Instrument Specifications for Aural Acoustic Immittance

The specification for Aural Acoustic Immittance is described in ANSI S3.39 – 1987 and revisited and simplified in this article below.  According to ANSI S3.39 – 1987,

“The purpose of this standard is to ensure that acoustic-impedance or acoustic-admittance measurements will be substantially the same for a given ear when these measurements are obtained with any instruments that meet the specifications and tolerances outline in this standard, and when comparable test conditions prevail.  This standard is not intended to inhibit development, to restrict incorporation of new features (such as probe frequencies other than 226 Hz), or to discourage other improvements that may become useful in the measurement of aural acoustic impedance or acoustic admittance.”

Acoustic immittance machines are categorized into specific types (1-4) according to its features and capabilities, similar to how audiometers are categorized.  Those devices that offer the greatest number of features and capabilities are classified as Type 1 and the most basic devices are categorized as Type 4 (see chart).  A type 1 acoustic immittance device generally is more precise than a type 2 or 3.  There are however, mandatory characteristics and features acoustic immittance devices must have in order to meet ANSI S3.39 standards.  

Ambient temperature and barometric pressure are important variables which affect the measurement of acoustic immittance.  Since these variables are likely to change on a periodic basis due to environmental conditions, it is important that Audiologist's calibrate their machines for changes in these conditions, on a daily basis prior to use.  According to ANSI S3.39 -1987, the manufacturer will provide the hard walled test cavities and data necessary for this calibration.  There should be at a minimum, three calibration cavities provided with Type 1-3 devices with volumes of 0.5, 2.0 and 5.0 cm³.    

Usually the daily calibration procedure simply requires the Audiologist insert the probe end of the immittance device into the correct sized test cavity and press a calibrate button, however each manufacturer may adapt different methods of calibration.  It is up to the manufacturer to provide the recommended procedure for using the calibration cavities.  In addition, each manufacturer must also specify the temperature and barometric pressure range in which the immittance device will be able to maintain optimum performance.  ANSI S3.39-1987 requires that all devices be able to operate within a range of + 15 ^oC to + 35 ^oC, over a relative-humidity range of 10% - 90%.          

Type 1, 2 and 3 devices must be able to change and indicate the air pressure in the external auditory meatus and in the manufacturer calibration cavities.  The maximum amount of pressure permissible by any immittance device is  - 800 daPa and + 600 daPa.  The minimum pressure requirements is dependent on the type of device.  A type 1 and 2 immittance device is minimally required to operate in the range of – 600 daPa to + 200 daPa.  A type 3 devices minimum range is – 300 daPa to +100 daPa. 

It is the manufacturers responsibility to indicate the accuracy and limitations of their device, as well as the limitations with regards to the atmospheric pressure and altitude above sea level in which the device will operate.  ANSI S3.37 – 1987 indicates that type 1 and 2 devices must meet an accuracy of +/- 10 daPa or +/- 10% of the reading on the display (which ever one is greater).  Type 3 and 4 devices must be accurate to +/- 10 daPa or +/-15% (which ever is greater).  

All immittance devices regardless of type, must have a 226 Hz probe signal with an accuracy within +/- 3% of that signal with no more than 5% total harmonic distortion.  Other signals may be present within a device, but only 226 Hz is required for compliance with standard.

The total harmonic distortion level allowable for frequencies other than the 226 Hz probe tone differs depending on the transducer being used.  A  chart summarizing the maximum total harmonic distortion for pure-tone acoustic reflex activating signals can be found by clicking here.

For signals that fall at higher level settings, the max total harmonic distortion is not to exceed 5% for supra aural earphones or 10% or insert style earphones.  The fixed frequency reflex activating stimuli on any immittance device should be any number of the following:  250, 500, 1000, 1500, 2000, 3000, 4000, 6000, or 8000 Hz.  Type 1 devices must have the following; 500, 1000, 2000, and 4000 Hz for both contralateral and ipsilateral acoustic reflex measurements.    

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