
11
Auto Delay Application Background
Compared to light and electricity, sound travels along at a leisurely 1130 feet/second. This relatively slow speed
affects many aspects of sound system design and operation. Reverberation as we know it would not exist if sound
traveled at the speed of light. Many of the other challenges would be different, as well.
One such challenge occurs in many churches, auditoriums and venues where a talker is addressing a large group of
people either unaided or through the use of a central sound system. Audience members may have difficulty hearing
or understanding what is being said when seated in certain areas of the room. They may be seated in an area that the
main loudspeaker system does not cover well, such as under a balcony; they may be in a location that is physically
connected but acoustically isolated, such as an overflow seating area; or they simply may be too far away. Hearing can
be difficult in these areas because the sound level from the talker or main loudspeaker system is too low compared
to ambient noise or room reverberation. In most cases auxiliary loudspeakers are added locally to obtain sufficient
sound levels in these areas; however, without proper consideration for the slow speed of sound, intelligibility can still
be a problem. If the auxiliary loudspeaker system is sufficiently far from the main loudspeaker system, the auxiliary
system will be heard first followed by the main system, which is heard as an echo. Depending on the delay time and
volume, the echo can interfere with listening. Additionally, if the auxiliary loudspeaker is not between the listener
and the original sound source, the sound will appear to come from the wrong direction (the auxiliary loudspeaker)
and can distract the listener.
In the 1940s, Helmut Haas researched the disrupting effect delayed sound can have on the listener and how delay
affects sound localization. Using principles based on his research, and with the use of modern digital delay lines, the
echo and localization problems described above can be overcome and the integration of auxiliary loudspeakers made
virtually seamless.
Haas’s experiments involved listening tests where one talker’s voice was reproduced by two loudspeaker systems,
one using a magnetic audio-tape delay. He observed that if the sound arrives from the delayed loudspeaker between
1 and 30 ms after the original, the delayed loudspeaker is not heard at all, even if the volume from each loudspeaker
is the same. The sound appears to come only from the non-delayed loudspeaker. However, the perceived volume
will be louder due to the combined power of the loudspeakers. He further observed that if one loudspeaker is
delayed 5 to 30 ms, the delayed loudspeaker needed to be 10 dB greater in volume than the reference loudspeaker
for the listener to perceive the volume from the two loudspeakers as equal. As the delay time was further increased,
the volume difference must be decreased for the two loudspeakers to appear to be at the same level. Although
the sound quality changed somewhat with the delay, it was not considered disturbing, and actually made listening
less tiring and more natural. As the delay reached approximately 50 ms, it was possible to discriminate the delayed
loudspeaker as a separate echo.
Haas further observed that depending on the rate of speech, if the amplitude of the echo were equal to 10 dB greater
than the original sound, delays of 40 to 50 ms would disturb only a small percentage of listeners. If the echo signal
was reduced in amplitude to 10 dB below the original, no amount of delay disturbed the listeners.
Based on his research, auxiliary loudspeakers benefit from sufficient delay of their signal so that its sound arrives
at the listener 5 to 30 ms after the arrival of the original sound. Even as the amplitude of this local loudspeaker is
increased, the additional delay will help move the sound image toward the original source. As long as the difference
in arrival time within the coveragence area of the two loudspeaker systems is 45 ms or less, an echo will not be
perceived and listeners should not be disturbed.
Traditionally there are several standard approaches to achieving the alignment goal discussed above. The installer can
measure the distance from the main loudspeaker to the listener nearest the auxiliary loudspeaker system and the
distance from the auxiliary loudspeaker to that same listener. The needed delay could be calculated by the following:
(required delay distance) = (main to listener in feet) – (auxiliary to listener in feet) + 10 feet. To convert the distance
into the approximate delay time, divide (required delay distance in feet) by 1130. (Sound travels approximately
1130 feet per second.) The delay time can also be set by ear. Most installers now set delay time using audio test
equipment.
AUTO DELAY™