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Differences in the performance of metal-and plastic-cased TDH-39 and TDH-49 audiometric earphones, and consequences for their calibration.

Sherwood, T R; McNeill, H A; Torr, G R (1995) Differences in the performance of metal-and plastic-cased TDH-39 and TDH-49 audiometric earphones, and consequences for their calibration. NPL Report. RSA(EXT)0058

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Abstract

Since the Telephonics type TDH-39 gained its position as one of the main audiometric earphones in use in Britain, there have been several superficial changes to its design, the most significant of which was the change from a metal to a plastic cased version, the TDH-39P. It was assumed at the time that the new plastic cased version was equivalent to the original metal cased version as both types exhibited similar frequency responses when measured using the IEC 303 acoustic reference coupler.
Canning (1991) conducted an investigation into the two types of TDH-39 using data compiled from his own audiometer calibrations. Transfer sound pressure level (SPL) values, ie. the difference between the sound pressure levels generated by an earphone when measured using the IEC 303 acoustic reference coupler and the IEC 318 artificial ear, were calculated for both earphone types, and he concluded that the two types of earphone exhibited significant differences at 6 kHz.
The object of the work reported here was to investigate further the metal and plastic cased TDH-39 and quantify any differences between them. On examining various earphones it was discovered that the original metal cased TDH-49 had also been superseded by a plastic cased version, the TDH-49P, and these earphone types were also included in the investigation. Five samples of each earphone type were measured on two IEC 303 acoustic reference couplers and two IEC 318 artificial ears. The transfer SPLs were used to assess the differences between the earphone types.
Our results for the type TDH-39 and TDH-39P earphones verify Canning's results, with a difference in transfer values of the order of 5 dB at 6 kHz. The TDH-49 and TDH-49P earphones do not exhibit such large differences in transfer values, the difference being of the order of 3 dB at 6 kHz. Differences of the order of 2 dB were observed over the range 3 - 4 kHz for both TDH-39/39P and TDH-49/49P earphones. It is concluded that a move away from the use of TDH39/39P earphones for audiometry would be beneficial both to avoid this problem of differing performance and to facilitate the adoption of the IEC 318 artificial ear as the sole calibration device for supra-aural audiometric earphones.

Item Type: Report/Guide (NPL Report)
NPL Report No.: RSA(EXT)0058
Subjects: Acoustics
Acoustics > Sound in Air
Last Modified: 02 Feb 2018 13:19
URI: http://eprintspublications.npl.co.uk/id/eprint/422

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