Comparison of the NAL(R) and Cambridge formulae for the fitting of linear hearing aids

This paper describes a laboratory-based comparison of the effectiveness of two formulae for fitting linear hearing aids, the NAL(R) formula and the Ca mbridge formula. The formulae prescribe the desired insertion gain as a fun ction of frequency, based on the audiometric threshold. The two formulae ha ve a similar rationale; both are based on the goal that, for speech with a moderate level, all frequency bands should be equally loud (equal loudness per critical band) over the frequency range important for speech (400-5000 Hz), and the overall loudness should be comfortable. However, the formulae differ; generally the Cambridge formula leads to slightly more high-frequen cy gain (above 2 kHz) and slightly less mid-frequency gain (between 500 Hz and 2000 Hz) than the NAL(R) formula. The two formulae were implemented usi ng an experimental digital hearing aid whose frequency-gain characteristic could be controlled very precisely. A loudness model (Moore and Glasberg, 1 997) was used to adjust the overall gains for each subject and each formula so that a speech-shaped noise with an overall level of 65 dB SPL would giv e the same loudness as for a normally hearing person (according to the mode l). The adjustments were, on average, smaller for the Cambridge than for th e NAL(R) formula. A condition was also used with all insertion gains set to zero, simulating unaided listening. Evaluation was based on: (1) subjectiv e ratings of the loudness, intelligibility and quality of continuous discou rse presented in quiet at levels of 45, 55, 65 and 75 dB SPL and in babble at an 0-dB speech-to-babble ratio, using speech levels of 55, 65 and 75 dB SPL; (2) measures of the speech reception threshold (SRT) in background noi se for two noise levels (65 and 75 dB SPL) and four types of background noi se. Neither the subjective ratings nor the measures of the SRTs revealed an y consistent difference between the results obtained using the two formulae , although both formulae led to lower (better) SRTs than for simulated unai ded listening. It is concluded that the differences between the NAL(R) form ula and the Cambridge formula are too small to have measurable effects, at least in a laboratory setting.