The plot of AM detection thresholds as a function of modulation rate is called the temporal modulation transfer function. Threshold is plotted as modulation depth in dB with low (good) values at the top. The carrier in this case was a broadband noise. Each curve in the graph represents the data of one listener.
	Notice that for modulation rates below 50-60 Hz, thresholds are about -23 to -26 dB, about 5% modulation depth. Above 50-60 Hz thresholds get worse. At 500 Hz modulation rate, something like 50% modulation is needed before we hear it.
	So the auditory system acts like a low-pass filter when it comes to temporal fluctuations in sounds-- with a cutoff frequency (3-dB down point) around 50-60 Hz. We can follow fluctuations up that modulation rate fine, but at higher rates we donŐt follow as well.