Digital Delays

What were you hearing? Is there a way the sound system could be changed to improve the previous situations? As you probably guessed, there are simple reasons for the bad sound you heard. Fortunately, it is now possible to correct the poor sound.

Sound Travel Takes Time

Sound travels quite slowly. In fact, sound travels much slower than light. This phenomenon is noticeable during a thunder storm. First, you see the lightning flash, then the thunder is heard later.

The speed of sound is about 1130 feet per second. Or, it takes .899 milliseconds for sound to travel one foot. (One millisecond is 1/1000 of a second.)

This relatively slow speed is the cause for the confusing sound under the balcony.

Figure 1

Let's look at Figure 1. The sound comes from the loudspeaker in the front and travels to the back of the auditorium. In our example, the distance is 73 feet, so it will take 65 milliseconds for the sound to arrive at the back of the room. Our example also has an under balcony loudspeaker which is only six feet from the listener's ears. Since the electronic signal from the amplifier arrives at the front loudspeaker and the under balcony loudspeaker at virtually the same time, the listener will hear the under balcony loudspeaker first.

The sound from the under balcony loudspeaker arrives at the listener's ears in only 5 milliseconds. Then, the listener hears the sound from the front loudspeaker 60 milliseconds later. The late arriving sound will be heard as a distinct echo. It can also cause the spoken word to be difficult to understand.

How Can the Delayed Sound Problem be Fixed?

Figure 2

So what can be done to correct the situation if you have this problem in your auditorium? In the past, the solution was quite expensive, but it is now affordable for almost every school. The solution is to add a piece of electronic equipment called a digital delay (Figure 2). You may also need to add another amplifier depending on how your sound system is designed. A digital delay allows the electronic signal to be delayed to the under balcony loudspeaker. Now, the sound from the under balcony loudspeaker arrives at the listener's ears in sync with the sound from the front loudspeaker. In fact, if the sound from the under balcony loudspeaker is at the correct level and arrives from 10 to 25 milliseconds after the front loudspeaker, the sound still appears to come from the front. So not only have we fixed the intelligibility problem, but we have improved the localization (the direction from which the sound seems to originate) of the sound system.

The Haas Effect

The sound level from the under balcony loudspeaker can actually be set 4 to 10 dB louder than the sound from the front loudspeaker. It will still seem as though the sound comes from the front. This phenomenon is called the Haas effect, named after the researcher Haas.

Additional Applications of Digital Delay in a Sound System

Figure 3

Another use of a digital delay is to provide better localization in a distributed loudspeaker system (Figure 3). By setting up different delay zones, the sound can appear to originate from the front. However, the degree of effectiveness is very dependent on the strength of the voice and the acoustics of the room. The listener at the rear must be able to hear sound coming from the front before the Haas effect will be effective.

A digital delay can be used to provide synchronization between any front loudspeaker system and secondary loudspeakers that are farther away, such as an overflow area, under balcony, or upper balcony. With proper engineering and calibration, a delayed loudspeaker system can solve many problems.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

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