| 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 donÕt
follow as well. |