RT60 Target Chart for Churches

Four targets, chosen by what the room is mostly used for rather than by its size. A room serving two very different services has to compromise, and the honest answer is to favour the service that carries the most spoken word.

Reverberation time rises with volume and falls with absorption. These are computed for the room geometry this site models at each seat count, using the Sabine relationship, across the range of average absorption coefficients real sanctuaries fall into.

The arithmetic that turns a target into a purchase order. Panels are counted as 2 inch fabric-wrapped fiberglass at 2 feet by 4 feet, which is about 6.8 sabins of absorption each.

Intelligibility depends on the ratio of direct sound to reverberant sound at the listener, not on the absolute level. Direct sound is what travels straight from the loudspeaker to the ear. Reverberant sound is everything that has bounced. Turning the system up raises both equally, so the ratio does not move and the words do not get clearer. They get louder and just as muddy.

The distance at which those two are equal is the critical distance. Inside it, the direct sound dominates and speech is clear. Outside it, the reverberant field dominates and adding level achieves nothing. In a reverberant stone church the critical distance can be under fifteen feet, which is precisely why an unamplified preacher is perfectly clear to the front rows and unintelligible at the back.

Two things genuinely move the ratio. One is absorption, which reduces the reverberant field directly. The other is loudspeaker directivity: a box with a tighter pattern, aimed so its energy lands on people rather than on walls and ceiling, puts more direct sound on the listener and less into the room. A well-aimed 60 by 40 degree cabinet in a reverberant church outperforms a wide one at the same power, which is covered in how to aim and place church speakers.

This is why the order of spending matters. A church with a 2.5 second RT60 that spends its entire fund on loudspeakers has bought a louder version of the same problem. The same money split between treatment and a directional system produces a room where people understand the sermon. Credibility requires saying this even when the loudspeaker sale is the easier one.

Sabine is an approximation. The relationship assumes absorption is spread evenly around the room and it over-states RT60 in very absorptive spaces, roughly once the average coefficient passes about 0.3. For deciding how many panels a sanctuary needs it is the right tool. For a recording studio or a highly treated broadcast room it is not.

RT60 is one number describing six. Reverberation time varies with frequency, and most churches have far longer decay in the low end than in the midrange, because the hard surfaces that absorb speech frequencies do almost nothing below 250 Hz. A room measuring a respectable 1.2 seconds at 1 kHz can run 2.5 seconds at 125 Hz, which is heard as boom rather than as reverberation. Measure by octave band.

Occupancy changes the answer substantially. A congregation is a large absorber, so an empty room measures considerably longer than a full one. Treat a measurement taken on a Tuesday afternoon as the worst case and expect the occupied figure to be noticeably shorter, particularly in a room with hard wooden pews.

It says nothing about echoes and flutter. A distinct slap off a rear wall or a flutter between parallel side walls is a separate defect that a good average RT60 will hide completely. Those need targeted treatment at the offending surface, and they are often more damaging to intelligibility than the overall decay time.

Treatment is not free of pastoral consequences. Bringing a traditional sanctuary from 2.4 seconds to 1.0 will make congregational singing feel exposed and thin, and people will notice. If a church values its singing and its organ, the honest target is the traditional one plus a directional speech system, not a dead room. See acoustic treatment for a sanctuary.