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How to Control the Acoustics of Your Worship Space
By: Ben Davenny

Whether a church has “good acoustics” or not depends on the particular style of worship and music for that particular church. There is no universal quality of “good acoustics,” because the physical measures that define the acoustics of a space have wildly different optimum values for different uses.

Reverberation Time
Different types of music require different values of reverberation time. Reverberation time is the time required for sound to decay to inaudibility in a space. The optimum reverberation time depends on room volume and the types of sounds expected in the space. Gothic organ music sounds best with reverberation time of 6 seconds or longer. Contemporary worship music with a rock band requires clarity to match or approach the precision of a studio recording, and a reverberation time of 1.5 seconds or lower is required. With this type of music, the lower the reverberation time, the better. If the sound engineer feels like there is not enough reverberation, they can usually add it as an effect to the music signal sent to the loudspeakers.

Sound Absorption
The sound absorption coefficient is a measure of the percent of energy not reflected back into the space by a surface. This coefficient varies from 0 (perfectly absorptive) to 1.0 (perfectly reflective). Sound absorption coefficients higher than 1.0 are sometimes reported by manufacturers due to peculiarities in the testing procedure. Sound absorption coefficients are often reported in terms of octave bands from 125 Hz to 4000 Hz. The most common reporting of sound absorptive performance is the Noise Reduction Coefficient, which is the average of the 250 Hz, 500 Hz, 1000 Hz and 2000 Hz octave band sound absorption coefficients. 

Sound absorption performance usually varies with frequency. Most sound absorptive materials that are called “acoustical” have absorption coefficients of 0.5 or higher from 500 Hz and above and have absorption coefficients below 0.5 at frequencies below 500 Hz. It is possible to get higher absorption coefficients at low frequencies with sound absorptive materials, but this usually requires either very thick materials (4 to 6 inches thick) or a very deep airspace behind the sound absorptive material. 

Another type of sound absorptive device is a “bass trap,” also known as a “panel resonator.” These dissipate bass energy by positioning a thin membrane or mass layer a distance away from the wall where the bass will make the membrane vibrate. This vibration reduces the level of bass sound reflected from the surface. This low frequency absorption can reduce the bass buildup and “boominess” experienced with contemporary worship.

Not all sound reflective surfaces have the same acoustical performance. Massive constructions like painted masonry or polished stone are very reflective across most of the audible frequency spectrum. Thin reflective surfaces like plywood or gypsum board with a deep airspace behind are reflective at mid and high frequencies but somewhat absorptive at low frequencies. Thin plywood or gypsum board layers can act as bass traps. 

The frequency dependent sound absorption of surface finishes means that the RT must be calculated for low, mid and high frequencies. It may be necessary to add or remove sound absorptive materials to balance the RT across the frequency range. Spaces designed for un-amplified music like choral or organ music generally require long reverberation times in low and mid frequencies and somewhat shorter reverberation times in the high frequencies. The high frequency absorption for large spaces is generally taken care of by the absorption of the air. Spaces for amplified music generally require short reverberation times across the entire frequency range.

For spaces that use amplified music, some sound absorptive materials should be placed on the walls. The congregants provide sound absorption on the floor, and the ceiling can be left partially reflective to support congregational singing. Music reinforcement systems can be designed to target most direct sound energy away from the ceiling, but it is can be difficult to keep the direct loudspeaker sound from hitting the walls.

The placement of sound absorption is very important. To reduce the reverberation in a large space, the ideal placement of sound absorptive materials is in a uniform distribution around the room. However, practical considerations affect this distribution. The seating area on the floor always constitutes a large area of sound absorption. Equal distribution of sound absorptive materials alternated with sound reflective materials may not have the best aesthetic result. Also, the timing of individual reflections can determine the placement of sound absorption. One example of this is “slap-back echo,” which occurs when a sound travels across a long distance, reflects, and arrives at the listener as a distinct echo compared to the direct sound. 

Sound Diffusion
Sound absorption is not the only tool that can be used to control the acoustics of a space. Sound diffusion occurs where the sound is scattered in multiple directions when reflected from a surface, as opposed to being specularly reflected in one direction like a billiard ball. The current standardized measurement of diffusion is the scattering coefficient. The scattering coefficient is a measure of the amount of sound not reflected specularly. While not many manufacturers report scattering coefficients for their products, a rule of thumb is to size the depth variations at least 6 inches. Examples of diffusive finishes include half-cylinders, pyramids and quadratic-residue diffusers. The goal is to get sound to reflect in all directions, so to envelop the listeners with sound. Diffusion can help with congregational singing in all worship styles and to aid even coverage and distribution of un-amplified music. Avoid concave surfaces, because they focus sound.

Background Noise Levels
The level of background sound in a worship space affects how well the congregants hear speech and music. High levels of background noise can distract from the service, detaching the congregants from the worship experience. The primary source of background sound is usually the building mechanical system. The elements of a mechanical system are the fan, ducts, dampers, air devices and refrigerant compressors. 

Fans either move air in and out of the space or re-circulate air within the space. Fans produce a broadband spectrum of sound that usually has a lot of low frequency noise. Generally, fans that are bigger and run slower than fans with the same mechanical parameters are quieter. Fan noise can be reduced by duct silencers and by acoustical duct lining. Duct silencers are rated according to their insertion loss and pressure drop. Insertion loss is the level by which they reduce sound energy and is frequency dependent.  The pressure drop is a measure of the resistance the silencer adds to the system. 

Turbulence noise can be generated in the ducts depending on how fast air flows through the ducts. This turbulence noise can also be generated at diffusers. Turbulence noise can be reduced by enlarging ducts and/or using acoustical duct lining. 

Compressors used to provide cooling produce noise and vibration. The radiated noise from these devices--along with air handling units, pumps, and stand-alone fans--should be addressed by locating mechanical rooms away from noise-sensitive spaces. The sound isolating effectiveness of wall and floor-ceiling constructions surrounding the mechanical rooms should be considered. One example of a high sound isolation wall is a double stud wall with two layers of gypsum board on each side and insulation in the stud cavity. 

Mechanical equipment can transmit vibration to the building structure. Vibration transmission can be reduced by placing the equipment on vibration isolation mounts that could be spring and/or neoprene based. These isolators are selected based on their static deflection performance. Static deflection is the difference in length of the isolator between the isolator on its own and when the isolator is loaded by the weight of the equipment. The appropriate static deflection for an isolator depends on the operating speed and power consumption of the isolated equipment. Springs have static deflection capabilities between 0.5 inches and 3.5 inches. Neoprene isolators have capability between 0.05 inches and 0.50 inches. Components that need to be connected to vibration isolated equipment, like ducts and electrical conduit, should be flexible. The exception to this is piping, which should be vibration isolated for a distance away from the equipment.

Background noise is rated in terms of Noise Criteria (NC), and the optimal ratings for churches should be between NC-20 and NC-35. Churches that feature un-amplified music and/or speech require lower noise levels (NC-20 to NC-25) than those churches that amplify speech and music (NC-30 to NC-35). 

Church acoustics are affected by the products and materials used in their construction. Sound absorptive, reflective and diffusive surfaces affect the reverberation time. The building mechanical system design affects the level of background sound, therefore affecting the listening environment. All of these product decisions should be made with the type of worship in mind.   

Ben Davenny has worked for six years on acoustics and noise control for many building types, including churches. Acentech is a multi-disciplinary acoustical consulting firm whose areas of expertise are architectural acoustics, audiovisual and sound system design, noise and vibration control, building dynamics, and quiet product design.

Product Roundup

Echo Buster Panels
Echo Buster absorption panels are specifically designed to absorb echoes that reverberate around a room. By reducing these reflected sounds, a quieter, more revealing environment is created, just the background needed for intelligible sermons; crisp, clear music; and awe-inspiring choirs. Echo Buster panels are:
* Attractive
* Extremely effective
* Affordable in any budget
www.echobusters.com

Sound Made Simple iCD
Walthall & Associates has announced the release of their latest training application, Sound Made Simple iCD, an interactive computer-based application that uses eye-catching animation paired with an easy-to-understand presentation that teaches the basic fundamentals of audio and acoustics using a unique, academic approach the industry has yet to see. Sound Made Simple iCD addresses the ever-growing need for training in the fields of:
* Acoustics
* Audio
* Sound system operation
www.SoundMadeSimple.com

Ceiling Grid Systems from illbruck
illbruck acoustic provides a ceiling grid system in colors that match the company’s ceiling tiles for a smooth, sophisticated look.
* Standard colors include white, almond, grey, black and satin chrome.
* The system features a double web design for added strength.
* All components are manufactured from hot dipped galvanized steel for superior strength and stability. 
* The grid can be installed easily in any drop ceiling application and works with standard ceiling tiles.
www.illbruck-acoustic.com

Pro Acoustics Speakers
The new SD4 speaker from Pro Acoustics takes ceiling speaker performance to a new level. The SD4 is an innovative speaker concept that meets the demanding needs of audio and systems contractors. SD speakers:
* Are quick to install
* Are competitively priced
* Fit into standard ceiling tile grids
* Provide 360 degrees of hemispheric sound
These speakers are also economical to install.
www.proacousticsusa.com

Tectum Finale SoniCor
Tectum Inc. has introduced a new version of the successful Finalé Wall Panel, Finalé SoniCor.  The Finalé system consists of these elements united in a single product:
* Tectum Wall Panels
* Tectum furring strips
* A new SoniCor fiber core
Whenever your project calls for absorption of undesirable noises, and where the activities demand abuse-resistant panels, new Tectum Finalé is the answer.
www.tectum.com

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