SABINE Graphi-Q2 3122-S User Manual
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9.4.2.1. 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/(2t) Hz, where t = the delay time in seconds. The cancellations are separated
by (1/ t) Hz. Delay Fig. 3 shows how the comb filters change with the delay time.
9.4.2.2. Comb Filter Amplitude
If the original signal and the delayed signal are the same amplitude, the reinforced frequencies increase
in amplitude by 6 dB, 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:
Stack two identical full-range loudspeakers as shown in Delay Figure 4. Carefully align the HF horns
and wire the speakers in mono. Stand in front while listening to your favorite full-spectrum CD. Ask a
friend to move the top speaker slowly away from you. The degradation in sound quality you hear is
caused by comb filters. The experiment is most dramatic when you use good quality speakers.
9.4.2.3. Correcting Comb Filters
Comb filters are inevitable to some degree in every live sound system, and they cannot be
corrected with equalization. Fortunately, most comb filter problems can be reduced to a minimum
by synchronizing the signals and reducing the amplitude of the delayed signal. The examples below
show several practical applications.
9.4.3. The Precedence Effect: Aligning the Acoustic Image
Helmut Haas published a study in 1951 describing a series of experiments that demonstrated how people
perceive delayed signals and echoes. In his experiments, a listener was positioned between two speakers
placed 3 meters away; one was placed 45 degrees to the right and the other was placed 45 degrees to the
left. When the same program was played through both speakers simultaneously, the listener perceived
the acoustic image (the direction from which the sound seemed to be coming) centered between the
speakers.
When Haas delayed the signal going to one of the speakers by somewhere between 5 to 35
milliseconds, the listener perceived a shift in the acoustic image to the speaker heard first. While the
delayed speaker did not contribute to the apparent direction of the sound, it did make the program seem
louder and “fuller.”
Haas showed that you must increase the loudness of the delayed signal by about 8 to 10 dB (twice the
perceived loudness) in order for the acoustic image to move back to the original center position.
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Delay Figure 3: Comb
filters get closer as delay
time increases.
Delay Figure 4: Comb
filters noticeably affect
your sound.
Clean Sound
Comb Filter
Sound