Treatment of a small boxy room - REW file attached

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Greyhound
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Treatment of a small boxy room - REW file attached

#46

Postby Greyhound » Fri, 2022-Dec-09, 14:14

Of course it's likely impossible to know, with any certainty, the actual flow resistivity values for locally available materials so the only value of these simulations is to help come up with a design that can be tuned to account for a potentially wide range of (unknowable) material parameters.



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Treatment of a small boxy room - REW file attached

#47

Postby gullfo » Sat, 2022-Dec-10, 15:26

having a variable "mouth" approach can assist. build with approx % openings on two layers and slide to adjust. this approach works on even a single long slot. then it becomes an empirical exercise instead of simply being a theory :-)
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EXAMPLE HH BOX WITH ADJ SLOTS.skp
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EXAMPLE HH BOX WITH ADJ SLOTS.skp
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EXAMPLE HH BOX WITH ADJ SLOTS0004.jpg
EXAMPLE HH BOX WITH ADJ SLOTS0003.jpg
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EXAMPLE HH BOX WITH ADJ SLOTS0001.jpg



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Treatment of a small boxy room - REW file attached

#48

Postby Greyhound » Sun, 2022-Dec-11, 19:21

Thanks Glen!

Man that's a clever idea, particularly as it seems like the shot of hitting resonance on the first shot would be like winning the lottery!

Now where did I put my CNC router... :lol:



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Treatment of a small boxy room - REW file attached

#49

Postby Greyhound » Mon, 2022-Dec-12, 14:12

I'm thinking of taking a first stab at the resonator using a hole-based configuration, as this will be easier to fabricate at the moment.

I'll start with a small hole size, and work upwards in 1/64" increments to "tune" it. I think it will be important to drill the holes with a sacrificial backing piece to avoid messy "tear through", and maybe do a quick de-burring pass with a countersink or similar.

It would look something like this - I can get 3/4" MDF in quantity at low cost, so experimenting within reason is acceptable (I have lots of other projects where I could recycle any "reject" panels). The panel dimensions are 22 inches wide by 6 feet high, based on what I could fit in the corners of my room. The cavity would be 5.5" deep.

If there are any physics reasons why this arrangement looks "off" then I'd be grateful to hear before I go nuts with the drill! I suppose Glen's "variable mouth" approach could also work with circular apertures (unless something about this geometry/symmetry is fundamentally different).

Image 2022-12-12 at 12.03 PM.jpeg


Image 2022-12-12 at 12.00 PM.jpeg


The plan would be to put these "coffins" in the back corners of my room - tests using the "walking mic" approach indicate very high SPL at 47 Hz in these locations, presumably due to the near-identical length and width axes.



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Treatment of a small boxy room - REW file attached

#50

Postby Greyhound » Mon, 2022-Dec-12, 14:37

One concern is that the low frequency (47Hz) of the mode I'm trying to address, along with other design constraints, lead to this 4" hole spacing as being pretty much the minimum separation for achieving the desired resonance. If they are any closer together, the required hole sizes become very small to an extent that seems to place unrealistic demands on precision.

I wonder if the relatively low density of holes may translate into a relatively small absorption effect even given the relatively large area. I'm curious enough to just give it a shot, but still interested in any feedback! One possibility would be to double the depth of the cavity, although this would then start blocking a window...

Another way of saying this is: I wonder if a shallow Helmholtz resonator, due to the small fractional hole area required to hit my 47Hz mode, will have an inherently low efficiency? I guess looking closer at the equations should give some idea, since the output is 'absorption coefficient'. It's probably relevant to figure out how the absorption is normalized to unit area...



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Treatment of a small boxy room - REW file attached

#51

Postby Greyhound » Mon, 2022-Dec-12, 17:16

Here's a simulation showing how increasing the hole size, by using successively larger sizes of fractional inch drill bits, should affect the resonance frequency (as before this is for 4" hole spacing in a 3/4" front plate, with a 5.5" cavity in back filled with mineral wool).

Image 2022-12-12 at 3.10 PM.jpeg


Each blue line shows the resonance curve for a different drill bit size, as indicated in the legend. The lines are not colour coded, but the resonant frequency increases systematically with increasing drill bit size (up to 9/32"). The simulations account for the fact that these are diameters, whereas the HH equations are based on radius.

The title at the top of the plot is for the largest bit size of 9/32".



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Treatment of a small boxy room - REW file attached

#52

Postby Greyhound » Sun, 2022-Dec-25, 13:09

So I threw together a couple of boxes as described above, and they actually do resonate!

It was just a preliminary test, without any absorber and without final tuning, but it's cool to see the effect in action.

To be clear, there is no measurable impact on room acoustics at all yet. I had to use a candle flame to visualize the pressure disturbance occurring at resonance:

Helmholtz 40Hz Candle.jpg


Here is a YouTube Video showing the effect.

I think this will probably warrant a dedicated thread at some point...

Merry Christmas!



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Treatment of a small boxy room - REW file attached

#53

Postby RJHollins » Sun, 2022-Dec-25, 21:34

cool idea for using the candle 8-)



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Treatment of a small boxy room - REW file attached

#54

Postby Greyhound » Sun, 2022-Dec-25, 22:26

RJHollins wrote:Source of the post cool idea for using the candle 8-)


Thanks! If I'd gone by room measurements alone, I'd have sworn it was doing absolutely nothing. I am trying to do this systematically, one step at a time. Knowing that it actually resonates is really helpful. Adding absorbent material will shift the resonant frequency (probably slightly).

I'm still not sure if this will actually help with my room modes, but at I'd like to at least understand why they don't if that is the outcome!



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Treatment of a small boxy room - REW file attached

#55

Postby gullfo » Mon, 2022-Dec-26, 12:18

the box needs to sit at the modal pressure point (e.g. 50% of the room length to address side-to-side, ot 50% of back (or front) wall to address end-to-end) -- in effect -- neutralise the offending frequency which then drops the amplitude of that frequency and related harmonics. adding some insulation into the box will increase the bandwidth (widen Q) but reduce the intensity of the primary frequency.
as a note: nothing can fix a null except to shift where they happen and counter the frequency to alter its phase levels which form the null (hence room shaping with angled panels etc) and generally "remove the room" where possible (shapes, positions, listening volume, etc)



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Treatment of a small boxy room - REW file attached

#56

Postby Greyhound » Mon, 2022-Dec-26, 15:01

gullfo wrote:Source of the post the box needs to sit at the modal pressure point (e.g. 50% of the room length to address side-to-side, ot 50% of back (or front) wall to address end-to-end) -- in effect -- neutralise the offending frequency which then drops the amplitude of that frequency and related harmonics. adding some insulation into the box will increase the bandwidth (widen Q) but reduce the intensity of the primary frequency.
as a note: nothing can fix a null except to shift where they happen and counter the frequency to alter its phase levels which form the null (hence room shaping with angled panels etc) and generally "remove the room" where possible (shapes, positions, listening volume, etc)


Thanks Glen,

I thought that the pressure maxima for this (1,0,0) mode would be at the room boundaries, and that 50% back-to-front was the velocity maximum (and pressure minimum). I am still getting my head around what causes modal nulls, and appreciate the comment.

Perhaps I have misunderstood something in regards to modal pressure points - it would be interesting to move the box around the room, to see what effect this has. My understanding was that the boxes should be at the maximum pressure points, which would be the vertical corners in my case since front/back and left/right dimensions are the same (although with lesser symmetry along the left/right axis due to doorway/windows/bulkhead).



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Treatment of a small boxy room - REW file attached

#57

Postby gullfo » Mon, 2022-Dec-26, 18:46

however, the nulls occur in the room when you have the pressure wave rarefication at multiples (e.g. 50%, 33%, etc) depending on the wavelength. we use the term "waves" but its pressure of particles being compressed and released. and because the pressures are not simply linear - you have oblique and tangential pressures influencing the overall modal strength at particular location. so corners are converged points but not necessarily the best spot for pressure / resonant trapping if you're focused on a single problem. corners are ideal for resonant traps which support multiple frequencies - top and bottom - LF, middle - midrange. divide the sections to improve efficiencies rather than a single compartment (the differing openings / spacings will result in less effective absorption for a given set)



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Treatment of a small boxy room - REW file attached

#58

Postby Greyhound » Tue, 2022-Dec-27, 11:08

Ah ok that clarifies a couple of things, thanks.

One source of confusion is that we can talk about "spatial nulls", where the SPL passes through a minimum - for a specific modal frequency - as we move through the room in the direction of wave propagation, and also "frequency nulls" where the SPL curve drops (often abruptly) at the listening position for a particular frequency.

It seems clear, and is borne out by experience, that the SPL around a spatial null - for a specific frequency - will vary sinusoidally as we walk in and out of the null position. Frequency nulls (or dips) at the listening position seem to have a much sharper profile but I guess that's to be expected since these may be the result of destructive interference from different frequencies/wavelengths.

I suppose these are two manifestations of the same thing, but I'm still trying to reconcile how one can get such sharp frequency nulls/dips at the listening position. Does this mean that there just happens to be a spatial null, at the listening position, for the frequency in question? or is it a more complicated destructive interference between several modal frequencies? I guess it could be some combination of both.



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Treatment of a small boxy room - REW file attached

#59

Postby gullfo » Tue, 2022-Dec-27, 13:45

size matters. so does frequency. smaller rooms, higher frequencies, sharper null and peaks. and yes, oblique and tangential modes as well as SBIR cause the unevenness. a perfect (or really flat response) room is a large canvas tent in a slightly curving hills open field of tall grass. and depending on if you're doing anything other than nature recordings, it can be problematic - no isolation in or out, canvas kind of sounds meh for drum recordings, etc. so one of the first questions we ask people - how much isolation do you need? because with great isolation comes great acoustic evils...



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Treatment of a small boxy room - REW file attached

#60

Postby Greyhound » Tue, 2022-Dec-27, 14:24

gullfo wrote:Source of the post because with great isolation comes great acoustic evils...


That certainly applies in this room, which is very well isolated by residential standards. Front wall, side walls, and floor are enveloped by concrete (outside the drywall and hardwood floor). Front left/right walls are 2x 5/8" drywall on wood studs, back wall is 2x5/8" drywall on resilient channel. Ceiling is 3x5/8" drywall on resilient channel. The isolation is more than adequate for my needs (and I need it for both containment and isolation).

I can meet many of my immediate needs with headphones, but the room treatment is kind of an ongoing physics experiment.




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