Helmholtz formula, is this incorrect?

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Re: Helmholtz formula, is this incorrect?

Postby Scott R. Foster » Thu Jul 30, 2009 12:47 pm

I don't know.
SRF
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Re: Helmholtz formula, is this incorrect?

Postby Savant » Thu Jul 30, 2009 2:44 pm

msikio wrote:Who's right ? who's wrong ? It won't be the first time the mh-audio.nl site is wrong (remember the now corrected slat resonator formula).

I would put my faith in Kinsler-Frey. (Hasn't failed me yet...)

FWIW, the reference is only for fr. The quality factor, Q, is not discussed, and no equation is given for it in the AIP Handbook. (If you notice on the website, the citation is given before the boxes for flow and fhigh.) I would suspect either the calculator is flat-out wrong for flow and fhigh, or it's using some other definition of Q to arrive at flow and fhigh. As you pointed out, the former is probably a little more likely. :?

Regardless, I'm still right so far (thanks to Kinsler-Frey)! :)
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Re: Helmholtz formula, is this incorrect?

Postby msikio » Thu Jul 30, 2009 6:07 pm

Savant wrote:I would put my faith in Kinsler-Frey. (Hasn't failed me yet...)

FWIW, the reference is only for fr. The quality factor, Q, is not discussed, and no equation is given for it in the AIP Handbook. (If you notice on the website, the citation is given before the boxes for flow and fhigh.)
Regardless, I'm still right so far (thanks to Kinsler-Frey)! :)

Fair enough Jeff. Thanks a lot.

BTW I found some very interesting things in an old (1951 !) BBC research center paper called "Helmholtz resonators in the acoustic treatment of broadcasting studios" (... not part of the free papers available at the BBC site, don't ask me why...)
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Re: Helmholtz formula, is this incorrect?

Postby Max Headroom » Tue Jan 12, 2010 8:14 pm

f = (1130 / 2 x 3.14) * sqrt(S / (L * V)

the basic equation is all you need
sound doesn't bounce forever & there's a ring time graph for a perfect helmholtz, not quite a full second
any "cloth" introduced into the resonator makes it ring for a shorter time & therefore less efficient
it doesn't change the volume or formula of f @ all
therefore if ring time equals the ring time of the mode, physics dictate the bandwidth must be the same
bandwith = helmholtz efficiency = ring time

a few weeks ago, i experimented with several in my concrete basement
& now it sounds like an amphitheater
http://forums.audioholics.com/forums/sh ... hp?t=61857
i dont have 5 post so no pics
i read somewhere that the difference in sound between an anechoic concrete room & outside is
like shining a flashlight, @ a wall, in a white sphere instead of into space & that's what i have
i have no waterfalls, but i'm sure close to Kansas

my bassement is 7.6' x 22.6' x 38.6'
freqs @......... 75hz x 25hz x 14.6hz

i've got an ideal helmholtz scenario
i have a concrete basement, modes are clear, strong, narrow & no resonance from sheetrock
i have 3 resonators, located @ the sub for the most energy, they have the strongest effect over the entire basement there

the floor to ceiling @ 75hz was an easy box to tune
i just used a board with 1 screw as an ajustable port, then screwed it in place with a second screw
75hz in a 2.0 cubic foot box is an open port
of course you use 3/4" as port length for the thickness of the wood
the floor & ceiling are the two largest surfaces in my basement & this sucker is getting hot :mrgreen:
it's the root freq & boy does it take a load off your ears, not to mention the nulls
i played with locations & this box sounds the best on top of the corner sub, near the ceiling with the port nearest the corner
i'm kind of a phase & point source bass junky so that's naturally where it should go
my thread states how i set the ring time

i have more dead speaker cabinets so 50hz was next
helmholtz set a 3.8" id port to around 3"
so i used the 3/4" wood as the telescopic portion of the tuning process & doubled rolled cardboard
Joe Nickell mentioned the 50hz being a tangible problem cause it was an octave over 25hz, the width of the room
& that box is alive & hot too
he also mentioned that the 38.6', 14.6hz or 29.2hz would be my smallest problem
set the ring time with more rags than the other two
although this box helped, it left a most pronounced & fatiguing ring @ 25hz & it almost sounded worse than the blend of resonances b4 any helmholtz

although i initially though i was tuning the third box for 14.6hz, it ended up tuning to 25hz
although it's hard to imagine this thing doing much, it was very obvious & it didn't need hardly any padding down

the room (the magnepans are 1st reflection treatments :roll: )
my pics clearly show my front speakers being two 6" bookshelfs stacked per side, tweets in middle
phase linear 700b sounds nice @ 16ohms through those

Image

& you can ghetto them if your sub blows them away

Image

2 corner firing 15" bandpass subs, 1 port toward corner @ ceiling height, 1 port toward corner floor height
75hz helmholtz on top w adjustable port valve

Image

50hz & 25hz on bottom

Image

Image

front concrete phase plane sounds awesome :mrgreen:

my sub sounds better than any i've heard indoors, very very precise
those tiny resonances destroy any freq near them & everything sounds like the room
all of the room's nulls are gone, the sub still has perfect clarity @ the back of the room,
but the rings are back there too

on the other side of the room, treatments can't be heard really anywhere the stairway hole in the ceiling is between you & the sub
i can easily throw a rag over any of the ports & hear what it sounds like with each
i'm convinced room modes ought to be handled @ the base freqs this way, then all that other **** afterwards

i used to wonder about why a room sounds better on a song than another
now all i hear are modes & my living room system sounds like **** :|
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Re: Helmholtz formula, is this incorrect?

Postby Don T » Sun Jan 17, 2010 9:10 am

Hello,
My best use of resonator tubes has been in the mid bass frequencies. Located behind the mix position and aimed towards it (usually in the ceiling). I think of them not as absorbers but as tunable diffusion, an energy gain at the resonant frequency. Allows me to tune them so sound energy arrives at the mix position at the proper phase and frequency to counter room anomalies. I use angular cuts of the mouth to broaden the Q. Have to use some damping in the tube to shorten the ring time to the RT60 of the room. Can also place it inside at high particle velocity positions to dampen harmonics.

Don T.
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Re: Helmholtz formula, is this incorrect?

Postby Max Headroom » Thu Jan 21, 2010 6:59 am

Thanks for the response & helping me understand RT60 & Q.

with my tuned subwoofer resonator array, all in the corner:
*all of the nulls in the room r gone.
*when i move about the room, the nulls seem to be filled in by the resonators.
(that's why i'd recommend using resonator cabinets that sound good as a speaker cabinets)
(it's as though the nulls weren't there, just as loud)
*the resonators tune like a fork to the room.
how could i explain this with no education, test eq or $...
like american idol, where i convince my friends it sounds good, but have no professional opinion...
that'll have to do.

it's easy to imagine the positive sound pulse coming from the sub & the other side of the room
@ the exact same time, @ that room mode freq only
& it getting pushed into the resonator, tuned to that freq
& the resonator sending that energy back out when the rest of the room goes negative, @ that particular freq
phase reversal, thrown back into the room, only @ that narrow freq

**the only physical way possible to be able to hear the room mode freq @ a canceling location
would be if the resonator was making the opposite phase
& offering a differential of sound pres, a place for standing waves to head, to be able to hear cancel mode freq @ all
giving it the effect of coming from the resonator
this is where i'd suggest filling the room, with the phase inverter effect, just like the sub
in the corner, all together @ 1 point.
THX calls it point source bass imaging i think, a word to explain what i like sound wise

*the resonators will only enable an actual energy increase
& an increase in sound clarity with room dimensionally induced canceling frequencies only,
so they have a tuning fork effect, a definate sweet spot

who knows if i even have the phase inverter thing right - oh well
i only post to learn & be corrected hopefully "is this correct helmholtz thread" - TIA

there's probably a proper term for what i'm trying to explain with sub applications

i use resonators, @ the sub location to eliminate nulls & improve bass definition
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Re: Helmholtz formula, is this incorrect?

Postby e-sonic » Mon Jan 10, 2011 8:41 am

Hello, just blow a breeze over this bottle :D

did some reading on the MHOA, pages 221-222, with respect to the BBC modules for their small voice studios, a report is on the bbc site

http://downloads.bbc.co.uk/rd/pubs/reports/1980-12.pdf

A conclusion for the low frequency absorber, main absorption is due to panel action, and not via helmholtz principle.
So for a correct helmholtz absorber, you will need a stiff panel, rigid back box, and no damping in/on the tubes.

regards jurjen van der velde
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