Quote:
Originally Posted by
Oli_W
➡️
Yes, as Jason says, I am asking about a broadband(ish) absorber (which might have a 0.4 absorption coefficient at 90Hz) with a high q tuned absorber behind, on the wall/floor.
As Andre keeps mentioning, and I hinted at too, a high-Q device needs to be tuned after it is installed. The reason is simple: High Q! Since your room mode is high Q (implying very narrow bandwidth, just a few Hz wide) and your device is also high Q (once again very narrow bandwidth), getting those two "very narrow" peaks to align is no easy task. First, just measuring the actual frequency of the problem accurately is an issue: precision measurements at low frequencies don't even happen too well in an acoustic lab! Take a look at the huge error bars in that paper that Andre linked you to. Just because REW shows a resonant peak at 85.27 Hz does NOT mean that it really is at 85.27 Hz.
Then, assuming that you carefully calculated and carefully built a device theoretically tuned to 85.27 Hz, that does NOT mean it actually will resonate at 85.27 Hz. Materials are not perfect (your plywood or drywall might be more dense or less dense than the average that you assumed, for example, and might vary across the surface of the same panel), building techniques are not perfect (you might have been a couple of mm off at one end, or the wood might not be dead straight, bending a little in the middle, etc), the temperature, humidity, and air pressure in your room might be different from what the equations assumed (thus, the wood might have different dimensions and mass, due to absorbed humidity or higher/lower temperatures), the equations you used are probably simplified versions that make a number of other assumptions, etc. There are so many variables that could throw off the actual tuning of your device, quite far from where it should be just from using the simple equations.
So it might turn out that your real modal frequency was 81.9 Hz, and your device is actually tuned to 95.1 Hz.... Way too far apart to be of any use. Therefore, you need to build the device is such a way that you can adjust the tuning AFTER it is complete, to match the actual modal resonance. If not, your chances of it working very low.
The there's the issue of location: If the mode in question is axial across the room, but you put the device on the rear wall, then it won't actually do anything useful! It has to be located in a pressure peak for that mode.
But let's say you eventually do manage to tune the device so it actually does hit the mode precisely, and then you put your broadband absorber in front of that: the mere presence of that thick insulation in front of the device can once again change the tuning, so you might need to re-adjust the tuning one more time, to account for the difference. That's the point I was getting at with my initial comment: "... you might want to experiment before you commit to that."
As I said, your plan can work, but it will need some experimentation with the tuning to actually achieve it.
And one more issue: Let's say you do manage to do all of the above, and tune it just right, located in the correct spot.... Is it big enough to kill the problem? It might work just fine, but if it isn't physically large enough to remove enough energy from that mode, then you wasted your time! Let's say, for argument's sake, that the mode is causing a 12 dB peak at 86 Hz, and you build a device tuned to exactly that, but it is only big enough to absorb 25% of the total energy in that mode (unlikely, but possible): that's only going to knock it down a point or two.... Think of it this way: that mode affects the entire wall... it is sloshing back and forth across the entire room, from side to side, affecting all of the surface area of that wall. Let's assume your wall is 5m long and your ceiling is 3m high, this that wall is 15m2. If your device is 50cm wide by 50cm high, it only covers 0.25 m2, or 1.6% of the wall area... How much of the total energy in that wave do you think it can absorb? Let's say you make a dozen such devices and stack them six high in the room corners: that's a lot better, yes, but you are still only covering 10% of that wall area. Is that enough to kill the mode?
I'm sure Andre can tell you exactly how much area you'd need to cover to have a decent effect, but a small device is not going to achieve a lot.
Which leads to the next issue: Let's say you do achieve a good result with your dozen devices stacked floor to ceiling at both ends of the wall: But now you find there's another axial mode in that direction that is also strong, plus a couple of tangential modes affecting that same wall, plus a smaller issue with the first oblique mode.... Where are you going to put the additional tuned traps to deal with those? You have no more space left, since you used it all up already. You can't layer several tuned membrane traps over each other...
But all of this begs the question: Why do you need to target that specific frequency? Have you measured the room response, and determined that your only problem in there is that one specific width mode? Or are you just predicting that you might have a problem there, using a room mode calculator?
All of the above is why I seldom used tuned membrane traps as the first option to treat a room. I only use them if there is still a stubborn modal issue left after I have gone the usual route of deep "bass trapping", and I just can't seem to hit that mode any other way. And then I suffer greatly, trying to tune the damn thing where I want it... and swear and curse because I can't make it big enough to get all the effect I want...
Hopefully Andre will be able to summarize what I just said, in one of his classic comments of about seven words or less, as he normally does!
Diffusely,
- Stuart -
The No.1 Website for Pro Audio
Latest
Trending
