Audiometer

An audiometer is a machine used for evaluating hearing loss. Audiometers are standard equipment at ENT (ear, nose, throat) clinics and in audiology centers. They usually consist of an embedded hardware unit connected to a pair of headphones and a test subject feedback button, sometimes controlled by a standard PC. Such systems can also be used with bone vibrators, to test conductive hearing mechanisms. Audiometer requirements and the test procedure are specified in IEC 60645, ISO 8253, and ANSI S3.6 standards. An alternative to hardware audiometers are software audiometers, which are available in many different configurations. Screening PC-based audiometers use a standard computer and can be run by anybody in their home to test their hearing, although their accuracy is not as high due to lack of a standard for calibration. Some of these audiometers are even available on a handheld Windows driven device. Clinical PC-based audiometers are generally more expensive than software audiometers, but are much more accurate and efficient. They are most commonly used in hospitals, audiology centers and research communities. These audiometers are also used to conduct Industrial Audiometric Testing. Because these audiometers can typically be calibrated to an accuracy of fractions of a decibel, calibration is more accurate than hardware audiometers. Some audiometers even provide a software developer's kit that provides researchers with the capability to create their own diagnostic tests.

Calibration of Audiometers

Any audiometer that is used to test hearing should be calibrated regularly to ensure that the level shown on its display is equal to the actual stimulus the subject is exposed to. Accurate and reliable measurements are the first stage in characterising and quantifying hearing loss; furthermore, proper calibration ensures that the measurements are consistent, regardless of the clinic or indeed the district or nation where the measurements are carried out. Audiometers are calibrated using an Ear Simulator System (or ‘Audiometric Calibration System’). For testing air-conduction hearing mechanisms, the systems include an ear simulator or acoustic coupler, conforming to either IEC 60318-1 (a.k.a. artificial ear) or IEC 60318-3 (reference coupler) respectively. These devices essentially consist of a calibrated microphone with an associated coupling volume, which is open on one side to allow application of headphones when testing. For testing bone-conduction hearing mechanisms, the ‘system’ will include a mechanical coupler (a.k.a. artificial mastoid) conforming to IEC 60318-6. This device uses a series of rubber layers to couple the bone-vibrator (which in testing delivers the stimulus to the patient) to a force transducer. This enables the device to mimic the way in which sound is transferred through the mastoid part of the temporal bone, to which the bone-vibrators are applied when testing.

Artificial Ear / Reference Coupler (Air-Conduction Ear Simulator Systems)

Both the ‘artificial ear’ and ‘acoustic reference coupler’ are designed for the purpose of calibrating (air conduction) audiometer outputs. When calibrating, the audiometer headphones are applied to the open side of the coupler, and the internal (calibrated) microphone is used to detect the resulting sound pressure level in the cavity. The electrical output from the microphone is usually measured with a sound level meter, and gives an indication of the sound pressure level that a real listener’s eardrum would be exposed to by the same signal and headphones when used in hearing tests. It is the complete combination of artificial ear, sound level meter and associated leads etc. that is referred to as the ‘Audiometric Calibration System’.

The microphone and associated coupling volume comes in two distinct types: the IEC Acoustic Reference Coupler (specified in IEC 60318-3) is simply a cylindrical coupling volume (volume depending on application) with a 1-inch microphone at the far end. The second, the IEC Artificial Ear (specified in IEC 60318-1) is designed to be used with a ½-inch microphone, and has various extra cavities and portholes adjoining the main coupling volume between the headphone and microphone. These serve to alter the acoustical transfer impedance (with varying frequency) in the main cavity in a way that mimics that of a human outer ear. Though both devices are still relevant, and comply with ISO 389-1 (for the calibration of supra-aural headphones), the more sophisticated Artificial Ear provides a better indication of the actual sound pressure level at the average patient’s eardrum.

Calibration Of Air-Conduction Ear Simulator Systems

The calibration of audiometers using an ear simulator is a form of secondary calibration; the ‘audiometric calibration system’ (ear simulator system) is used to validate / calibrate the output from an Audiometer. Bear in mind, however, that the sensitivity of the testing system itself is liable to drift with time (due to factors such as instability in the sound level meter, microphone drift) and hence also requires regular calibration. The calibration of ear simulator systems is often carried out by specialist test and calibration laboratories, or a National Measurement Institute, such as NPL in the UK.

Calibration is by comparison; in the case of air-conduction systems this means comparison of the ‘system’ output against known levels as measured by a calibrated reference microphone. Note that this includes specifying a method for setup of the sound level meter, as well as testing the sensitivity of the microphone and associated connectors and pre-amplifier. Once the ear simulator system has been calibrated, it can be used to calibrate audiometers ‘in the field’.

A similar calibration procedure to the above takes place for bone conduction audiometry, using a device called an artificial mastoid. As per air-conduction ear simulator systems, the system can be used to test the force hearing level delivered by an audiometer at a particular frequency, but the artificial mastoids must themselves be regularly calibrated device.

Artificial Mastoid / Mechanical Coupler (Bone-Conduction Ear Simulator Systems)

An artificial mastoid (or IEC Mechanical Coupler, as specified in IEC 60318-6) is an acoustic device that is designed to present a frequency dependent mechanical impedance approximating that of the mastoid part of the temporal bone. They are primarily used to calibrate the bone-conduction channel of an audiometer, which themselves are used to test patient’s hearing in response to bone-conduction mechanisms by presenting known force levels at specified frequencies.

The device consists of a ‘dome’ that represents a mechanical simulation of the human mastoid, in terms of its frequency dependent mechanical impedance, and has a force transducer to measure the force level delivered when bone-vibrators are applied. The electrical output from the force transducer is usually measured with a sound level meter, and gives an indication of the force level, (or force hearing level when referred to the RETFL, ‘Reference Equivalent Threshold Force Level’) that a real listener would be exposed to by the same signal and bone vibrator when used in hearing tests. It is the complete combination of artificial mastoid, sound level meter and associated leads etc. that is referred to as the ‘Audiometric Calibration System’.

Calibration of an Artificial Mastoid

Whilst the artificial mastoid, as part of an audiometric calibration system, is used to calibrate the bone vibrator force output of audiometers, the sensitivity of the testing system and the mechanical impedance of the ‘dome’ is prone to drift and hence also requires regular calibration. The calibration of artificial mastoid systems is usually carried out by specialist test and calibration laboratories, or a National Measurement Institute, such as NPL in the UK.

A bone-conduction audiometric calibration system must be calibrated in two specific ways: firstly to ensure that the mechanical impedance of the ‘dome’ is within the tolerance specified in IEC 60318-6, and secondly to ensure that the reading on the sound level meter is representative of the force hearing level which would be delivered to the patient. Calibrating the system includes specifying a method for setup of the sound level meter, as well as testing the sensitivity of the force transducer and associated connectors and pre-amplifier. Once the ear simulator system has been calibrated, it can be used to calibrate audiometers ‘in the field’. It is also worth noting that the artificial mastoid is very sensitive to temperature; special precautions such as temperature isolated enclosures are usually necessary when transporting the device in hot or cold conditions.

A similar calibration procedure to the above is used for air conduction audiometry, using either an Artificial Ear or Acoustic Reference Coupler. As per bone-conduction ear simulator systems, the system can be used to test the sound pressure level delivered to the patient by an audiometer at a particular frequency, but the system itself requires regular calibration.

References

External links

• National Physical Laboratory Guidelines on Audiometric Calibration
• MedlinePlus Encyclopedia 003341
• Department of Communication Disorders at BYU - Audiology department at BYU
• DK-Technologies A/S - Audio meters for studio and broadcast use. (Commercial)
• NS02 audiometer - USNewSound Hearing
• Online Audiometric Test - Warble tones, 250 to 8000 Hz, -5 to 80 dBHL