Treatment of a small boxy room - REW file attached

[Greyhound]

In further praise of soda bottles, and just for the sake of wasting time, I did a measurement with a 2L bottle (the other one is 500ml).

The resonant frequency should be inversely proportional to the square root of the volume, and voila:

Interestingly, running an RTA with 1/48 octave bins while blowing across the two bottles gives peak frequencies of exactly 100 and 200Hz, respectively. The resonance points seen in the REW curves seem to occur at slightly higher frequencies. Not sure why that might be so (nor the exact band frequencies used for 1/48 octave RTA - maybe it's a resolution thing).

So clearly there is a regime where the physics of this works predictably, and yields tangible effects that can be measured. Now if only I could bridge the gap with my MDF boxes...

[gullfo]

i didn't expect much change as your hand was performing most of the damping already, but just cross-checking to be sure...

so what's next? maybe identifying the main null at the listening position, crafting a pair of HH for that frequency, and then placing them on either side / end? (presuming the null is due to side walls or length)

[gullfo]

when you're ready to get adventurous...

[Greyhound]

That's a lotta holes!!!

[Greyhound]

Source of the post i didn't expect much change as your hand was performing most of the damping already, but just cross-checking to be sure...

so what's next? maybe identifying the main null at the listening position, crafting a pair of HH for that frequency, and then placing them on either side / end? (presuming the null is due to side walls or length)

Yeah it (bottle walls vibrating) was a reasonable thing to rule out!

It's true that I've strayed way out from the practical room treatment objectives that are nominal to this thread... even just thinking through speaker placement etc would probably be a more productive use of time in that regard. Right now I want to work through the physics of the Helmholtz concept, as it's relevant to some other work interests I have.

Thanks for posting the plans - I am just trying to learn and eventually will integrate everything into a practical room solution!

[gullfo]

Source of the post That's a lotta holes!!!

pegboard material. didn't want to make all the individual holes...

[Greyhound]

A bit more possible insight into the bottle "phase flip" behaviour.

I re-ran some Matlab simulations (with the modified code from Cox), and looked at plots of the magnitude and phase of the (complex) reflection coefficient. This kind of reconciles the wild "bipolar" swings seen with the empty bottle with the notion that the same construct might achieve a bell-shaped absorption curve.

Looking at the magnitude and phase for an empty HH resonator, you can see that the reflected phase swings wildly on either side of resonance:

This is consistent with the REW frequency response curves seen with my empty bottles (the phase swings go almost to plus or minus 180 degrees!)

Running the same simulation, for an HH cavity filled with absorber, gives the following result:

You can see the phase is a lot smoother, and doesn't vary more than 20-30 degrees either side of resonance. In both cases (with and without absorber in the cavity) you can see the dip in the magnitude of the reflection coefficient (which is another way of expressing the absorption coefficient).

Playing around, I was able to achieve a cotton ball "loading" of the bottle that seemed to reduce the bipolar swing and give a few dB of attenuation at the centre frequency (relative to a "control" sweep without the bottle):

In the above graph the orange curve is the "control" without any bottle in front of the mic, while the light blue curve is with the empty bottle (showing the "bipolar" swings). The green curve is with the bottle loaded with 25 cotton balls. It's not earth shattering, but at least it's an example of being able to control the phenomenon in a principled way. Keep in mind that this is not taken at the listening position - the mic and bottle are in front of my left main speaker (with all other sources turned off).

I'd speculated earlier on the possibility that simulation software using the transfer matrix approach might not be correct for "empty" HH resonators, but it seems the math works fine.

The issue is that these calculators and simulations usually just output the absorption coefficient, which is based on the magnitude of the reflection coefficient (alpha = 1 - |R|^2 where |x| is the complex magnitude of x). This does not provide a full depiction of how the device interacts with the sound field, as the phase also clearly matters. This is probably more important at low frequencies.

[gullfo]

also note that a "mouth" correction is often applied (if you haven't already) since the friction on openings can impact it.

[Greyhound]

Source of the post also note that a "mouth" correction is often applied (if you haven't already) since the friction on openings can impact it.

That's a good point - the simulations I'm running include the correction for "radiation impedance" at the hole ends, but it sounds like you're talking about something different. C&D'A refer several times to the fact that burrs around the holes can affect the behaviour, and although I used a sharp drill bit it seems unavoidable to cut MDF without producing edges that are slightly "furry".

This may or may not account for the total lack of acoustic effect from the MDF boxes, despite their ability to move a candle flame (could be related to turbulence?). It might be relevant that the bottle mouths, which do create an acoustic effect, are smooth plastic. Also the body of the bottle cavity flares smoothly into the neck, whereas holes in 3/4" MDF have a sharp corner (which may or may not matter, like everything else).

[gullfo]

somewhere someone once published a list of "typical" mouth correction values - probably in studiotips - but in general a factor of 1.2 is generally acceptable, some as low as 1.05, and some as high as 1.45 iirc...

[Greyhound]

Source of the post somewhere someone once published a list of "typical" mouth correction values - probably in studiotips - but in general a factor of 1.2 is generally acceptable, some as low as 1.05, and some as high as 1.45 iirc...

Thanks Glenn! I really need to track this down in archive.org as it sounds like a goldmine.

[Greyhound]

So I found an even larger bottle to add to my menagerie - this one is 3.8L (it's an empty lab bottle that had distilled water in it).

This one resonates down around 70Hz, and the bottles continue their undefeated streak against the MDF boxes in changing the local sound field around the mouth:

To me this is more evidence that there is no problem achieving resonance at low frequencies. I am increasingly wondering if there is an inherent limitation in the "drilling holes through MDF in front of a cavity" approach. This is not to say that it can never work, but that it might get really hard at low frequencies.

[gullfo]

LF are hard all around until you're over 40K ft3... and resonant absorbers aren't the only options - panel traps, limp mass, slat-slot, deep velocity, broadband, etc depends on amount of space and realistic expectation on "flatness"

[Greyhound]

Source of the post LF are hard all around until you're over 40K ft3... and resonant absorbers aren't the only options - panel traps, limp mass, slat-slot, deep velocity, broadband, etc depends on amount of space and realistic expectation on "flatness"

That's a good point Glenn, and I don't mean to suggest that LF treatment is hopeless (or that there is a magic treatment that will make a crappy room like mine great). I'm just wondering if there is a basic physical reason why LF Helmholtz traps seem, so often, to fall so far short of expectations.

[gullfo]

most times, they're not really the best for LF compared to limp mass and panel membrane (and of course VPR) trapping.