Stack two identical full-range loudspeakers as shown in Fig. 12. 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.
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.
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. Increasing the loudness more than this, or increasing the delay somewhat more than 35
milliseconds, makes the delayed signal sound like an echo.
The phenomenon describing how the acoustic image follows the signal we hear first is called the
Precedence Effect. The phenomenon that makes two distinct sounds heard less than 35 msec.
apart seem like only one sound is call the Haas Effect. However, the terms are often used
interchangeably in the sound industry.
THREE APPLICATIONS FOR DIGITAL DELAYS
APPLICATION I: Under-The-Balcony Speakers
Section 8: Digital Delay
17
Fig. 12: Overhead view of
under-balcony application.
