Calculating Comb Filter Frequencies
The frequencies of the reinforcements and cancellations depend on the delay time (the time
difference between the arrival time of the original signal and the delayed signal). The
frequency of the first cancellation occurs at 1/(2 x t)Hz, where t = the delay time in seconds.
The cancellations are separated by (1/ t)Hz. Fig. 3 shows how the comb filters change with
the delay time.
Comb Filter Amplitude
If the original signal and the delayed signal are the same amplitude, the reinforced frequen-
cies increase in amplitude by 6dB, while the out-of-phase frequencies cancel completely to
-4 dB.
Comb filters cause a lot of problems. The frequencies that are reinforced are prone to excite
feedback, while the out-of-phase cancellations make the program sound thin and over
equalized.
Try this simple experiment to hear what comb filters do to your sound.
5
Fig. 2:
COMB FILTERS. Input
signal mixed with a 2 msec.
delayed signal. (Delayed
signal has 10dB less
amplitude. Max. filter gain is
+2.5dB, and max. depth is
-3.) Reducing the amplitude
of the delayed signal reduces
the comb filters' effect.
Fig. 4:
Comb filters
noticeably affect
your sound.
Fig. 3:
Comb filters get
closer as delay
time increases.
