To develop a new version of the threshold-equalizing-noise (TEN) test for the diagnosis of dead regions, with levels calibrated in dB HL rather than dB SPL, and with levels corresponding to the dial readings on the audiometer.
The spectral shape of the noise required to give equal masked thresholds at all frequencies, when expressed in dB HL, was derived by two calculation methods and by empirical measurements of the electrical output of audiometers using TDH50 earphones and TDH39 earphones. To reduce the loudness of the noise and to minimize distortion generated in the audiometer and/or earphone, the noise was bandlimited between 354 and 6500 Hz. In addition, the noise was synthesized using a method that leads to a low crest factor (ratio of peak to root-mean-square value). This further reduced audiometer/earphone distortion and allowed higher levels per ERBN; ERBN is the equivalent rectangular bandwidth of the auditory filter at 1 kHz, as determined in young, normally hearing subjects. The test tone frequencies were limited to the range 500 to 4000 Hz. Subjects with normal or near-normal hearing were tested by using a noise level of 60 dB HL/ERBN to assess whether the noise did lead to equal masked thresholds in dB HL for all audiometric frequencies from 500 to 4000 Hz. Thresholds in the TEN were measured by means of manual audiometry with a 2 dB final step size.
The mean masked thresholds were almost constant across frequency when expressed in dB HL and were within 0.5 dB of the noise level per ERBN. For a single noise level, the test takes approximately 5 minutes per ear to administer.
The new TEN test has the following advantages over the original version (which used levels calibrated in dB SPL): (1) All levels are expressed in dB HL. Thus, absolute thresholds only need to be measured once. (2) Calibration is such that both the noise level/ERBN and the test tone levels correspond to the values indicated on the audiometer. This makes the test simpler to apply and reduces the likelihood of errors. (3) The noise bandwidth is restricted, and the noise has a low crest factor. This allows the noise level/ERBN to be increased while avoiding distortion, excessive loudness, and possible further damage to hearing.