When designing a sound system, physics still applies, no matter how hard I might wish that it wouldn't.  Many venues that we work in have low ceilings and comparatively far distances to the last row of the seating area.  Think of the typical 1970's Nazarene church -- long and low, and it could be built inexpensively with standard wood trusses and asphalt shingles.  60' from front to back, 10' side wall height, 14' roof peak.

The Inverse Square Law says that sound pressure level (SPL) or acoustic volume drops 6 decibels (dB) every time you double the distance from the sound source.  If we measure 94dB at four meters from the speaker, the SPL will be 88dB at eight meters, 82dB at 16 meters, 76 dB at 32 meters, etc.

When designing systems, if we're asked, what we hope to find is a room that's about twice as deep as the ceiling is high.  In that type of space we can keep the SPL difference to about 3-4dB over the seating area with conventional speakers.  Note that in the example above, there's a 12dB difference for the person sitting 13.2' from the speaker and the person sitting 52.8' away. That's why the 1970's Nazarene church style building is a tough place to install a sound system, and to do it well (not to mention inexpensively).  That difference is 12dB represents more than a 50% apparent reduction in acoustical volume.

That halving of volume is not a big deal, if you want people to be able to sit in relative audio comfort at the back of the room, but more often than not, the audio technician (who typically sits in a corner in the very back of the room) adjusts the SPL to his taste making the acoustic volume much louder (read as "too loud") up nearer the platform or stage.

Unfortunately, system performance is close to the last thing that many churches and other performance venues discuss prior to design and eventual construction.

What we hope to do as designers is to keep the distance between the listeners and the speakers as close to the same as possible.  That's why speakers should generally be mounted as high as possible in a given worship or performance space.

As a system designer, another goal is to make sound go where I want it to, and not where I don't.  That's easier said than done, sometimes.  Low frequency pattern control (the ability to direct lower frequencies toward the listener) is absolutely essential to having your system work the way you want it to.  And in low-ceiling rooms, it's even more important.  In that hypothetical church with a 14' ceiling, don't forget that the platform or stage is at least 1' high, and that the person speaking generally has a microphone that's about 5' off the floor.  At best, the highest point in the room is about 8' above that person's microphone.  Hang the speaker there to normalize volume, you'll lose another 2' to speaker mounting hardware and the size of the speaker, and that speaker is 5' above the head and 6' above the microphone.

A typical loudspeaker has a high frequency horn to direct midrange and high frequencies toward the listeners or congregation.  That's a good thing.  What happens to the frequencies that are reproduced by the woofer?  For the most part, they radiate 360 degrees around the speaker box itself, which means that there's as much low frequency energy 6' from the speaker below, above, behind and toward the listener.  Do you ever wonder why finger taps on a podium produces a low-mid frequency ring or feedback?  That's why getting some distance between the speaker and the microphone is important, and why low ceilings make our job more difficult.

If you have questions about integrating new equipment into an existing facility (even a long, low room), we have solutions and can help.  There are two particular loudspeaker manufacturers that pay lots of attention to low frequency pattern control and that's why we use their products.

Please call anytime, and we'll be glad to help.


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