Helmholtz Resonators

[lukejs]

I´m building a studio and want to keep the live room live, but take out the mid and low frequencies, and came across the SAE website showing helmholtz resonators. Here´s the link... ******//www.saecollege.de/reference_m...requencies.htm

I´m considering building one entire wall as a helmholtz resonator, but am not sure how wide to make my slats and how to decide on slot distance (gaps between slats) . The formula that is given just seems like there are so many alternatives in terms of slat and slot width and depth. Can anyone post some photos of their helmholtz resonators, or give some advice on how to design this ? Thanks, Luke

[Andreas G]

Hi Luke,

you need this two formulas for a Helmholtz Slat Resonator.
If you want to absorb freq. below 200Hz you need a different formula... just let me know.

Andreas

Edit: Sorry upload attachment doesn't work I'll try it tomorrow on a different PC

[Riad]

Check out John Sayers forum for the spreadsheet.

Good tips:
******//www.johnlsayers.com/phpBB2/viewtopic.php?t=2125

helmholtz calculator:
******//www.johnlsayers.com/phpBB2/viewtopic.php?t=1363

[jayfrigo]

Originally Posted by lukejs

I´m considering building one entire wall as a helmholtz resonator, but am not sure how wide to make my slats and how to decide on slot distance (gaps between slats) . The formula that is given just seems like there are so many alternatives in terms of slat and slot width and depth. Can anyone post some photos of their helmholtz resonators, or give some advice on how to design this ? Thanks, Luke

Consider making a perforated absorber instead of a slat absorber. Simple and effective. Either way, pick up a copy of Everest's "Master Handbook of Acoustics" to get more details on the best way to build them, along with "as-built" response measurements of actual constructions. Also search for Mankovsky, a Russian acoustician who did a lot of work on this subject.

[lukejs]

thanks for the tips ! I´ll look this stuff up !

[Tim Farrant]

In order to maintain diffusion of sound in the room (equally as important to absorbtion) do not make an entire wall a resonator. You are better off making smaller resonators placed in the corners around the room. Absorbtion in patches aids diffusion.

Slat resonators are easier to make than perforated ones - who wants to drill 500 holes? Slat widths of 50 to 200mm and slots of 5-10 mm are typical. Adjust the depth to get the frequency of absorbtion. A resonator that has wide slats and not many slots will be less efficient than a deeper box with thinner slats and more slots.

Tim.

[Anderson]

Originally Posted by lukejs

I´m building a studio and want to keep the live room live, but take out the mid and low frequencies, and came across the SAE website showing helmholtz resonators. Here´s the link... ******//www.saecollege.de/reference_m...requencies.htm

I´m considering building one entire wall as a helmholtz resonator, but am not sure how wide to make my slats and how to decide on slot distance (gaps between slats) . The formula that is given just seems like there are so many alternatives in terms of slat and slot width and depth. Can anyone post some photos of their helmholtz resonators, or give some advice on how to design this ? Thanks, Luke

Maybe you should think about membrane resonators. Helmholtz works fine, but with a membrane it will be much more efficient. Also many smaller resonators (no less than (65/70L)) but tuned on various freq will give way better results. Helmholtz can be tuned to one center freq only, and efficiency is rarely more than + or - 15% from that center freq. One big simply won't make it...

Cheers!

[Andreas G]

Here the formulas...

[jayfrigo]

Originally Posted by Tim Farrant

Slat resonators are easier to make than perforated ones - who wants to drill 500 holes?

I feel just the opposite. For low frequencies there aren't that many holes, far less than the "peg board" you buy at the store. However, you can sometimes get pre-drilled sheets with density less than the typical pegboard. Even if you need to DIY, it couldn't be easier than putting piece of plywood over a box (preferabley half filled with insulation to widen the Q) and drilling holes on anywhere from 3 - 6 inch centers (depending on the freq. range you desire to attenuate). That drill goes through 1/4" plywood pretty darned quickly! They're easy to cover with fabric and they can disappear into a room if you wish. They can easily be built modularly and you can move them around, take them with you when you leave, and they offer some diffusion above the bass absorption band. You can make them more broadband by using variable perf percentage or varying the depth (along with half-filling with insulation). These things are useful, versatile, cheap, and easy. Slats pretty much fit the bill too. Either way will get you there, and each offers different aesthetic possibilities as well.

The caveat, of course, is that you want to have a bass trapping plan that covers more than a single tuned frequency. Use broadband trapping in conjunction with these tuned traps, broaden the Q of the tuned traps, and if you are making several tuned traps, don't necessarily tune every trap the same, and place them appropriately spaced on different surfaces. You need to run the numbers in the room and come up with an overall plan. Just nailing one frequency in one place won't make a great room.

The corner thing is helpful often, but it's not magic. Sure, that's where the pressure is greater as two boundaries meet, but you don't listen in the corners after all. The pressure is even greater where three boundaries meet, in the corners at the floor and ceiling, but people don't seem to focus on putting extra trapping there. Depending on the room geometry and listening position (reflections from corners more or less come back to where they originated which can be problematic depending on your listening position choice), the corner traps may not be any more effective at making your listening position better than other placements, and indeed may be less effective than some other placements. Of course figuring this out assumes some knowledge of acoustics. If you're just treating a project studio on your own, the corner thing is a good palce to start, especially considering that a variable depth trap is pretty much automatic, but don't assume that's all there is to it.

[lukejs]

Thanks again for all those ideas ! I think the idea about not making the whole wall a resonator is pretty good one, since I don´t think a whole wall as a resonator could take out a wide enough bandwidth. Smaller absorbers tuned to specific frequencies is probably the best way to go about it. Here are three photos I took of the live room, spliced together in photoshop in a hurry...so don´t laugh !!
There were windows behind where the wooden frames are situated, and I plan to put a peice of wood over those frames, like you see on the far right side wall. I was considering making new wall on the left side ( the slat resonator wall ) . Those walls are concrete ! That´s why I figured covering the whole wall with a slat resonator may be a good idea... I guess I´ll have to have bass traps in about every corner !!! thanks for any more ideas you may have !! Luke

[Andreas G]

Originally Posted by lukejs

thanks for any more ideas you may have !! Luke

A cool example for a slat resonator at Nucleus Berlin

[lukejs]

Mann... that makes my studio look like crap !! But thanks for the example !!
It gives me inspiration !!

[lukejs]

Originally Posted by Anderson

Maybe you should think about membrane resonators. Helmholtz works fine, but with a membrane it will be much more efficient. Also many smaller resonators (no less than (65/70L)) but tuned on various freq will give way better results. Helmholtz can be tuned to one center freq only, and efficiency is rarely more than + or - 15% from that center freq. One big simply won't make it...

Cheers!

Thanks Anderson ! Can you explain what a membrane resonator is ? Thanks !

[Anderson]

Originally Posted by lukejs

Thanks Anderson ! Can you explain what a membrane resonator is ? Thanks !

A membrane-type of resonator looks like a big box or semi-cylinder, usually rather tall/large, which usually has a membrane as a front panel. Sometimes the membrane is hidden behind a wooden front panel. There can also be two membranes.

Anyway, a standing wave at the resonant frequency of the resonator, will cause the membrane to vibrate in sympathy to the standing wave, and "absorb" the standing wave with a much higher efficiency than a Helmholtz resonator. The larger the volume of air, the lower the frequency.

They are harder to calculate, but worth it. Since they are very efficient!

Cheers!

[Cojo]

Originally Posted by Anderson

A membrane-type of resonator looks like a big box or semi-cylinder, usually rather tall/large, which usually has a membrane as a front panel. Sometimes the membrane is hidden behind a wooden front panel. There can also be two membranes.

Anyway, a standing wave at the resonant frequency of the resonator, will cause the membrane to vibrate in sympathy to the standing wave, and "absorb" the standing wave with a much higher efficiency than a Helmholtz resonator. The larger the volume of air, the lower the frequency.

They are harder to calculate, but worth it. Since they are very efficient!

Cheers!

For control room use I've heard that membrane resonators don't work well at low volume since they need high energy to vibrate. Is this so? Can anyone confirm this?

/Cojo

[lukejs]

Originally Posted by Anderson

A membrane-type of resonator looks like a big box or semi-cylinder, usually rather tall/large, which usually has a membrane as a front panel. Sometimes the membrane is hidden behind a wooden front panel. There can also be two membranes.

Anyway, a standing wave at the resonant frequency of the resonator, will cause the membrane to vibrate in sympathy to the standing wave, and "absorb" the standing wave with a much higher efficiency than a Helmholtz resonator. The larger the volume of air, the lower the frequency.

They are harder to calculate, but worth it. Since they are very efficient!

Cheers!

Thanks...but what´s the membrane made of ? I´m considering something like this.... ******//www.saecollege.de/reference_material/index.html go to where you see curved panel absorber. is the curved panel like the membrane ? It´s a thin peice of plywood....

[Ethan Winer]

Conny,

> I've heard that membrane resonators don't work well at low volume since they need high energy to vibrate. Is this so? <

No, it's not so. If that were true then adding such a bass trap would literally add distortion into the room acoustically. And they don't do that. At least not if they're made properly. Every bass trap design I know of is completely linear, meaning they do exactly the same thing at low and high volume levels.

--Ethan

[Cojo]

Originally Posted by Ethan Winer

Conny,

> I've heard that membrane resonators don't work well at low volume since they need high energy to vibrate. Is this so? <

No, it's not so. If that were true then adding such a bass trap would literally add distortion into the room acoustically. And they don't do that. At least not if they're made properly. Every bass trap design I know of is completely linear, meaning they do exactly the same thing at low and high volume levels.

--Ethan

Ok, that's good to hear! Thanks!

/Cojo

[Anderson]

Originally Posted by lukejs

Thanks...but what´s the membrane made of ? I´m considering something like this.... ******//www.saecollege.de/reference_material/index.html go to where you see curved panel absorber. is the curved panel like the membrane ? It´s a thin peice of plywood....

Well, there is not only "one type" of membrane, you can choose between many materials. From some woods to most of the polyvinyls (semi-flexible PVC for ex). All those materials will have certain specifications and depending on the resonator's structure, size etc, you will use one or another type of material. I guess you can find charts on the net giving you resilience and other various coefficient of those different materials.

If you're not absolutely sure of what you're doing, it's better to ask an acoustician in your region. DIY gives VERY random results in that field unless you know what you're doing!

Good luck with your project!

[lukejs]

Don´t laugh, but I was in sam´s the other day buying toilet paper, and when I picked up this package of 48 rolls, I thought to myself ( since I´m such an audio fanatic.... ) I bet stacking three of these packages on top of each other , in a corner, with a fiberglass sheet in front...... and covered with cloth...would make a great bass trap !! Just think about the absorbant material covering the cardboard cylinder .. with air in the middle of each roll ! It has to work ! They say sound doesn´t like going trough different masses of materials, and that air is like a spring for absorbing sound... so there you have it ! As$wipes as bass traps !! I´m gonna try it out..hey what do I have to lose ? If it doesn´t work, I won´t have lost anything..... but.... gained a year´s supply of buttwipes !!! You think it would work ?

[Riad]

If the material isn't dense enough it wont work. Remember bass traps should have some weight to them to be effective!

[mig27]

Hey Lukejs,

the buttwipe idea sounds tempting, but in the end, 705 should be even cheaper.
I did the maths the other day and figured out that a 3' by 3' area covered with 4" thick 705 costs only $12.
That's for the european work-alike, but I reckon prices are similar in the US.

My 2 € cents.


Michael

[mig27]

On the other hand - if you do try out these as$wipe absorbers, let us know how it works out for you.
And a nice advantage over mineral wool is that you don't get to deal with all this unhealthy glas fibre stuff.

M

[shangoe]

Originally Posted by mig27

Hey Lukejs,

the buttwipe idea sounds tempting, but in the end, 705 should be even cheaper.
I did the maths the other day and figured out that a 3' by 3' area covered with 4" thick 705 costs only $12.
That's for the european work-alike, but I reckon prices are similar in the US.

My 2 € cents.


Michael

thats right, plus rockwool or fiberglas is fireproof. one litle spark on your toilet paper and you will burn your ass very badly.....

[lukejs]

Originally Posted by shangoe

thats right, plus rockwool or fiberglas is fireproof. one litle spark on your toilet paper and you will burn your ass very badly.....

Yes..that´s right ! I never thought about that one ! But there will be no electrical cables anywhere near these basstraps, so I think I can get away with it .

[shangoe]

Originally Posted by lukejs

Yes..that´s right ! I never thought about that one ! But there will be no electrical cables anywhere near these basstraps, so I think I can get away with it .

seriously, i would not play with that. imagine you have a small fire somewhere else in your studio, a situation you can still handle. but this is about SECONDS and you burn in hell... i saw a fire going on in a house once, since then my respect grow 300% just a burning mailbox in the entry,you can not think how fast the 6 floor building was complete on fire.