Why The Big Delay?

Jim Brown, Audio Systems Group; Chicago, IL

Delayed loudspeakers are used for several reasons, but nearly always to supplement the main front loudspeakers at the side or rear of the congregation. Their successful use depends on what is popularly known as the Haas Effect, named for one of several scientists who independently "discovered" it.

The Haas Effect is simply the ability of the human ear-brain combination to assign direction to the first arrival of the same sound it hears, rather than the strongest. For example, say a guy is hunting in the forest, and he hears a bear. The sound may come to him directly from the bear, and it may also come to him bouncing off a cliff 20 feet away. Even if there are trees between him and the bear to muffle the direct sound, the direct sound gets there first, and his brain correctly tells him where the bear is, even though the sound from the cliff may be louder (because the sound traveled farther over the path from the bear to the cliff to the hunter).

We use this ability with sound systems by adding an electronic device called a Signal Delay in the feed to the amplifier for the loudspeakers at the rear of the congregation. We then set the delay time so that sound from the closer loudspeaker gets there slightly later than the sound from the main loudspeakers (10-20 milliseconds is about right).

Sound travels at about 1150 ft/second (1.15ft/millisecond) [see note at end of article], so we can use basic trig or a tape measure to determine the distances involved, then set the delay unit accordingly.

There are several reasons for using delayed loudspeakers. Under a balcony, for example, listeners won't hear loudspeakers that are flown above an altar. In a balcony, delayed loudspeakers can be used to allow the main cluster(s) to be smaller.

Delayed loudspeakers can also be used to eliminate problematic reflections from rear walls back to the altar area, or to the front of the audience. They can do this because they can be placed overhead, but further back in the congregation, so that they hit the back wall at a steeper angle. Think of it this way. A loudspeaker on the altar hits the back wall and comes straight back to the altar, and if the back wall is 100 ft away, the reflection will be about 180 ms late. Real nasty. A loudspeaker above the altar hits the back wall and comes down near the front of the congregation, perhaps 120 ms late. Still real nasty. But if you put the loudspeaker only 50 ft from the back wall and keep it high, you might be able to make it come down only 15 ft from the back wall. Considering the angles involved, it might be only 25 ms late. The ear has no problem with this short an arrival time difference.

This ability to tailor the angle at which the sound hits the back wall is a big part of why the delayed loudspeaker was recommended for the outdoor system. But there's another reason.

The direct sound from a loudspeaker drops off by 6 dB for each doubling of distance between the loudspeaker and the listener. So to get 95 dB at the back row of a 200 ft audience, you need 101 at 100 ft, 107 at 50 ft, 113 at 25 ft, 120 at 12.5 ft, 126 at 6.25 ft. This is called "inverse square law," and is another reason why it's good to get the loudspeakers high in the air. If you're 25 ft in the air and 10 ft from the front row, the loudest it will ever get in the front row will only be about 112, instead of about 122. Big difference.

And this is with a non-directional loudspeaker. If you have a loudspeaker up in the air which is able to focus the sound into a narrower vertical angle, you can achieve 102 in the front row with 95 in the back row. An even bigger difference. And because you have this control of the system, you can run the entire system hotter because you're not making ears bleed in the front rows to get the level you want at the rear of the congregation.

It's important to understand that it isn't easy to get directivity from the relatively compact boxes used in popular concert clusters. Most are too small to have any useful directivity below 1 KHz, and many lose control below 2 KHz. These systems are popular because the folks who design them don't sit in the lousy seats that are too close to them -- they hang out back by the mix console in the back row and crow about how wonderful they sound!

NOTE: The speed of sound is dependant on the air temperature and humidity in the air. In Jims' text, he gives the speed of sound at 92.5 degrees Fahrenheit. When it's twenty degrees cooler (72 degrees Fahrenheit) sound travels at 1130 feet per second. Why is this important? Simply put, you initially set your delay time based on the speed of sound at one temperature, when the temperature changes, you'll need to change the delay time to compensate to keep the Hass Effect working for you. This is true for large rooms, and not as critical in smaller rooms.