Peters R W, Moore B C, Glasberg B R, Stone M A
School of Medicine, and Department of Psychology, The University of North Carolina at Chapel Hill, USA.
Br J Audiol. 2000 Feb;34(1):21-36. doi: 10.3109/03005364000000115.
This paper describes a laboratory-based comparison of the effectiveness of two formulae for fitting linear hearing aids, the NAL(R) formula and the Cambridge formula. The formulae prescribe the desired insertion gain as a function of frequency, based on the audiometric threshold. The two formulae have 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-frequency 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 using an experimental digital hearing aid whose frequency-gain characteristic could be controlled very precisely. A loudness model (Moore and Glasberg, 1997) 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 give the same loudness as for a normally hearing person (according to the model). The adjustments were, on average, smaller for the Cambridge than for the NAL(R) formula. A condition was also used with all insertion gains set to zero, simulating unaided listening. Evaluation was based on: (1) subjective ratings of the loudness, intelligibility and quality of continuous discourse 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 noise for two noise levels (65 and 75 dB SPL) and four types of background noise. Neither the subjective ratings nor the measures of the SRTs revealed any consistent difference between the results obtained using the two formulae, although both formulae led to lower (better) SRTs than for simulated unaided listening. It is concluded that the differences between the NAL(R) formula and the Cambridge formula are too small to have measurable effects, at least in a laboratory setting.
本文描述了在实验室环境下对两种用于适配线性助听器的公式——NAL(R)公式和剑桥公式——有效性的比较。这两种公式根据听力阈值规定了所需的插入增益作为频率的函数。这两种公式的基本原理相似；二者均基于这样的目标，即对于中等强度的语音，在对语音重要的频率范围（400 - 5000赫兹）内，所有频带应具有同等响度（每个临界频带响度相等），并且总体响度应令人舒适。然而，这两种公式存在差异；一般来说，与NAL(R)公式相比，剑桥公式在高频（高于2千赫）产生的增益略多，在中频（500赫兹至2千赫之间）产生的增益略少。使用一种实验性数字助听器实现了这两种公式，该助听器的频率 - 增益特性能够被非常精确地控制。采用一种响度模型（Moore和Glasberg，1997）为每个受试者和每种公式调整总体增益，以便总体声压级为65分贝声压级的言语形状噪声能产生与正常听力者相同的响度（根据该模型）。平均而言，剑桥公式的调整幅度小于NAL(R)公式。还使用了一种所有插入增益都设为零的情况，模拟未佩戴助听器的聆听。评估基于：(1) 对在安静环境中以45、55、65和75分贝声压级呈现的连续话语以及在0分贝语音 - 噪声比的嘈杂环境中，使用55、65和75分贝声压级的语音进行响度、可懂度和质量的主观评分；(2) 针对两种噪声水平（65和75分贝声压级）以及四种类型的背景噪声，测量背景噪声中的言语接受阈值（SRT）。尽管两种公式得出的言语接受阈值均低于模拟未佩戴助听器聆听时的结果，但无论是主观评分还是言语接受阈值的测量结果，均未显示出使用这两种公式所获结果之间存在任何一致的差异。得出的结论是，NAL(R)公式和剑桥公式之间的差异过小，以至于至少在实验室环境中没有可测量的影响。
