What’S GoinG on?First among these is the type of performance.This can range from natural acoustic to high-powered reinforced electric. Is it music that istraditional or a cappella? Is it to be speech only,music only, or a compromise of acousticalambiance to allow both? One size doesn’t fit all.

A well-designed theater from the vaudevilleera will project an unamplified performance butis easily overwhelmed and acoustically over-loaded in the age of today’s high-powered linearrays.

Sound that did not reach to back wall, backin the day, now hits it with storm force, produc-ing annoying slap-back to interfere with themusician’s timing and the speaker’s concentra-tion. If an existing space is to be a naturalacoustic venue, little treatment and less soundreinforcement will be necessary.

What’S the Problem?Next, is there a sound problem or a noiseproblem, with noise being the sub-categoryof sound that is unwanted? If there is a noise

issue, is it noise coming into the auditoriumor leaving it to disturb neighbors or others inthe building?

Higher-pitched noise will generally be lessintense, with lower energy content and moreeasily controlled with dense, heavy barriermaterials, such as gypsum board and massloaded vinyl layers.

Low frequencies have longer wave-lengths and generally have more power. Thelonger wavelengths take a greater distanceto fully develop and may explain why theneighbors perceive a sound as louder thanthe folks close to the source. Low frequen-cies tend to be structure borne and usually

44 November 2011 | cspmag.com

B Y N I C K C O L L E R A N

When acoustically treating a performance space or

auditorium, there are several considerations. Some

are often obvious, but nevertheless often ignored.

Auditorium AcousticsConsiderations

AS SEEN IN

require decoupling from the source withresilient isolation. This treatment is lowercost if implemented at the start of construc-tion where it is easier to split a concrete slabor float a floor on pads..

iS ComPromiSerequired?The discussion of noise pollution, especiallythe long waves, is also a long discussion, bestreserved for a more in-depth article. The pri-mary consideration for an auditorium isalmost always its sound quality.

Again, sound quality is not the same forevery source or every listener. A typicalspace will sound best for spoken word if thereverberation time, RT60, is between 0.90second and 1.00 second. (Reverberationtime is the interval between when the soundis made and its decay becomes inaudible, 60dB down.) Traditional music may want tohear the reverberation in the same room atbetween 1.5 seconds and 1.6 seconds, whileelectronic music may want to be “dry” withno reverberation or echo other than what isadded artificially by the sound engineer.

These situations require compromise atperhaps 1.25 seconds for a traditional per-formance, while electronic music allowsspeech to be clear by using a dry room withartificial ambiance added to taste for themusic.

PeoPle CountAcoustical taming of a performance space ismost often achieved by adding soundabsorption to room surfaces, usually on wallsand ceiling. Some added sound control canbe accomplished with padded seats and car-pet on the floors. In general, the thicker thematerial, the more absorption is achieved,provided it is fluffy and porous.

When adding material, it should be notedthat the difference in sound absorption maynot equal the specifications of the product,depending upon the surface being covered.The full rating of an acoustical material couldbe “as advertised” over rigid gypsum drywallbut may only equal the difference in absorp-tion between an existing acoustical surfaceand the added acoustical panels (for exam-ple, if the existing walls are porous or coveredwith materials already providing someacoustical properties).

People count in this cover-up as well.Padded seating will add to the acousticalabsorption of an empty room and allow thespace to be similar in sound with a full or par-tial audience.

However, if the seating is hard, theacoustics will change as the seats are filledwith people. The ambiance will dry up drasti-cally in winter, when those attending wearheavy, fluffy coats.

Failure to account for this has producedacoustical difficulties in some well-knownvenues.

Form, FunCtion, Fire,and FiniShWhile symmetry of acoustics left-to-right isdesirable, even multiples in the steppedincrements of a theater’s rising listener areamay not. The ancient outdoor amphitheaterswere not spaced on even multiples.

At least one recent outdoor “bowl” hasuniform math and equally uniform badsound due to the commonality of wave-lengths, reinforcing or canceling the samefrequencies, rather than disbursing themover the full range of hearing. Good looks canhave, but do not necessarily produce, goodacoustics.

This leads into the topic of “form function,fire and finish.” If form is the principal consid-eration, good looks can lead to trouble andare almost always the path to greaterexpense, if acoustical function is not an equalconsideration. Both may be achieved if givenequal weight at the start.

The sound absorption function alone canbe achieved with a bale of hay, but there areallergy and fire considerations. Once firespecs are met, the final consideration drivingprice will be finish. A $20 panel can meetthese minimum requirements and performas well as the panel costing six times asmuch.

laSt iS FirStSound absorption placed anywhere open tothe air in a room will reduce reverberationtime. Acoustical panels have been mountedunder seating in places where there is notenough open wall area.

However, absorption lowering reverbera-tion alone will not solve all problems. Oncereverb is reduced, annoying direct reflections

are more easily heard. The most troublesome of these reflec-

tions are those bouncing back from the rearwall that are out of time sequence with themusicians and cause the person speaking torepeat himself involuntarily, reducing intelli-gibility and increasing distraction from thesubject of the presentation.

Following second is treating the stagewall, if there are floor monitors aimed towardit, and the sound reflects to interfere with themain source. This can confuse the perform-ers and be more annoying to the audiencelistening.

SCienCe to artBeyond controlling and containing sound isthe shaping of it. This is achieved by spread-ing sound uniformly with diffusion that low-ers intensity without eliminating the soundby absorption. Boom can be controlled withbass trapping that requires size and depth forlow frequency control.

Once the sound is tamed, it can betrained by these methods, but that is an artrequiring a more in-depth discussion exceed-ing the space of this article. CSP

Nick Colleran is a principal ofAcoustics First Corporation,www.acousticsfirst.com.

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