LESSON 113

Audiometer Calibration : Why and How

Name: Audiometer Calibration : Why and How
Uploaded: 2026-11-04T04:00:00+03:00
Duration: 11 min 19 s
Description: Audiometer calibration verifies that the sound produced by earphones, bone vibrators and loudspeakers agrees with standards such as ISO 389 and IEC 60645. In this lesson we look at the RETSPL concept, the role of the artificial ear, the coupler and the sound level meter, the properties that are measured, and the daily, periodic and exhaustive levels of calibration.

Audiometer calibration verifies that the sound produced by earphones, bone vibrators and loudspeakers agrees with standards such as ISO 389 and IEC 60645. In this lesson we look at the RETSPL concept, the role of the artificial ear, the coupler and the sound level meter, the properties that are measured, and the daily, periodic and exhaustive levels of calibration.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we will learn why audiometer calibration is needed, which standards it is based on, and how it is done.
We will talk about the role of the artificial ear, the coupler and the sound level meter, and the three levels of calibration.
If the audiometer display says forty dB HL, the earphone must really produce that level.
Calibration means measuring how well the sound produced by the device agrees with standard values, and correcting it if needed.
Earphones can be dropped, cables can wear, and electronic parts can change over time.
If these small changes go unnoticed, every patient's thresholds will be systematically wrong.
Being able to compare the same patient's results across clinics or over the years also depends on calibration.
This is why calibration is part of both clinical accuracy and professional responsibility.
Calibration is done against values defined in international standards.
The ISO 389 series gives the reference values that correspond to zero dB HL for each earphone and frequency.
IEC 60645-1 defines the technical properties of the audiometer, such as frequency, level and distortion.
In the United States, the same topics are covered by the ANSI S3.6 standard.
The IEC 60318 series defines the ear simulators and couplers used in measurement.
ISO 8253-1 covers the test method and how often the equipment should be checked.
Let us recall from earlier lessons: zero dB HL is the average threshold of young adults with normal hearing.
The sound pressure level matching this threshold at each frequency is the reference equivalent threshold sound pressure level, or RETSPL.
For example, with the TDH-39 earphone at one thousand hertz, zero dB HL corresponds to about seven dB SPL.
For the bone vibrator, force is measured instead of sound pressure; this is the reference equivalent threshold force level, or RETFL.
The aim of calibration is to verify that the audiometer adds the correct amount above these reference values at every setting.
Because the reference values depend on the transducer and the measurement setup, the table for the earphone in use must be chosen.
To measure the sound an earphone produces, a device that takes the place of the human ear is used.
An artificial ear is a measurement cavity that mimics the volume and acoustic properties of the ear covered by the earphone.
Simpler, hard-walled measurement cavities of a set volume are called couplers.
A common example for supra-aural earphones is the six milliliter coupler.
For insert earphones, a two milliliter coupler or an occluded ear simulator is used.
The bone vibrator is measured on an artificial mastoid, which mimics the human head.
A sensitive microphone inside the coupler passes the sound to the sound level meter, and the level is read in dB SPL.
A sound level meter is a measuring instrument that converts the signal from a microphone into a sound pressure level.
Calibration usually uses a class one meter, the highest accuracy class in the IEC 61672 standard.
Octave or third-octave filters are needed to measure noise signals and ambient noise.
Before measuring, the sound level meter itself is checked with an acoustic calibrator that produces sound at a known level.
For example, an acoustic calibrator produces a fixed reference sound such as ninety four dB SPL at one thousand hertz.
The calibrator, the microphone and the sound level meter are also verified regularly by an accredited laboratory.
In an exhaustive calibration, output level is first measured at every frequency and for every transducer.
Standards usually allow a tolerance of plus or minus three decibels for this level, and somewhat wider at high frequencies.
Frequency accuracy is checked; a tone labeled one thousand hertz must really be close to one thousand hertz.
The purity of the tone, that is, low harmonic distortion, is verified.
The attenuator, or level control, is checked for linearity: each five decibel step must really be five decibels.
The bandwidth and level of the masking noises are also measured.
The rise and fall times of the tone and the timing of the pulsed tone are also compared with the standard.
Calibration is not a single procedure but a process thought of at three levels.
The first level is the listening and visual check done every day before testing; we will see it in detail in the next lesson.
The second level is periodic acoustic checks; output levels and basic properties are checked with measuring instruments.
These interim checks can be done several times a year, depending on how heavily the device is used.
The third level is exhaustive calibration, in which all technical properties are compared with the standard.
In many clinics, exhaustive calibration is done at least once a year and after every repair.
ISO 8253-1 also defines the checks as stages A, B and C; the recommended intervals differ between standards.
The output of the bone vibrator is measured on an artificial mastoid.
Here, it is not sound pressure but the vibratory force applied by the vibrator that is measured; the reference is the RETFL values.
The force of the headband pressing the vibrator to the head also matters; standards define a specific static force.
In sound field calibration, the measuring microphone is placed where the patient's head will be.
This measurement is made with no patient in the room; reference values are given in standards such as ISO 389-7.
In loudspeaker testing, warble tones or narrowband noise are used to avoid standing waves in the room.
Let us see the effect of a calibration error with an example.
The blue line shows the true thresholds of a person with normal hearing.
On this device, the earphone output at four thousand hertz has dropped by ten decibels; the threshold is measured ten decibels worse.
The result is a false four thousand hertz notch that suggests noise-induced hearing loss.
The same error repeats in every patient tested with that device and is hard to notice.
Daily checks and regular calibration with measurements catch errors like this early.
Exhaustive calibration is done by a qualified technical service or laboratory with the necessary measuring equipment.
The measuring instruments themselves must also be linked to a traceable calibration chain.
The role of the audiologist and audiometrist is to do the daily checks and notice anything unusual.
Being able to read a calibration report and see that values are within tolerance is also part of this responsibility.
With a doubtful finding, the device is taken out of use and the service is called.
A practical tip: calibration is done for the audiometer together with its own earphones and bone vibrator.
Swapping earphones with those of another device, or fitting a new earphone, can make the calibration invalid.
After such a change, do not start testing patients before a measured check.
Checking the serial numbers of the earphones and the vibrator on the calibration certificate is a good habit.
The result of every calibration is recorded in a written document.
The document includes the date, the person or company who measured, the instruments used, and their calibration dates.
The measured values, the target values from the standard and any corrections made are written in a table.
Attaching a label to the device that shows the last calibration date is a common practice.
No measurements should be made with a device whose calibration has expired.
These records protect the clinic during quality audits and in case of any inquiry.
Calibration verifies the sound produced by the audiometer against standards such as ISO 389 and IEC 60645.
Earphone output is measured in dB SPL with a sound level meter, inside an artificial ear or a coupler.
Output level, frequency accuracy, distortion and attenuator linearity are checked.
Daily listening checks, periodic measurements and, usually, a yearly exhaustive calibration are used together.
Test yourself: In calibration, what is the output level of a supra-aural earphone measured on?
The correct answer is B: an artificial ear or a coupler.
The artificial mastoid is for the bone vibrator; the earphone is placed on a measurement cavity that mimics the ear.
In the next lesson, we will go step by step through the daily biological check, done in five minutes every morning.
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Audiometer Calibration : Why and How

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