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Acoustics. Reference zero for the calibration of audiometric equipment - Reference equivalent threshold sound pressure levels for pure tones and circumaural earphones Ingestion-build-48024 Three main methods are described1 Scope This part of ISO 389 specifies reference equivalent threshold sound pressure levels (RETSPLs) for pure tones in the frequency range from 125 Hz to 8 kHz, applicable to the calibration of air conduction audiometers equipped with a particular model of circumaural earphones (SENNHEISER HDA 200). NOTE Some notes and references on the derivation and the test conditions used to determine the recommended reference levels are given in Annex A and the