Output level
At each frequency and hearing level, every earphone, insert and bone oscillator must produce the reference level for that transducer, within tolerance. This is the heart of every calibration.
Audiometer standards
Audiometer specifications, explained
Audiometers are built and calibrated to published standards. ANSI S3.6 in the United States and IEC 60645 internationally set what an audiometer must do and how accurately it must do it; ISO 389 sets the reference levels its outputs are measured against.
The standards
Which standard covers what
| ANSI S3.6 | The American National Standard specification for audiometers. It defines audiometer types, from full diagnostic audiometers to screening audiometers, the features each type must have, and the tolerances for output level, frequency, distortion and the other specifications below. The ACS has the 1969, 1989, 1996, 2004 and 2010 editions built in. |
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| IEC 60645 | The international audiometer standard, published in parts: pure-tone and speech audiometers, plus separate parts for acoustic impedance (tympanometry), otoacoustic emission (OAE) and auditory brainstem response (ABR) instruments. |
| ISO 389 | The reference threshold levels: the sound pressure (or, for bone conduction, the force) that corresponds to 0 dB HL at each frequency for each type of earphone and bone oscillator. These are the targets an output level calibration is measured against. The ACS has the 2002, 2012 and 2017 editions built in. |
| ANSI S3.39 | The specification for instruments that measure aural acoustic impedance and admittance: tympanometers and middle ear analyzers. |
| OSHA 29 CFR 1910.95 | For occupational hearing conservation programs in the United States, Appendix E sets how often audiometers must be checked: a functional check before each day’s use, an acoustic calibration at least yearly and an exhaustive calibration at least every two years. More on calibration schedules. |
The specifications
What an audiometer specification covers
The exact tolerances depend on the audiometer’s type and the edition of the standard. The ACS has them built in, so each measurement is compared with the right limit and anything out of spec is shown in red.
Frequency
Each test tone must be within a set percentage of its nominal frequency, including warble (frequency modulated) tones.
Distortion
Harmonic distortion must stay below the limit, so the listener hears a pure tone and not its harmonics.
Attenuator linearity
Each step of the hearing level dial must change the output by the stated amount, across the whole range from the quietest to the loudest setting.
Masking noise
Narrowband, white and speech noise must have the right level and bandwidth to mask the non-test ear effectively.
Tone switching
When a tone is presented, its rise time, fall time and overshoot must be controlled so there is no audible click, and with the tone off nothing should be heard.
Crosstalk
Sound meant for one ear must not leak into the other earphone above the allowed level.
Speech
Speech audiometry channels must deliver speech at the calibrated level.
Tympanometers, ABR and OAE
Probe tone level, compliance and pressure for tympanometers; output level, distortion and linearity for ABR and OAE probes and headsets.
Testing against the standard
How the ACS checks each specification
Acoustic measurements are made with a measurement microphone in a coupler that matches the transducer: NBS-9A/IEC-303 or IEC-318 for headphones, IEC-126/HA-2 or IEC-711 for insert earphones, and IEC 60318-2 couplers for high frequency phones. Bone oscillators are measured on an artificial mastoid. Electrical tests such as frequency, rise and fall time, crosstalk and linearity are made directly through the ACS interface.
On 73 audiometer models the ACS controls the audiometer through its serial or USB port, sets each test point, measures it against the selected standard and adjusts it, so an exhaustive calibration takes under 10 minutes.
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