Inside Scoop: Subwoofer Isolation For Church Project
The following instructional article was graciously provided by Jeff Lange, who heads up Sonic Art LLC, based in Oshkosh, WI, a firm specializing in the development of audio, video and lighting systems for houses of worship. Our thanks to Jeff (pictured at left) for the article and photos.
The design of a new sound system for the sanctuary of The Pentecostals of the Fox Cities in Menasha, WI, was particularly challenging. This 500-seat auditorium has a 12-foot ceiling, dictating loudspeakers that are very compact in order to not detract from the room’s aesthetics.
To meet this requirement, we chose EAW MK8100 Series loudspeakers, with six of these cabinets flown in their horizontal cabinet position (with horns rotated), carefully placed in a distributed, decentralized arrangement. (Shown in the photo at right.)
Largely due to compact design of these loudspeakers, frequency response doesn't extend much below 90 Hz. Therefore, we also needed subwoofers to reinforce frequencies beneath the 90 Hz point to meet a stated design goal of providing a system offering true full-range capabilities.
Finding a location for the subwoofers in this sanctuary proved to be a challenge. Placed on the platform, they would tend to distract the musicians and singers while flooding the area with feedback-inducing LF energy. And the ceiling was far too low to consider flying them.
Fortunately, the construction of a sound isolation drum booth on the platform necessitated extending the platform outward. We seized this opportunity, deciding to put the subs under the platform extension.
Of course, this direction presented its own set of challenges. The platform height would be a tight 24 inches, leaving much less than that to realistically work with. Evaluating possible solutions, we chose a pair of EAW SBX220 subwoofers that are only 18 inches tall while capable of handling the 5000 watts we wanted to deliver to them.
The next challenge: Isolating the LF energy from the platform. If the subs aren’t properly isolated, the entire space under the platform could tend to act as a “resonation chamber” that causes uneven frequency response and feedback.
Working with a Mark McLernon, a skilled carpenter from the church, we constructed an isolation box. This box effectively isolates the low frequencies from the platform via two chambers - the platform extension is isolated from the existing platform, and the subwoofers are isolated from the extension.
The subwoofer “inner chamber” is constructed of wood and fastened tightly together with glue and screws to prevent rattles and buzzes. All seams are caulked air-tight, furthering isolation properties. The “outer chamber” – the airspace between the inner chamber and the platform - is isolated from the platform by using wood and concrete board. The space between the inner and outer chambers is hard-packed with insulation. Once the insulation is installed, two layers of plywood close the top.
The end result has been as predicted – flat and very smooth low frequency presentation, with output that keeps up with the high impact (pun intended) of the church’s youth services.
Again, thanks much to Jeff for this report. Click here to read about another innovative Sonic Art LLC project.
