Thread: BBC Report now available: Absorber for Very Low Frequenc

1. BBC Report now available: Absorber for Very Low Frequenc

Those guys at the BBC are very cool. I noticed the other day that they had listed but not yet posted a report called "The Design of a Modular Sound Absorber for Very Low Frequencies."

There was a little notice on their website saying if you see a report which hasn't yet been converted to PDF, request it. Well, I requested it just a few days ago, and John Barrett of the BBC kindly responded, and lo and behold, it's already converted and posted. Here's the link:

LF Absorber PDF

Lee

2. Re: BBC Report now available: Absorber for Very Low Frequenc

very good stuff lee.
thanks

3. Re: BBC Report now available: Absorber for Very Low Frequenc

Is it just me, or is there a mistake in section 7.3.2? Fletcher states that the "theoretical frequency of the limp panel resonance over the enclosed air space of this box would be 22 Hz." But according to my calculations, the expected resonance frequency would be 63 Hz, and in fact it was measured to be 56 Hz.

The flexible barrier mat had a mass of 5 kg/m2 (about 1 pound/sq ft). The box is 184 mm deep (about 7.25 inches).

If you plug these numbers into your formula, what do you get?

Lee

4. Re: BBC Report now available: Absorber for Very Low Frequenc

On another issue, section 7.2.3 on Multiple Hole Boxes provides some new information. Until now I used to think that to calculate the frequency of a Helmholtz resonator, you just use the sum of the openings. Like Fletcher says, "A Helmholtz resonator with two ports is theoretically simlar to one with a port of the combined area." Yet his tests show that "the behaviour seemed to be as if the box were divided internally into two halves with one hole in each."

This suggests that if you build a slat resonator wall, using smaller slots in one area and larger slots in another may in fact make it operate in a more broadband fashion - even without building separate chambers internally. Same goes for angling it, so that the depth behind the slats varies.

Or, as a wise man once said, "In theory, theory and practice are one and the same. But in practice they are not."

Lee

5. Re: BBC Report now available: Absorber for Very Low Frequenc

Thanks Lee. I just downloaded the file and haven't had a chance to look at it yet. Will look for those items you detailed.

6. Re: BBC Report now available: Absorber for Very Low Frequenc

The thing I found interesting was that on the membrane absorber, the version that had mineral wool inside was actually the WORST. This goes against the conventional wisdom here, and also pisses me off because I've already built a bunch of traps with rockwool inside.

Rockwool inside = Not much absorbtion, period.
Fluffy fiberglass inside = Good balance between Q and absorbtion.
Nothing inside = Q is way too peaky, though it absorbs the most.

Should we all be putting fluffy fiberglass in our panel traps, instead of 703 or mineral wool?

7. Re: BBC Report now available: Absorber for Very Low Frequenc

dasbin, I think the problem with their rockwool traps may be that the stuff was touching the membrane, wasn't it?

An advantage to rigid fiberglass is that it can be spaced a 1/4" away from the membrane so that the membrane can still vibrate at max rate, yet still be damped by the choked off air inside the trap.

But it may be interesting to use fluffy fiberglass - unfaced - just touching the membrane to damp it ever so gently. I wonder if anyone else has done this and what they came up with?

Lee

8. Re: BBC Report now available: Absorber for Very Low Frequenc

Lee, not sure how well that would work - the theory behind membrane/panel traps is that the membrane is placed at highest sound pressure (not velocity) - this vibrates the membrane, which in turn sets the inside air in motion, which then has to pass through the inside absorbent - trapping is accomplished both by the flexing of the panel (friction) and secondarily by the absorbent's acoustic resistance converting part of the wave to heat (same as a purely absorbent trap in that respect)

Sooo, it seems like allowing the fiberglas to touch the panel would kill too much of the flex as well as reduce the inside air wave that's trying to get through the absorbent.

One possible way to "rube goldberg" a test of this might be to build a panel absorber but fasten the BACK to cleats using screws and sealing only with weather strip - make two sets of holes in the rear of the sides to fasten the cleats holding the back - fasten all the "guts" of the trap, including the insulation, so that they are referenced to the back of the trap - then, test the trap with the back shoved forward enough to just engage the panel with insulation touching it - finally, remove the screws holding the back and move it to its rearmost position (covering the old screw holes with a couple layers of duct tape temporarily) and test again. Obviously the center frequency would change due to different trap depth, but you might be able to tell how much difference there was in the Q of the trap that way. There might even be a way to just move the absorbent without changing the volume of the trap, but it would be a bit trickier.

Just a thought... Steve

9. Re: BBC Report now available: Absorber for Very Low Frequenc

That seems like a good experiment.

I've also wondered what would happen if you left a portion of the inside unfilled with anything. Does the insulation need to "block off" any possible air movement path within the trap?

It seems like there's still an incredible amount of room for real-world test with bass traps. Kind of interesting. I would be interested in maybe one day getting a bunch of like-minded individuals together to fund the testing for all these kinds of things, and perhaps selling the resulting article to a recording magazine or something.

10. Re: BBC Report now available: Absorber for Very Low Frequenc

Dasbin,

> on the membrane absorber, the version that had mineral wool inside was actually the WORST <

I'd take that with a grain of salt. I've measured wood panel traps, and the ones I've built have very high absorption at the center frequency.

--Ethan

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