricing for your configuration, or see the ACS in action.