Wall counterweight to reduce LF room mode

[bachatero]

I'm working on my gym hifi's acoustics and I'm tackling the LF range.

When its subwoofer makes sound, the gym's corrugated steel walls literally vibrate like a house in an earthquake. I know this is due to a room mode down in the low range (the space is a hundred feet wide), but covering everything in acoustic treatment is basically impractical due to cost.

So, I was wondering if it's possible to reduce LF room modes by mounting counterweights to the walls to create an impedance mismatch and thereby eliminate this resonance. What the counterweights are made of probably doesn't matter, but an acoustic material would also as a bonus help keep the HF/MF reverb down.

Is this a reasonable strategy?

 

[rationaltime]

Before making modifications you might verify the wall sheeting is the source
of resonance. Did you try striking the wall with a soft hammer (such as the sole
of an athletic shoe) to see if that excites the same resonance? If you think that
is the problem I suggest driving more screws to stiffen the wall. That should
raise the resonant frequency.

You might make a drawing of the room with measurements and post it here.

 

[kemmler3D]

More screws is a good idea if there are good spots to attach the walls to something.

I think the weights idea sounds reasonable in theory... maybe you could try it on 1-2 panels first to see if it works.

Have you done any acoustic measurements in the space?

 

[bachatero]

Did you try striking the wall with a soft hammer (such as the sole
of an athletic shoe) to see if that excites the same resonance?

You might make a drawing of the room with measurements and post it here.

I'll try this out and see if there actually is resonance along with some real measurements.

If you think that is the problem I suggest driving more screws to stiffen the wall. That should
raise the resonant frequency.

More screws is a good idea if there are good spots to attach the walls to something.

This might be a very practical option as it just comes down to finding something to attach the walls to. There are already some steel beams that the walls attach to, but not much else is available. However, a very stiff item such as a steel plate with some acoustic foam/fiberglass on it might help stiffen the wall if it's attached at multiple points.

Have you done any acoustic measurements in the space?

I've only done a few trivial things like listening to the audible reverb after playing an impulse and pink noise. When I get the next opportunity, I'll bring a UMIK-1 and a laptop to get some proper measurements.

Regarding the reverb, it sounds diffused and not particularly intense in one area of the spectrum. That's likely due to so much stuff sitting in the gym and its different shapes helping diffuse the sound. This got me thinking that a simple treatment like rugs can help a lot because much of the sound will point downward and mainly leave LF left to deal with.

 

[Audio Science Pal]

So, I was wondering if it's possible to reduce LF room modes by mounting counterweights to the walls ...

Have you looked into having spray foam wall insulation applied to some of your metal walls?

 

[rationaltime]

If there is no structure on the back of the wall, adding some would help
stiffen the wall. The corrugation adds stiffness in one direction. Adding
horizontal members would help in the horizontal direction. I think adding
plates is not the answer.

However, in my opinion it would be good to get some local expertise,
maybe a metal building place. Also before making modifications give
some thought to what other improvements might be useful, electrical
distribution, thermal insulation, plumbing, windows, that sort or thing.

That is my opinion.

 

[DonH56]

Room modes are due to room dimensions. Screwing the walls down may help keep them from vibrating as much, but making the walls stiffer would actually increase the amplitude of the modal frequency since a stiffer wall means stronger reflections. I do not see how a counterweight would help; it would have to react at the same frequency and in opposition to the waveform. Pressure is highest right at a boundary (and velocity is zero) so adding a number of thick (4"~6") absorber panels on standoffs (maybe 2"~4") would likely do more to reduce the rattling. You can mount the walls on isolators such as Kinetics Noise Controls IsoMax clips so they can move (vibrate) to reduce the reflections. Or just use an equalizer to cut (notch) the offending resonance frequency (that's probably what I would do). Or turn down the volume.

 

[raindance]

Eq out the resonance. Done. But obviously you'd need to measure to find it.

 

[bachatero]

Or just use an equalizer to cut (notch) the offending resonance frequency (that's probably what I would do). Or turn down the volume.

Eq out the resonance. Done. But obviously you'd need to measure to find it.

I'd love to do this right now, but there are a couple hitches:

• I need something simple that can just do EQ without fuss, and the options aren't looking so good right now. Check out https://www.audiosciencereview.com/...le-inline-dsp-equalizer-is-it-possible.50759/. There is one more option I didn't mention, and it's the built in EQ to the EV ETX speakers once I upgrade to those later this year. But because I don't have those yet, I'm holding out on EQ as a contingency.
• EQ will stop the resonance just fine but only in the one spot where I measure it. As the mat where I train in front of the speakers is HUGE, the sweet spot (or area?) should be as large as possible.

 

[DonH56]

I'd love to do this right now, but there are a couple hitches:

• I need something simple that can just do EQ without fuss, and the options aren't looking so good right now. Check out https://www.audiosciencereview.com/...le-inline-dsp-equalizer-is-it-possible.50759/. There is one more option I didn't mention, and it's the built in EQ to the EV ETX speakers once I upgrade to those later this year. But because I don't have those yet, I'm holding out on EQ as a contingency.
• EQ will stop the resonance just fine but only in the one spot where I measure it. As the mat where I train in front of the speakers is HUGE, the sweet spot (or area?) should be as large as possible.

There are many choices for analog and digital equalizers. I am surprised a gym system does not have one already since you usually need to time-align multiple speakers to fill such a large area. You could use an inexpensive (~$100) miniDSP to create a notch filter for the resonance frequency. That may be enough and reducing just one frequency will likely have minimal impact on the overall sound. Since it is a room mode, at least that is our assumption, that is the only frequency that matters and it matters at the vibrating walls and not anywhere else in the room.

Room mode calculator: https://www.harman.com/audio-innovations -- scroll to the bottom for calculators, but it's worth reading the articles on room modes and subwoofer placement (sub or mains, the info is applicable).

HTH - Don

 

[bachatero]

Ok, I got some real measurements and the gym is only 50 feet wide and not a hundred like I said. That equates to a room mode of about 1 / (50 / (3.28 * 342)) or 22.5 Hz. The third harmonic of that is about 67 Hz and that corroborates what I found using my tone generator. Therefore, I can just use the subwoofer's built in PEQ (it is already an EV ETX) to reduce 67 Hz and now we're a lot better.

Update: I implemented the fix (change 66 Hz by -3 because 66 was closer to the actual harmonic) and I don't know if it's a placebo or not, but music sounds different.