Helmholtz formula, is this incorrect?

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Helmholtz formula, is this incorrect?

Postby DanDan » Wed Jan 14, 2009 6:17 pm

Is this formula correct? I have seen the formula elsewhere with c divided by 2 pi, not multiplied.
download/file.php?id=344
Dan FitzGerald
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Wed Jan 14, 2009 10:07 pm

Hey, that's my file! :)

Yes, it should be c/(2*π). But I think this came up when I posted the original file and I subsequently (and quickly :oops: ) corrected it. Of course, there have been several forum changes since '02. I'll check my archived files later on, but going back 7 years might be asking too much. :?

Is the original thread still on here somewhere?

Regardless, good catch! :)
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Thu Jan 15, 2009 5:22 am

Woojahbuhleevit? Original file found, fixed, and attached. Hopefully, the mods can update the link you gave. (I'm not savvy about such things.) :wink:

Side note: Seven years, almost to the day, and no one around here caught that little gaff. Is anyone awake 'round here 'sides DanDan? :roll: :)
Attachments
Helmholtz Cube.pdf
(25.25 KiB) Downloaded 1372 times
---Σοφός---

10*bLog(P²)

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Using Helmholtz

Postby DanDan » Thu Jan 15, 2009 1:50 pm

Hi Savant, funny little story eh? Thanks for fixing that but much more thanks for putting it up there in the first instance.
I have taken on a little project. In the Master Handbook pages 229-229 I see a Tube Helmholtz resonator/absorber used to very very successfully shorten the stongest and lowest room mode 47Hz. Most of the small semi domestic studios I have measured have a similar issue, so a solution would be most welcome. I asked Doug of ETF about that example and he assures me it did happen.
I have been looking and asking for some time for some decent formulae. That may sound a bit lightweight but my Math, Algebra etc. are rarely used and well rusted. I found all sorts of versions of the formula, none directly applicable to a Tube and none with the adjustment for loading of the duct. Thus yours was the best I have found.
I am about to do this as an experiment. I expect calculations to vary from measurement, they seem to always do.
I expect the device will actually change the frequency of the room mode. I will use the numbers as a starting point.
I expect that the diameter of the Duct with corresponding length affects Q. Do you know of a formula for this?
I am considering a sliding Duct , i.e. a slightly longer Duct which slides in and out of the Cylinder to tune it. I don't have any theory or numbers for this, it is a wild guess. Do you think this would work?
Best Regards, DD
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Re: Helmholtz formula, is this incorrect?

Postby Scott R. Foster » Thu Jan 15, 2009 2:17 pm

Please post test results and pics!

Also, can you provide a link to the forum page where the flawed calcuation was found? I will replace it with S's correction.
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Thu Jan 15, 2009 4:19 pm

DanDan,

Equations for Q are also in the source referenced in the PDF above (Kinsler-Frey), as well as in various other sources. (Not in the MHoA, if memory serves.)

If you don't have access:

Q ~= 2*π*sqrt[V*(L'/S)³]

This is an approximation (see text for details). Note that any sort of fuzz in the cavity (cube) changes (widens) the Q. Trial and error are best to determine the amount of fuzz you might need. In my experience, some fuzz is recommended. Otherwise, the Q is too narrow and real-world differences between calculated-f and actual-f could put the resonator in an unusable range of frequencies. YMMV.

BTW - fáilte go studiotips - I haven't seen you on here before. :wink:
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Re: Helmholtz formula, is this incorrect?

Postby DanDan » Thu Jan 15, 2009 6:12 pm

Hi Scott, the formula is at the bottom of this page viewtopic.php?f=20&t=150
I will try to do this experiment and post the results reasonably rigorously, with respect to the research quality on this forum. I have a good source of Tubs, sewer pipe is holding me up as they sell it in 18 foot lengths.
Savant, go raibh maith agat (ta) for the link, the equation, and the welcome in Gaelic. I have been lurking around for a while. I have some nice fuzz, Eco-Wool. I believe everything will affect everything else, thus my sliding duct idea.
Do you think sliding the duct in and out will change pitch?
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Thu Jan 15, 2009 6:44 pm

DanDan wrote:Do you think sliding the duct in and out will change pitch?

Unlikely for the frequencies we're talking about. The inserted portion of the tube would have to significantly change the volume of the cube to significantly change f.
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Re: Helmholtz formula, is this incorrect?

Postby bert stoltenborg » Thu Jan 15, 2009 7:58 pm

Savant wrote:
DanDan wrote:Do you think sliding the duct in and out will change pitch?

Unlikely for the frequencies we're talking about. The inserted portion of the tube would have to significantly change the volume of the cube to significantly change f.


Make a telescope duck. Effectively making the duck longer and shorter (or wider and narrower) will modify fs.
Although I seriously doubt this is an effective way of treating modes.
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Thu Jan 15, 2009 10:36 pm

Bert: Like this one? I agree; probably not good for modes. :) :) :)
Attachments
telescopeduck.jpg
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---Σοφός---

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Re: Helmholtz formula, is this incorrect?

Postby bert stoltenborg » Thu Jan 15, 2009 11:06 pm

Little Bill Daggett: [talking to English Bob, and refering to a book] That you here, Bob, on the cover? "The Duck of Death?"
W.W. Beauchamp: Duke. It's the Duke. "Duke of Death."
Little Bill Daggett: [correcting W.W Beauchamp] Eyewitnesses?
W.W. Beauchamp: Yes, sir.
Little Bill Daggett: Like the Duck himself, I guess.
W.W. Beauchamp: The Duke.
Little Bill Daggett: Duck, I says.
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Re: Helmholtz formula, is this incorrect?

Postby Scott R. Foster » Fri Jan 16, 2009 12:53 am

All fixed
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Re: Helmholtz formula, is this incorrect?

Postby DanDan » Sat Jun 27, 2009 2:19 pm

It has taken a while but I have finally started the experiment quoted at the start of this thread. I have done some rough attempts and can report the following:-
I did a resonance test in the centre of a different room, in an audible null. Tub resonance is easily detectable. I use a hardware sine generator with a DPA 4061 mic in the tub. This is fed to a preamp and a level meter in ProTools.
Even a small amount of fuzz makes resonance much more difficult to detect. I reckon I got the device close to or at 34Hz and tried it in the problem room. It was placed in the worst corner.
With the mic about a foot from the port, the device seems to decrease the 34Hz room resonance by 3dB or so. Even a small amount of fuzz eliminates this effect.
I can detect no change in the length or shape of the mode in Waterfalls or EDT measurements. None.
I am about to repeat the experiment more methodically today. I intend cutting a series of port lengths from 10cm to 20cm at say 2cm intervals. This seems to be the best way to be sure of resonance, in situ, with and without fuzz. Calculations have not been very useful.
Overall it seems likely that I cannot replicate the very spectacular example mentioned in the MHOA. Way back, I did communicate with Doug of ETF. He did assure me that the experiment and results were genuine, but 'were done by someone else'. Perhaps it was done by a QUACK :-)
I started this little venture because I was so impressed with the result in the book, and hoped to put it to everyday use. Now it seems I am in the process of disproving that example. I have all the gear in place, so now is the time to prove or disprove this one. I would very much welcome any suggestions etc. to make this experiment very sure. I don't particularly like being in the position of rubbishing something in the Mr. Everest's wonderful book.
Best Regards, Dan FitzGerald
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Re: Helmholtz formula, is this incorrect?

Postby Scott R. Foster » Sat Jun 27, 2009 4:08 pm

Howdy Dan:

I can detect no change in the length or shape of the mode in Waterfalls or EDT measurements. None...


Nice empirical exploration


but



- wait for it -



Pics or it didn't happen!




:D

PS: FWIW I have had experience with a resonating construct messing up a room [open 4" wiring conduit in the corner coming up through the concrete floor slab hooting like a pipe off a church organ] but have never had any luck fixing anything with one. Never... none. Just as you report. My skeptical conclusion is that unless the resonating acoustic adjustment system is raised to a level of size and complexity that is a substantial fraction of the room itself - it aint gonna do squat - and that's a pretty silly way to go about the task. :roll:
SRF
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Re: Helmholtz formula, is this incorrect?

Postby Terry Montlick » Sat Jun 27, 2009 4:51 pm

Scott R. Foster wrote:
but



- wait for it -



Pics or it didn't happen!

Yes. And we want dimensions for everything. :)
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Re: Helmholtz formula, is this incorrect?

Postby DanDan » Tue Jun 30, 2009 5:49 pm

Doubting Thomases.......
P5310002.JPG
P5310002.JPG (153.5 KiB) Viewed 23411 times


Tub
H 65 cm
D 45 cm
Port
D 10.5 cm internal
Various lengths, probably finalise this later this evening.
Target frequency is 34Hz.
Fuzz is Ecowool, a light recycled plastic attic insulation.

This tub is quite big, although hardly comparable with the room, which is
4.4 x 3.39 x 2.45 M

DD
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Re: Helmholtz formula, is this incorrect?

Postby JohnPM » Tue Jun 30, 2009 7:01 pm

When I last played with Helmholtz resonators (which admittedly was double digit years ago) even a very small amount of lightweight damping material inside the resonator had a pretty dramatic effect in lowering the Q. They did have a measurable beneficial effect at the listening position once a suitable corner was determined by trial and error, but not proportionate to the effort involved in making and tuning them. Fiddling with the port lengths and the enclosure volumes to hit the particular modes they were aimed at was a very tedious process.
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Re: Helmholtz formula, is this incorrect?

Postby Terry Montlick » Tue Jun 30, 2009 9:55 pm

DanDan wrote:Doubting Thomases.......
Tub
H 65 cm
D 45 cm
Port
D 10.5 cm internal
Various lengths, probably finalise this later this evening.
Target frequency is 34Hz.
Fuzz is Ecowool, a light recycled plastic attic insulation.

This tub is quite big, although hardly comparable with the room, which is
4.4 x 3.39 x 2.45 M

DD

:D

Interesting. You are just outside the classical Helmholtz limit for 34 Hz, which is 1/16 times the resonant frequency wavelength of around 1009 cm. That means that the actual prediction given an effective port length of 21.6 cm (real port length plus end corrections) using the full power series expansion for a cylindrical resonator* is about 1 Hz shy of the Helmholtz prediction: 33.1 Hz.

Of course, since you are trying different port lengths rather than trusting Dr. Helmholtz, all will be well. :mrgreen:
But your tub shouldn't be much longer, or you could be way off from the Helmholtz formula.

As for the Q, fugetaboutit. :mrgreen: :mrgreen:

Regards,
Terry

*Panton, R.L. and J. M. Miller (1975), "Resonant frequencies of cylindrical Helmholtz resonators," J. Acoust. Soc. Am.,Vol. 57, No. 6, Part II.
Last edited by Terry Montlick on Wed Jul 01, 2009 12:58 am, edited 1 time in total.
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Re: Helmholtz formula, is this incorrect?

Postby DanDan » Tue Jun 30, 2009 10:18 pm

I have repeated the experiment more carefully. I placed the DDevice in the centre of another room, in the null of it's primary mode, on a chair to place the port at half height for the same reason.
Helmholtz resonance is quite clear now. Due to the very thick end plug of 40mm I can slide the port quite a bit to alter frequency. Trombone Duck has come to life!
A full tub of fuzz now has little or no effect. Adding a towel clearly broadens the Q.
I tuned to 34Hz as best I could. Move the DDevice to the problem room. Inverted the DDevice while raising it off the floor to allow sound in to the port. Microphone is on floor under port. Some FuzzMeasure sweeps show the following.
6dB decrease in level with the DDevice. 34Hz mode is shortened from 410mS to 380mS.
I will give this one last go tomorrow. I want to be entirely sure that I have exact resonance in situ. Not sure how to determine this yet. Ideas welcome.
Best, DD
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Re: Helmholtz formula, is this incorrect?

Postby Scott R. Foster » Tue Jun 30, 2009 10:51 pm

it aint that we doubt you - its that the intnerwebs are a visual experience... so if you don't post pics we don't "really" get to share you experience. :8

Cool tub.
SRF
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