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

#61

Postby Greyhound » Wed, 2022-Dec-28, 21:06

As a thought experiment, to try and better understand Helmholtz resonance, it might help to figure out the answer to the following question:

Will a resonator without any absorbent material (damping) act as an absorber at the resonant frequency? or will it act as an additional sound source due to excitation of the undamped spring/mass system?



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Treatment of a small boxy room - REW file attached

#62

Postby gullfo » Thu, 2022-Dec-29, 15:22

if the chamber is damped so it's not vibrating (much), and there is nothing inside the chamber, it will have a high Q (small bandwidth) and will also be a bit more efficient at its resonant frequency. if the chamber is not damped, you'll likely have coincident resonance as well as harmonics based on the material -- some of which will have reinforcement and some neutralisation. as a general design rule - start with more broadband attenuation strategies, and then when you have problems which need point solutions, you can use those. using PVC tubes which are tuned (often effective because they're adjustable and can have a number placed where most effective) are also good for "tuning" the room.



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Treatment of a small boxy room - REW file attached

#63

Postby Greyhound » Fri, 2022-Dec-30, 10:45

Thanks Glen,

I guess I was wondering specifically about the intended "harmonic oscillator" function of the Helmholtz cells, but you raise a good point that there could be other "coincidental" resonances that are unrelated to this. My big boxes are certainly likely to have many vibrational modes in addition to the intended Helmholtz resonance.

If I'm reading correctly, an "ideal" Helmholz resonator (with no resonances other than the intended one) with no damping would tend to amplify sound at the resonant frequency, rather than absorb it.

I see the logic of starting with broadband (assuming you mean porous absorbers), and will likely go this route for the practical treatment of the room. However, the physics of the Helmholtz resonator is interesting and it seems to produce physical effects that are easy to visualize directly (even when there is no impact on REW room curves).

I was thinking about PVC (or ABS) tubes, as this would probably be easier to control (in terms of tuning and damping) than a big box with undivided cells. One advantage is that one could presumably work out the tuning/damping parameters on a single cell, and then scale to multiple cells. With the "perforated box" approach, you kind of have to build the whole thing and incremental testing is a lot more laborious.



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Treatment of a small boxy room - REW file attached

#64

Postby gullfo » Fri, 2022-Dec-30, 14:17

the idea behind the HH resonator is to neutralise by the chamber trapping the energy at the specific frequency. i.e. it's an energy conversion since the device itself resonates (regardless of damping of overtones) which converts...
yeah, the tubes make placement and variety much easier but typically i'm only really using those in rooms where i have several feet of space above the "room" boundaries to hide them. and then they're usually for specific "notes" which need to be balanced (e.g. a D#3 is a bit low due to a slight attenuation, a 4" tube tuned to D#3 can bring that back so the listening position is nearly flat across the entire piano range.)
when you have enough general "flatness" to the room, more particular the critical listening position, then variations in notes becomes more apparent... similar to isolation - once you hit 45-50db in a single slab pair of rooms (example) then you start to hear all the other sounds coming through the structure, wires, plumbing, ducts, windows, doors etc. and then worse, some folks opt to nearly anechoic room which brings out the self-noise / tinnitus for which there is no room treatment options... :-)



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Treatment of a small boxy room - REW file attached

#65

Postby Greyhound » Sat, 2022-Dec-31, 02:01

Thanks Glen!

I did another experiment to see if I could understand this, partly because there have been so many threads (elsewhere) on unsuccessful HH traps.

You are right, and an undamped HH resonator will indeed amplify sound around the mouth. I flooded my room with a 200Hz test tone and played around with a 500ml soda bottle, which resonates very close to that frequency. As you add more damping, there is a specific range where the resonator seems to exhibit peak absorption.

Here's a plot of SPL, with the room "flooded" with 200Hz, as the mouth of the bottle is brought up to the mic with progressively more cotton balls inside:

Helmholtz 200Hz Balls.jpg


I made an extremely boring video of the tests.



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Treatment of a small boxy room - REW file attached

#66

Postby Greyhound » Sun, 2023-Jan-01, 17:20

As a control experiment, I did the same thing with the coke bottle but with test tones that were different from the undamped resonant frequency of 200Hz.

Interestingly, at slightly higher frequencies the SPL around the bottle mouth was attenuated with an empty bottle. The attenuation diminished as cotton balls were added. The series of dips in the SPL plot below occur when the mouth of the soda bottle was brought close to the measurement mic while a set tone of 225Hz was played in the room. The left-most dip is from the empty bottle, with five cotton balls added for each of the successive dips moving to the right.

D3B93648-E53F-47C1-9FE5-6836EE3FE923.jpeg


It's hard to say if the dips in this plot and the one shown earlier (for the 200Hz test tone) are due to resonant absorption or some other effect like a blocking of the sound field.

I did other control tests with the bottle flipped around so that bottom was presented to the mic, and this did not change the sound level at all. I also tried at other frequencies, with similar effects, but this is getting somewhat outside the scope of this thread and forum.



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Treatment of a small boxy room - REW file attached

#67

Postby gullfo » Sun, 2023-Jan-01, 18:48

phase reversal



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Treatment of a small boxy room - REW file attached

#68

Postby Greyhound » Sun, 2023-Jan-01, 19:27

gullfo wrote:Source of the post phase reversal


Cool - so sound at the test frequency is coming back out of the (not resonating) bottle, out of phase with the ambient sound field, and so is attenuating by destructive interference?

The wavelength of the 225Hz tone should be around 1.5m, but I suppose the reactance of the bottle must be budging the phase enough to cause cancellation. I guess this is just like the general comb filtering issue with any reflective surface?



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#69

Postby gullfo » Sun, 2023-Jan-01, 19:50

yes, only we plan it :-)



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Treatment of a small boxy room - REW file attached

#70

Postby endorka » Sun, 2023-Jan-01, 20:56

This is fascinating stuff, thank you for posting these trials and observations. I've been waiting some time to see this type of thing done for Helmholtz resonators.

Cheers!
Jennifer



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Treatment of a small boxy room - REW file attached

#71

Postby Greyhound » Mon, 2023-Jan-02, 14:14

gullfo wrote:Source of the post phase reversal


What's also interesting is that tones below the resonant bandwidth appear to increase the local SPL slightly, whereas tones above the resonance band decrease the SPL as shown in the plot above for 225Hz.

Since resonance is supposed to happen when the imaginary part of the resonator's impedance passes through zero, and since it's this imaginary component that determines the phase shift (if any), this is consistent with the lower frequencies getting a "constructive" phase shift while higher frequencies get a "destructive" phase shift.

I'm still trying to figure out how this fits into the absorption vs frequency behaviour of a practical HH resonator, and recall that Cox & D'Antonio point out that the concept of "absorption coefficient" is problematical at low frequencies since the sound field is not at all diffuse.

It's important to acknowledge that what I'm measuring with my bottle/cotton/mic tests is not "absorption" as it is formally defined, but there should be some relation between this observed behaviour and what is happening in a practical room treatment situation.

For one thing, the angle of incidence of the wavefront reaching the bottle mouth is not controlled at all. Clearly this does not matter in terms of exciting resonance, but it could be important in terms of absorption-like behaviour.



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Treatment of a small boxy room - REW file attached

#72

Postby Greyhound » Mon, 2023-Jan-02, 14:15

endorka wrote:Source of the post This is fascinating stuff, thank you for posting these trials and observations. I've been waiting some time to see this type of thing done for Helmholtz resonators.

Cheers!
Jennifer


Thanks I'm glad this is of interest!

It probably seems like I'm obsessing over little details, but I've read so many threads about HH traps that were carefully designed from online calculators (often with considerable effort) and which end up doing nothing. The resonant approach seems so promising, but it does not seem realistic to get a working trap without some experimentation and careful testing.



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Treatment of a small boxy room - REW file attached

#73

Postby RJHollins » Mon, 2023-Jan-02, 19:46

We enjoy your
Greyhound wrote:Source of the post obsessing over little details

I think that is one of the reasons for Sites like this 8-)



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Treatment of a small boxy room - REW file attached

#74

Postby endorka » Mon, 2023-Jan-02, 20:24

Greyhound wrote:Source of the postIt probably seems like I'm obsessing over little details, but I've read so many threads about HH traps that were carefully designed from online calculators (often with considerable effort) and which end up doing nothing. The resonant approach seems so promising, but it does not seem realistic to get a working trap without some experimentation and careful testing.


The details are hugely important here. Like yourself I'm intrigued by the concept, but most of the real world examples I've seen performed in an underwhelming fashion. Can you recall any examples that worked well?

Cheers,
Jennifer



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Treatment of a small boxy room - REW file attached

#75

Postby Greyhound » Mon, 2023-Jan-02, 22:47

Thanks guys!

For someone who is obsessing over little details, I mis-stated an important detail regarding my Helmholtz "box" tests. Instead of using 2x6" (nominal) framing lumber, as stated in my original post, I used 2x8" (which gives a cavity depth of 7.25"). I don't know why I had 2x6 stuck in my brain, as my simulations made it clear that deeper is viewed as generally better. A larger cavity volume allows for a higher hole area fraction for a given frequency, which may be a determinant in the actual absorption level seen in practice (I'm still not sure how the hole area factors into the total absorption that occurs across the face of the box).

I've been sneaking this stuff into some actual renovation work I'm supposed to be doing, and coming off a nasty post-Christmas flu, so apologies for the mistake. I think that's the only error, and it does not impact the overall narrative much. The Matlab simulation using the correct depth gives a resonant frequency very close to what I saw in practice with the candle:

Untitled (1).jpeg


(this simulation is without absorber, as in my candle tests, for the purpose of showing that resonance can be achieved at all)




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