Church almost destroyed acoustically

I just did a design for a church as a paid consulting engagement. It is roughly 50 x 25 x 5m.

There is already carpet, upholstered seats, curtains etc. However there are some concrete, some SIP and some plasterboard walls. Obvious flutter echoes can be heard between them. 

The calculations show, with about one third occupancy, a reverberation time (RT60) of 1.6 seconds. For a room volume of about 6500m^3 this fine and agrees with AS/NZS. Rudimentary measurements confirm the calculation.

AS/NZS 2107:2000 recommends reverberation times of;
1.0 seconds for Spaces for speech
1.3 seconds for Cabaret and Theatre restaurants,
1.7 seconds for Music Studios;
for rooms of approximately 6500 cubic metres

With the room full it approaches 1 second, which is a little too low. Worship spaces are used for purposes that commonly require all of the above functions, so a compromise is always needed. Generally, a high RT60 increases the engagement of the congregation. More problematically, a low RT60 isolates people and they miss the community experience of the congregation. So the objective is to maintain a suitable RT60 and eliminate problematic echoes that may interfere with speech intelligibility or clarity.

They had originally asked suppliers of acoustic absorption to quote on what they needed. Of course this is like asking the Ford dealer what car is best - an obvious conflict of interest! A large polyester supplier quoted them almost 200 sheets of a mixture of 50 and 25mm thick polyester. This would have destroyed the acoustics of the room in terms of a worship space. But they would have sold over $30k in polyester! Not only that but the calculations on this amount show that after a certain point, adding more absorption makes very little difference i.e. the room is "killed off" with a small amount of absorption and the rest is just wasted.

I have advised them to add NO absorption and construct some simple diffusers to reduce the flutter echoes. This will maintain energy in the room for an inclusive community experience and reduce problematic echoes. If for some strange reason the result demands a lower RT60 some absorption could easily be placed on the ceiling - but I very much doubt it will be needed.

Including my fees, and constructing the diffusers, they have probably saved over $25k and avoided a complete disaster. 

So the message is, get qualified advice and don't ask someone selling a product how much you need....

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