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
4.1 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 0 kHz, the overall sound pressure sensitivity level of the microphone and associated measuring system shall be known with an uncertainty not exceeding 0,3 dB for a level of confidence of 95 %. For measurements above 1 0 kHz, the overall pressure sensitivity level of the microphone and associated measuring system over the specified frequency range shall be known with an uncertainty not exceeding 0,5 dB for a level of confidence of 95 %. The make and model of the microphone shall be specified by the manufacturer of the occluded-ear simulator. NOTE The acoustic impedance of the microphone affects the overall acoustic impedance of the occluded-ear simulator.
4.4 Pressure equalization A vent shall be provided to equalize the static pressure in the cavity of the occluded-ear simulator. The vent shall have an acoustic resistance of (7,0 ± 5,5) GPa⋅s⋅m –3 .
4.5 Acoustic transfer impedance level The level of the acoustic transfer impedance modulus of the occluded-ear simulator and the associated tolerances shall be as specified in Table 1 .
NOTE 1 At 500 Hz, the specified transfer impedance level corresponds to the magnitude of the acoustic transfer impedance 35,9 MPa⋅s⋅m –3 and also to the magnitude of the effective volume 1 260 mm 3 of the ear simulator. NOTE 2 The tolerances have minimum values at the frequency 500 Hz, where the influence of leakage and wave motion is small.
4.6 Example of design An example of one specific design of occluded-ear simulator is shown in Annex A.
NOTE 1 Using the measurement method described in Annex B, it is not easy to measure the acoustical transfer impedance level below 1 00 Hz, due to the effects of an imperfectly sealed measurement configuration. However, the acoustical transfer impedance between 20 Hz and 1 00 Hz is governed predominantly by the volumetric elements of the occluded-ear simulator, and their contribution to the overall acoustical transfer impedance can be validated by the measurements at higher frequencies.