"&
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.
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.
9.4.4. Three Applications for Digital Delays
9.4.4.1. Application I: Under-The-Balcony Speakers
Delay Figure 5: Overhead view of
under-balcony application.
Section Nine: Suggestions for Optimal Use of the GRAPHI-Q
