Loudspeakers: Improve Your System with Digital Delay

Have you ever sat under a balcony and heard a very confusing, unintelligible sound? Or perhaps you have sat in the back of an auditorium then the rear door opened as somebody came in late. Suddenly,the sound became very unclear.When the door closed, the sound cleared up again. What were you hearing? Is there a way that 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 compensate for the reasons and to correct the poor sound.

Sound Travel Takes Time

Figure 1

Sound travels quite slowly. In fact, sound travels much slower than light. Everyone has heard this phenomenon during a thunder storm. First the lightning flashes and then the thunder is heard much later. The speed of sound is about 1130 feet per second. Or put another way, it takes .889 milliseconds for sound to travel one foot. This relatively slow speed is the cause for the confusing sound under the balcony. Look at Figure 1. The sound comes from the loudspeaker in the front and travels to the back of the room. In our example, the distance is 73 feet, so it will take 65 milliseconds for the sound to arrive at the listeners' ears. A millisecond is 1/1000 of a second. Our example also has an under balcony loudspeaker which is only six feet from the listeners' ears. Since the electronic signal from the amplifier arrives at the front loudspeaker and the under balcony loudspeaker at virtually the same time, we will hear the under balcony loudspeaker first. The sound from the under balcony loudspeaker arrives at the listeners' ears in only 5 milliseconds. Then, the listeners will hear the front loudspeaker 60 milliseconds later. This sound will be heard as a very distinct echo. It can also cause the spoken word to be very difficult to understand. This delayed sound is what also makes the sound poor when the rear doors of the auditorium are opened and we also hear the lobby loudspeaker.

What Can You Do To Fix It?

Figure 2

So what can you do to correct this situation if you have this problem at your church? In the past, it was quite expensive, but it is now possible for almost every facility to correct this problem. The solution is to add a piece of electronic equipment called a digital delay to the sound system. You may also have to add another amplifier depending on how your sound system is designed (Figure 2). 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 listeners' 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 loudspeakers ' sound, our ear/brain tells us that the sound is coming from the front. So not only have we fixed the intelligibility problem, but we have improved the localization of the sound system. That means that the sound appears to come from the front where the talker is, which is more natural.

The Haas Effect

The sound level from the under balcony loudspeaker can actually be 4 dB louder than the sound from the front and your ear/brain will still be fooled into thinking the sound comes from the front. The under balcony loudspeaker can even be up to 10 dB louder than the sound from the front loudspeaker before the under balcony loudspeaker will sound louder to your ears. This phenomenon is called the Haas effect, named after the researcher Haas.

Digital Delay In A Distributed 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 acoustics of the room. It is also dependent on the vocal strength of the talker. 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. If you think your sound system could be improved, please contact us.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This articles 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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