Test Results Of Acoustics In Monolithic Dome Gyms Delight The Tester

January 10, 2002

by Freda Parker

"I would say that the acoustics in these gymnasiums are reasonably astounding considering the rooms," said Porter Falcon, president of Falcon Audio Video, Inc. of Owasso, Oklahoma. The two gymnasiums Porter referred to are the Monolithic Domes at high schools in Italy, Texas and Avalon, Texas, where he recently tested each gym's acoustics.

Pleased with the test results, Porter said, "It is very unusual for gymnasiums as a whole -- even rectangular ones -- to sound good and it's even harder for a dome." He explained that, for acoustical purposes, you could think of a dome as a parabolic reflector, much like a satellite dish or a bowl. Sounds entering that bowl reflect or bounce off its surface. The larger the bowl or dome, the more bounces or reverberations the sound will make.

Porter said that the period of time during which a sound keeps reverberating within an enclosed area is called "reverberation or decay time." And, while it seems that it might be, decay time is not necessarily bad or unwanted. It all depends on the purpose of the building or room.

"You can use most living rooms as an example," Porter said. "If you clapped your hands in most living rooms, you probably would hear nothing in terms of reverberation or echo. The sound dies out quickly, so there is virtually no decay time. We call such a room or building soft. Hard rooms, on the other hand, have longer decay times."

He explained that a soft room is great for speech because the quieter or less-reverberating a room is, the better we can understand speech. But that same room may not be so great for other activities. "Music sounds really poor in most living rooms," Porter said, "because they don't have longer reverb times to support the music."

If you want speech to be clear and understandable and music to be enjoyable and sound good in the same structure -- such as a church, for example -- then, according to Porter, you need a decay time of no longer than 2.3 or 2.5 seconds.

But what about a gymnasium?

Most gymnasiums are noisy places -- especially ones constructed just for athletic events. There, the echoes of both the athletes' plays and their fans' cheers add to the excitement and are an integral part of that environment.

But most school gymnasiums do not fall into that category. Besides sports, they usually house various activities, such as commencement exercises, stage productions, musical reviews, art classes, dances, and school and community gatherings. Porter said that ideally such gymnasiums should have decay times that are as close to 2.5 seconds as you can get them.

Hooray for Avalon and Italy

Porter tested the acoustics at the Monolithic Dome, high school gymnasiums in Avalon and Italy. As indicated by the results of that testing, both domes did well, and the Avalon gym did exceptionally well.

"Avalon did better for a couple of reasons," Porter said. With a diameter of 124 feet, the Avalon dome is smaller than the Italy dome with its diameter of 148 feet. Then too, Avalon has a suspended, tile ceiling and rectangular walls, so there is less of a curved surface from which sound can focus. Thus, Avalon has shorter decay times.

But the acoustics at Italy also tested well, particularly after 4000 square feet of sound-absorbing material, called blankets, were hung in two rows from its ceiling. Porter said, "The blankets took two seconds off the decay time at Italy. That's very, very important. I've measured gymnasiums with decay times as long as 17 seconds. It's not unusual to find them that way."

Porter summarized his test results in the tables included here. Those tables indicate Hertz or Hz and seconds. Named after German physicist Heinrich R. Hertz, who died in 1894, Hertz are units of frequency of a periodic process equal to one cycle per second (Webster's New International Dictionary). Example: 63 Hz - 2.72 sec. means that a sound with a Hertz or cycle of 63 took 2.72 seconds to die out. The 2.72 seconds were its decay or reverberation time.

According to acoustical experts, the human ear is most sensitive to frequencies between 63 and 4000 Hertz. For that reason, at the Avalon and Italy gyms, Porter tested frequencies ranging from 63 Hz to 4 KHz (K=kilo or 1000).

Related Links:

• The Sound of Quality in Monolithic Domes
• New Monolithic Dome Multipurpose Center at Avalon
• Gladiator Coliseum
• Bouncing Off the Walls

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