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.
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.
Background Noise Levels
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.
Echo Buster Panels
Sound Made Simple iCD
Ceiling Grid Systems from illbruck
Pro Acoustics Speakers
Tectum Finale SoniCor