IEC 60318-3:1998 pdf download – Electroacoustics – Simulators of human head and ear -Part 3: Acoustic coupler for the calibration of supra-aural earphones used in audiometry

IEC 60318-3:1998 pdf download – Electroacoustics – Simulators of human head and ear -Part 3: Acoustic coupler for the calibration of supra-aural earphones used in audiometry.
IEC 60318-3 describes an acoustic coupler for loading supra-aural audiometric earphones as specified in Iso 389-1 If with a specified acoustic impedance. when calibrating audiometers, in the frequency range of 125 Hz to 8 000 Hz.
The Sound pressure developed by an earphone is not, in general, the same in the coupler as in a persons ear. However, the IEC recommends its use as a simple and ready means for the exchange of specifications on audiometers and for the calibration of specified earphones used in audiometry.
2 NormatIve references
The following normative documents contain provisions which, through reference In this text, constitute provisions of this part of IEC 60318. At the same time of publication, the editions Indicated were valid. All normative documents are Subject to revision, and parties to agreements based on this part of IEC 60318 are encouraged to investigate the possibility of applying the most recent edition of the normative documents indicated below. Members of the IEC and ISO maintain registers of currently valid International Standards
IEC 61094-4:1995, Measurement microphones — Part 4: Specifications for woi’*ing standard microphones
ISO 389-1, — Acoustics — Reference zero for the calibration 01 audiometric equipment — Part 1: Reference equivalent threshold sound pressure levels for pure tones and supra -aural earphones’
3 Definition
For the purpose of this International Standard, the following definition applies:
3.1
acoustic coupler
cavity, of predetermined shape and volume, used for the calibration of earphones in conjunction with a calibrated microphone adapted to measure the pressure developed within the cavity
4.3 Calibrated pressure microphone
The internal shape of the base of the coupler corresponds to that of certain commercial microphones wIhout protective grid but with a coupler ring, which can be used to form that base, eg. a WS1P microphone conforming to IEC 610944. Other microphones of same dimensions as types WSI can be used. provided that the dimensions and stipulated volume are preserved, for example by using an adaptor.
The microphone used shall have a high mechanical impedance, the equivalent volume being less than 200 mm3 of air, at frequencies between 125 Hz and 8 000 Hz. The manufacturer of the microphone shall state the equivalent volume.
The overall pressure sensitivity of the microphone and associated measuring system over the specified frequency range shall be known with an uncertainty of ±0,2 dB with a coverage factor of k • 2.
There shall be an effective seal between the coupler and the microphone achieved by using a suitable material, for example grease
Any obstruction in the static pressure equalization device of the microphone should be avoided.
4.4 StatIc pressure equalizer
Static pressure equalization shall be provided. To equalize the static air pressure Inside the coupler cavity with the external atmospheric pressure. a hole of 0.6 mm ± 0.05 mm diameter is drilled into the coupler. A wire of 0,4 mm t 0,05 mm diameter is inserted into the hole.
A glass capillary tube may also be used for pressure equalization, in wtich case care shall be taken to seal the tube along its complete length In such a way that air leakage occurs Only through the inner bore. The sound pressure difference, developed by a given voltage at the earphone terminals with the hole plugged or left open should not be greater than 0.2 dB at any frequency between too Hz and 10 000 Hz.