MartinC

'Bass Trap' panel tests

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Moving my subwoofer to a different corner of my lounge has given me space to experiment with a larger 'bass trap'. I have twelve acoustic absorber panels made from 1200 x 600 x 100 mm Rockwool sheets in open edged wooden frames. These are panels I made myself to experiment with but the basic absorbing material is not wildly different in commercial products. For anyone interested here are the results I measured using some example arrangements, which demonstrate the expected result that absorption needs to be big to do much at the lowest frequencies.

I'm going to show measurements using just my subwoofer as the sound source. With no absorbers in place the frequency response at my current listening position is as follows. This is with no filters or EQ applied to the sub but is with the listening position and sub location chosen to make this as 'flat' as possible. Note that I've unusually chosen to use a linear frequency scale so the lowest frequencies don't dominate the plot as much. No smoothing has been applied to this and all subsequent graphs, and the microphone was not moved throughout.

510481944_Nopanels.jpg.0585f2d8c3bd6a2c40851f2470b68f9b.jpg

First I hung up a single panel across the corner, made a measurement, and then added another two so that it looked like this:

754343551_3panelssmall.thumb.jpg.b109621b445a55a22d4988e712b12c44.jpg

The cat is called Raffles BTW :). The effects of one and three panels were as follows:

697910351_3panels.jpg.634033c4b4adb98c5e28444102b9e320.jpg

Below 100 Hz the changes are pretty modest (I'll post some waterfall plots later but the same holds). The most notable change higher up is the removal of a big dip at about 143 Hz but the addition of a big dip just below 130 Hz. I've seen changes like this before and I assume the appearance of new dips results from less destructive interference.

I then added a further three panels so that it looked like this:

16762678_6panels.thumb.jpg.dcceda2d8f5a3ca57c0be2d10509f7c3.jpg

Having measured the above setup, I added another three panels, making 9 in total. (The fronts of the last three have a thin foil layer on the front which was to make the panels reflect higher frequencies.) This was now taking up a fair bit of space! I added the cushion just to stop the panels falling over, although i guess it added a tiny bit more absorption.

1258736933_9panelssmall.thumb.jpg.4bd1ce54f452077429a914842c46e9cf.jpg

Excluding the one panel curve to make things clearer, here are the results:

1232324991_9panels.jpg.79f6c01bea9f9673ce5726961eb7dc51.jpg

Having the much larger amounts of absorber in place (and importantly further into the room) does show meaningful effects below 100 Hz. Really noticeably so at the 47 Hz peak, although less so at the lowest frequency peak (34 Hz).

(Waterfall plots to follow in next post.)

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As would be expected, waterfall plots show decay reductions corresponding to the reduction in peak amplitudes.

With no panels:

151239893_Nopanelwaterfall_linear.jpg.e58e12694187c7c5eb0b4bb5de37eb7a.jpg

With one panel:

1059613221_1panelwaterfall_linear.jpg.45b72837bffbf3949a5e85ea91e5237d.jpg

With 3 panels:

290065878_3panelswaterfall_linear.jpg.bf7307b81ab71443c169e821fde3254c.jpg

With 6 panels:

852119141_6panelswaterfall_linear.jpg.08d343be002bd19802aa36f23a085c76.jpg

With 9 panels:

1550988581_9panelswaterfall_linear.jpg.13088e70d8d177068d0d7d0687ac4371.jpg

Edited by MartinC
Log scales plots placed with linear scale plots for consistency.

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It would have been interesting to see measurements from another listening spot position.

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20 minutes ago, tuga said:

Martin, what's the amplitude level at 1kHz?

I didn't measure above 500 Hz but as I was just using my sub it wouldn't have been much :).

17 minutes ago, tuga said:

Results look promising by the way.

I mostly tried this out of curiosity rather than as a definite plan to have a large bass trap in that corner. If my sub doesn't go back into that corner it's just about possible I could decide to put a bass trap there as part of redecorating the room but I'll probably stick with just using EQ I think. 

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Actually @tuga I did make a few measurements with my main speakers run full range and no sub which you may be interested in. Here's a no panels vs all 9 comparison for you (with 1/24th octave smoothing):

1332460242_Leftmain.jpg.cf01d7b47e96aeba7693285d0255711f.jpg

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Thanks for taking the trouble to post your experiments and findings; very interesting, although looking at your photos I think ?I’ll stick to DSP!

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Just now, Camverton said:

Thanks for taking the trouble to post your experiments and findings; very interesting, although looking at your photos I think ?I’ll stick to DSP!

Indeed :). I mostly did this to give some context to panels vs EQ discussions.

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The 9 panels certainly seem to have had an impact a 45Hz. It’s hard to imagine the size and depth of panel needed for the 33Hz one...

interesting experiment, thanks for posting.

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33 minutes ago, MartinC said:

Actually @tuga I did make a few measurements with my main speakers run full range and no sub which you may be interested in. Here's a no panels vs all 9 comparison for you (with 1/24th octave smoothing):

1332460242_Leftmain.jpg.cf01d7b47e96aeba7693285d0255711f.jpg

The panels seem to be operating between 40 and 80Hz.

It would be interesting to move the mic back and forth and see if you can reduce the depth of the 70something null and/or the 34Hz peak, making the sub-100Hz a bit more even.

I think that a combination of panels and DSP EQ would go a long way.

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8 minutes ago, tuga said:

It would be interesting to move the mic back and forth and see if you can reduce the depth of the 70something null and/or the 34Hz peak, making the sub-100Hz a bit more even.

I did just this a few days ago :). The red curve below is roughly where the microphone was for the above panel tests, and the other positions were all further back (distances in the legend are from the back wall). The 34 Hz peak is the primary longitudinal room mode and so predictably the amplitude of this peak decreases the closer I get to the middle of the room.

1826349534_MLPtests.jpg.d3f2d28440be25429a171fb78b2b9901.jpg

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You got 2 sets of panels at the bottom. Would leaving a gap of say 4” - 6” between them help?

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38 minutes ago, tIANcI said:

You got 2 sets of panels at the bottom. Would leaving a gap of say 4” - 6” between them help?

Bringing the front set of three even further into the room? Yes, I'd expect that to increase absorption at the lowest frequencies more. It would take up even more space though :).

In case you're interested the end of the following link shows a measurement for a gapped corner absorber example, although I assume they got the legend on the graph wrong as it doesn't match the description in the text. I'm pretty sure the green curve is the nested result.

http://forum.studiotips.com/viewtopic.php?p=3969#3969

For info. the simple corner absorber approach in that link was what I had in mind when I made my panels. Putting these around the room across multiple corners had pretty minimal effect below 100 Hz though, with the results in this thread being much more significant, especially at 47 Hz. This makes sense in terms of having more absorbing material further from the walls (where particle velocity will be higher at the lowest frequencies).

Note that I just did this as a quick test rather than trying to find or demonstrate what might be optimal. Before someone mentions it, I'm well aware that as corner absorbers get deeper the optimal absorber material to use changes. My panels are more suited to being used in thinner layers than to totally fill a volume as large as the corner I ended up blocking off. I have done some other tests with lower density (strictly lower GFR) loft insulation but not over such a large volume.

Edited by MartinC

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