BS 60318-4:2010 pdf download – Electroacoustics — Simulators of human head and ear Part 4: Occluded-ear simulator for the measurement of earphones coupled to the ear by means of ear inserts 4 Construction 4.1 General The occluded-ear simulator shall be constructed of hard, dimensionally stable, non-porous and non-magnetic material. The general construction of the occluded-ear simulator and mounting of the microphone shall aim at minimizing the response of the microphone to vibration (for example from an earphone) or to extraneous sound outside the cavity. NOTE 1 The external diameter of the occluded-ear simulator should be kept as small as possible in order to minimize diffractional errors which might affect the measurements when the occluded-ear simulator has to be placed in a free sound field. NOTE 2 In order to avoid a possible sound attenuation caused by the dust protector (see Figure A.1 ), an earphone calibration should be performed above 1 0 kHz at least every two years with the occluded-ear simulator equipped with and without its dust protector in place. The two results should not differ by more than 0,2 dB at frequencies up to 1 6 kHz. If this is not the case at some frequencies, earphone measurements at those frequencies should generally be performed without the dust protector. The construction of the occluded-ear simulator shall permit the location of a transducer at the reference plane for calibrating the simulator. Where tolerances are specified in this part of IEC 6031 8, these shall be reduced by an amount equal to the actual expanded measurement uncertainty of the test laboratory before deciding if a device conforms to the stated requirement. 4.2 Principal cavity dimensions The diameter of the principal cavity shall be (7,50 ± 0,04) mm. The length of the principal cavity shall be such as to produce a half-wavelength resonance of the sound pressure at (1 3,5 ± 1 ,5) kHz. 4 .3 Calibrated pressure-type microphone A calibrated microphone is located at the base of principal cavity of the occluded-ear simulator. The acoustic impedance of the microphone diaphragm shall be high, so that the equivalent volume is less than 20 mm 3 over the specified range of frequencies. The microphone shall conform to the requirements of IEC 61 094-4 for a type WS2P microphone. The microphone shall be coupled to the principal cavity with a seal that prevents acoustic leaks. In the frequency range 20 Hz to 1...
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