Audiometric Earphone Calibration using an Artificial Ear/Reference Coupler 

When calibrating a TDH style earphone, the earphones must be detached from the headband portion and the audiocups must remain on the earphone.  

The earphone is placed on the NBS 9A coupler and a 500 g weight is placed on top of it.  The output is read in voltage and then transformed to dB (or read directly in dB SPL) re 20 mPa.  After the earphone is placed on the coupler, a low frequency tone (125 or 250 Hz) is introduced and the earphone is readjusted on the coupler until the highest output intensity is read.  This helps assure best placement on the coupler.  The output from the earphone may then be compared to the expected values at each frequency.  The standard SPL values that are used are given in ISO recommendation R 389 (1981) and ANSI S3.6 1989.  

In the diagram above, the NBS 9A coupler is attached to the sound level meter.  The 500 gram weight is located at the lower left of the coupler.  Although the diagram above depicts a TDH style earphone without the audiocups attached, the calibration requires that the audiocups be attached to the earphone when applied on the coupler.  The earphones are of course detachable from the headband for this procedure.  

The ANSI standard now gives reference equivalent threshold sound pressue levels for the Telephonics TDH-39, TDH-49, and TDH 50 earphones as measured using the NBS 9A and IEC 318 coupler.  The values below are taken from ANSI S3.6 1996.  

ANSI allows a tolerance of +/- 3 dB from 500 to 4000 Hz and +/- 5 dB for other frequencies.

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