Acoustic absorber placement ..

 Acoustic absorber placement.

Lets assume we have a massively lively room. One of those where you could clap your hands, go out to a local cafe for a cup of tea, and when you come back, still hear your hand clap gently decaying !. OK maybe that’s a little extreme, but you get my meaning … A room of this nature is going to need absorption to get it under control –

“What ?” is not such a great problem, I’m sure we all know about “Rockwool” and other “near perfect” absorber materials, but “where ?”.

“Where ?”  and “how much ?” go hand in hand.

“Where ?” or acoustic absorber placement, becomes more and more important as your chosen absorption material gets better at it’s job.  With a relatively weak absorption material you’d have to use so much of it, you could pretty well slap it everywhere. But with a highly absorbent material however, that could lead to an unnaturally dead sound.  Acoustic absorber placement: how much and and on which room surfaces, becomes pretty critical, and a fairly delicate balance. I can recall a rather classic occasion, when, after having specified replacing some (only some !) ceiling tiles with another particular brand of tile, for a cafe at a Marks & Spencer store. They went ahead and did the same in another store, obviously trying to “get out” of the necessary design cost, and, using the same tile we had previously specified, but, rather “on mass” instead of the correct amount. The result, for us, was quite pleasing and rather amusing – they made the space sound worse !. And, it cost them the time and expense of removal and corrected re-fit, design, and facility “down time” – which equals loss of earning.  Quite an expensive exercise when you consider the relatively low cost of design, and absolute proof that acoustic absorber placement does matter.   As it happens, our unique “calculative modelling” system is pretty cheap, and alarmingly accurate – embarrassingly accurate for the cost really !. We achieve this high “cost” to “value” ratio by the nature of the modelling system being “calculative” rather than a more exotic “reverse ray trace” or similar system. Calculative systems are far easier and quicker to “enter room data” into, than Ray Trace systems. so there’s your first saving, in, both time, and money. Also, Ray Trace systems (or similar) tend to “go off” for a while and do a whole load of calculations before delivering a result, where as Calculative methods work in real time. And I mean as “real” as …. “wonder what happens if we put a huge rug on the floor” ?  Whop … instant answer. I’ve even been in telephone conversations with clients before now, where they have said something like … “if we change the curtain material to xxx, will there be a problem” ?.  Ten computer keyboard depressions and 2 mouse clicks later, I can answer them – fully !. The failing of calculative methods and systems is that they are all derived from an ancient and arguably “over simple” calculation. Used on it’s own, it ignores the “geographic” absorber placement of materials in the space, resulting in massive errors, and even things like 2 completely different acoustic qualities at opposite ends of the room. However, we’re above all that ! Our system is unique and accurate because it runs 2 different calculations simultaneously. As in standard engineering practice, the 2 calculations start from as different a point of view as you can manage. Then, if the results are the same, you can pretty well guarantee they are right. The system actually delivers 2 reverberation time V frequency plots on a single graph axis. This gives us visual indications of:

A) Both plots the same = correct, and,

B) The two plots being different  = acoustic absorber placement imbalance.

While it’s not exactly artistic I know, we call this “Plot Drift” or PD. We can use PD to show, the same amount of absorber, but in differing placements, making a difference to the expected reverberation time, while, zero DP equates to an acoustic outcome exactly the same as modeled. We also use the frequency range of PD (should there be any) to warn of overall outcome inaccuracy and increased risk of things like flutter echoes. To prove accuracy, we both measured and modeled a small practice room, both before and after treatment. Graphs show …

1) before treatment) the RED trace is Modeled before treatment, and the GREEN Modeled after recommended treatment. & …

Reverb time graph jpg

2) (after treatment) the RED trace is Modeled after recommended  treatment, and the GREEN MEASURED after recommended treatment.

Reverb time graph jpg

NOTE how both modeled and measures plots “sit on top of each other”  ie: no Plot Drift and total outcome accuracy assured.

And, here’s a typical room:

This Graph shows the bare untreated room’s model in RED, with the modeled final outcome after recommended absorber treatment in GREEN, and a warning to the customer, that without just 1 of several recommended absorption elements, the result would be as in the BLUE trace !.

Reverb time graph jpg

One thing is for sure, if treated as recommended, the measured outcome would be exactly the same as the modeled outcome. In conclusion: The correct “geographic” placement of absorption to reduce long reverberation times is vital to get a workable and expected result. More so as absorber efficiency rises.

It’s not enough to simply “park” a lorry load of “Rockwool” in a room !