Schorer E
J Acoust Soc Am. 1986 Apr;79(4):1054-7. doi: 10.1121/1.393377.
The ratios between the modulation index (eta) for just noticeable FM of a sinusoidally modulated pure tone and the degree of modulation (m) for just noticeable AM at the same carrier and the same modulation frequency were measured at carrier frequencies of 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz. Signal levels were 20 dB SL and 50 dB SPL or 80 dB SPL. At low modulation frequencies, for example, 8 Hz, AM and FM elicit very different auditory sensations (i.e., a fluctuation in loudness or pitch, respectively). In this case, eta and m show different values for just noticeable modulation. Since both stimuli have almost equal amplitude spectra if eta equals m (m less than 0.3), the difference in detection thresholds reflects differences in the phase relation between carrier and sidebands in AM and FM. With increasing modulation frequency, the eta-m ratio decreases and reaches unity at a modulation frequency called the "critical modulation frequency" (CMF). At modulation frequencies above the CMF, the same modulation thresholds are obtained for AM and FM. Therefore, it can be concluded that the difference in phase between the two types of stimuli is not perceived in this range. At center frequencies below 1 kHz, where phase errors caused by headphones and ear canal presumably are small, the CMF is useful in estimating critical bandwidth.
在0.125、0.25、0.5、1、2、4和8千赫兹的载波频率下，测量了正弦调制纯音的刚可察觉调频的调制指数（η）与相同载波和相同调制频率下刚可察觉调幅的调制程度（m）之间的比率。信号电平为20分贝感觉级以及50分贝声压级或80分贝声压级。在低调制频率下，例如8赫兹，调幅和调频会引发非常不同的听觉感受（即分别是响度或音高的波动）。在这种情况下，对于刚可察觉的调制，η和m显示出不同的值。由于如果η等于m（m小于0.3），两种刺激的幅度谱几乎相等，检测阈值的差异反映了调幅和调频中载波与边带之间相位关系的差异。随着调制频率的增加，η - m比率降低，并在一个称为“临界调制频率”（CMF）的调制频率处达到 unity。在高于CMF的调制频率下，调幅和调频获得相同的调制阈值。因此，可以得出结论，在该范围内没有察觉到两种类型刺激之间的相位差异。在中心频率低于1千赫兹时，耳机和耳道引起的相位误差可能较小，CMF可用于估计临界带宽。
