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Think of Acoustics First, Not Last
By: Nick Colleran

Before presenting the new sanctuary design to the committee or congregation, determine that what you hear will be as good as what you see. Making a decision with only the eyes can hurt the ears (and attendance). Once the audience falls in love with a design, there is a higher hurdle to corrective changes. Doing it over is both costly and stressful.

The sound system cannot fight the room. The room is “alive.” Acoustics may be passive electronically, but an untreated, poorly shaped room is active acoustically. The distance between walls determines what frequencies will be emphasized and which will be cancelled. Complete cancellation will occur when a wall is one-half wavelength away from its source. The reflected peak, or forward motion (push) of the sound wave, travels back from the wall and meets the incoming trough (pull) of the cycle and sums to zero.

Boosting the “missing” sound’s frequency electronically with an equalizer will simultaneously boost its reflected out-of-phase component, retaining a net sum of zero. Hence, it is a zero sum game. Absorption, diffusion, and trapping can remove or redirect sound to overcome the basic room geometry and dimensions.

Why Trap Bass?
The term bass trap is counterintuitive. By trapping bass, the out-of-phase reflections are not allowed to reflect back and combine with the incoming sound to sum to zero. There are many listeners who hear bass build-up in corners, where it is reinforced at the wall intersections. In fact, it is an absence of audible bass in the middle of the room, resulting from out-of-phase cancellations, that produces this effect. A bass trap acts like an open window for the low frequencies, allowing them to keep going in one direction rather than returning to the room to cancel.

In a sealed room, with no acoustical treatment, bass may be inaudible due to internal wave cancellations, but, if the walls and windows are acoustically thin, the lows will travel throughout the neighborhood. This combination of the effects of physics will exacerbate the problem as the sound man compensates for what is heard (or not heard) inside the room, while being unaware of the sound outside.

For Bass: Size and Shape Matter
A standard fabric-covered wall panel made of 6 – 7 pound per cubic foot, compressed acoustical fiberglass board, or similar porous material will absorb (trap) all sound effectively above 500 Hz. The wall panels still work well down to approximately 250Hz, but, beyond that point, specialized traps become the most effective means of low-end control. One of the most common solutions is the polycylindrical diffuser, a curved panel that has been seen for years on motion picture scoring stages. In addition to being a spatial sound diffuser due to its curved surface, this device performs “double duty” as a bass trap. The diffuser’s front face flexes to absorb the lows but, beyond diffusion, has no real effect above 500 Hz. Their use in combination with standard acoustical wall panels extend the sound absorption range to effectively cover the full audible spectrum and eliminate the low-end bass “bump” in room response that is often left by the use of flat wall panels alone.

Now You Hear It
Recent times have seen increased use of sub-woofers. These are not used to create the annoying excess bass of the passing automobile but to allow the bottom octave in music to be heard. The best systems will have the listener unaware that there is a sub-woofer until it is turned off and the sound becomes thin. With the room no longer fighting the system and canceling bass, there is less need for massive amplifiers and excessive numbers of speakers to be thrown at the zero-sum situation.

(As a side note, excessive automotive bass has a common contributing cause. The sealed passenger chamber reduces audible bass inside the vehicle, while the thin car walls and windows allow the neighbors to feel the experience.)

One Size Does Not Fit All
If the story stops here, there is already enough information to treat a church having a contemporary worship service and a praise band. However, if the purpose is to treat acoustics first, it first requires knowing the type of service (performance) to be held within the space. Whereas a contemporary service may benefit from a “dry” acoustical environment, there are traditional services with organ and choir or unaccompanied congregational singing. Holding the latter type of service in a space designed to control high intensity amplified music is disappointing at best. Adding back acoustical ambience can cost an amount equal to the initial incorrect treatment.

While a proper acoustical environment can reduce the need for extra sound equipment, or several replacements to get the system right, knowing the intended use of the room can similarly reduce the need for acoustical materials to be purchased more than once. As with sound gear, adding enhancements is more rewarding than fixing a bad initial solution.

Diffusion Confusion
While an acoustically dry room may be desired for the modern praise band, a live reflective room can greatly enhance unamplified voices. There is always a need for clarity in speech. An interesting sermon should not cause fatigue for the listener having to separate the message from the background echo. Diffusion solves the problem of speech clarity by scattering sound in many directions and lowering intensity. At the same time, sound is not lost, as it is with purely absorptive materials. Acoustical life remains in the room and speech clarity improves, as sound no longer is allowed to build up and obscure the spoken word. In the unfortunate situation of having to bring a dead room back to life to match the service, replacing a quantity of ceiling tiles with equivalent diffusive units is often enough.

Isolation Items
It is good to have the interior of the sanctuary peaceful and quiet. At the same time, it is desirable not to have unwanted sound traveling from one room to another. Both of these situations are easier to fix and cost less if addressed sooner rather than later.

To prevent sound from traveling through walls requires two components: heavy sound blocking mass and light sound absorbing porous materials. Absorption won’t stop sound penetration but inside a wall, but fiberglass insulation will trap the air to prevent the cavity from behaving like a drum, with one wall vibrating when sound hits the other side.  

Any commercial building insulation will provide the necessary sound absorption. Multiple drywall layers can supply the necessary mass. For retrofits, a layer of mass loaded vinyl (one-eight-inch thickness) and a layer of acoustical wall covering (one-quarter-inch thickness) will usually solve the problem. The elimination of sound (as vibration) traveling throughout the building can be accomplished with isolation pads.

One of the lowest cost methods is to float the praise band’s stage. This is something to do during construction and, for dollars spent, has a high return in both room-to-room isolation and lower on-stage feedback. A stage floated in several sections will keep the bass from traveling up the stand to the singer’s microphone, as well as prevent the entire band’s performance from rocking the nursery.

Dealing with acoustics first, including defining the intended performance in the space, will cost less and allow the sound system to perform “as advertised.” If systems are up and running but underperforming, it is usually not the equipment, unless the system is improperly designed.

Today, there is very little bad equipment. Unfortunately, there are many bad rooms, especially if “bad” is defined as those not meant for their intended current purpose. If you get the room right, the sound system may heal itself.

Nick Colleran is past president of SPARS (Society of Professional Audio Recording Services), past president of the VPSA (Virginia Productions Services Association), a former recording artist, and recording engineer. He is a principal of Acoustics First Corporation, which designs, manufactures, and distributes products to control sound and eliminate noise for commercial, residential, and industrial uses, www.acousticsfirst.com.

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