Helmholtz questions

First off, thanks to everyone for this amazing resource don't know what i would do without it. I and a couple friends are building a small studio in Cincinnati, Ohio on a very limited budget, doing almost everything ourselves. (you can see some pics here: http://www.newfidelityproductions.com/STUDIO%20PICS.html) Construction is almost done, put up the monitors (free standing), desk, installed broadband absorbers/traps etc. Used the mode calculator to plan positioning of sweet spot and monitor placement. Found my frequency issue. At the mix position, 69Hz-83Hz is severely attenuated and 127Hz-161Hz is mildly boosted. My plan is to build a slat-type helmholtz resonator tuned to 74Hz using the wall as the back, and using insulation against the slat's to broaden the freq range a bit. First, does this seem like the appropriate step to take? Second, does it matter if the absorber is placed in the front or rear of room? Third, how large do you typically want the resonator to be? The calculator will tell you slat width and depth and so on, but i am hard pressed to find info on how large to make the actual unit. If anyone can help, I would greatly appreciate it. Chris

is this based on actual measurements or calculations? you can build some broadband slat resonators which may help but I'd recommend taking real room measurements in several locations (mix desk, producer couch, etc...) to see whats really happening...

Glenn, I have done tests, and used signal generators. at the mix position is where the dip occurs. The response at the producers couch is as relatively flat as you can hope for. Do you think this is a job for a helmoltz? Thanks, chris

Glenn, I have done tests, and used signal generators. at the mix position is where the dip occurs. The response at the producers couch is as relatively flat as you can hope for. when i place the reference mic half the distance to the wall directly to the right of the mix position, the low end gap fills in and all of the other frequencies are relatively flat as well. I seems that since i am monitoring in the center (width-wise) of the room, i am getting this cancelation. Do you think this is a job for a helmoltz? Thanks, chris

do you have the exact dimensions of the space? w-l-h + shape. I'm assuming based on the photos is all drywall over steel framing. how is the floating floor constructed? it sounds like a mode issue and maybe something where a panel trap may be one answer, as well as slats. let's try to determine where the mode source is...

Chris - because your speakers are free standing I suspect you are getting reflections back off your front walls that are cancelling/adding at the desk. Slots won't fix the 69Hz-83Hz area as it's too low for them but they would fix the 127Hz-161Hz if you can get enough depth to get that low. You could try adding some of Ethan's low absorbers across the front wall. What is behind the walls either side of the window? cheers john

Yes, steel frame w/ drywall. Unfortunately the floor is built on a steel frame that is attached to the frames of the walls for that room. (contractor we had didn't always listen to us or would say it couldn't be done. Unfortunately the owner of the building trusted him more and would side with him on issues...long story.) length (wall - wall) of the room is 20' 8" width at mix position(splayed walls) is 15' 10" and height at mix position (again splayed) is 9' 1".

John, Behind all of the walls is roxul afb. The rooms behind the walls are small amp chambers. So, even if you used a lot of depth and large slat widths, (as i've calculated on the "helmholtz calc" for a center freq of 74Hz) it would still not be affective at that lower freq? Do you have a link for ethan's low freq absorber? Thanks. chris

You can find plans for Ethan's traps here: http://www.ethanwiner.com/basstrap.html J

I'm one of the friends x-t mentioned that's been helping with this studio, although I haven't done research into the acoustics of the thing to the extent that he has. I'd like to thank all those here who have been filling my head with more info about acoustics and studio design than I ever thought possible. Maybe I just have a fundamental misunderstanding of how and why a helmholz resonator works, but why is it that a resonator wouldn't work at such a low frequency? Is it simply that the size the thing would have to be to be effective wouldn't be practical, or is there some limitation to the design of a helmholz resonator that prevents it from being effective below a certain point? If I need to do some homework on this, please let me know. I've searched through a handful of posts here about helmholz resonators, and think I've got the basic idea, but I also feel like I'm missing something. Sometimes in the many many many posts here full of good info, I get acoustic info overload. thanks, Steve

Basically a helmholtz resonator works like a coke bottle when you blow over the top and produce a sound. Ever seen someone with a line of bottles all lined up with different amounts of liquid in them and each makes a different note? When you put insulation into the bottle you no longer hear the note - the note is there but the insulation stops it. Similarly with a helmholtz resonator - the deeper the bottle the lower the note. The note is also determined by the depth of the bottle neck (the depth of the slat) and the width of the bottle neck ( gap between the slats) and the with of the slat (mouth of bottle) To get down to 75hz you would need very wide slats, very narrow gap, deep neck (slat) and deep cavity behind. cheers john

Enter room dimensions: 1st 2nd 3rd 4th Height: 9 ft -1 in 62Hz 124Hz 187Hz 249Hz Width: 15 ft -10 in 36Hz 71Hz 107Hz 143Hz Length: 20 ft -8 in 27Hz 55Hz 82Hz 109Hz maybe some 3'x6' panel traps on each side (constructed per Ethan's site) and a slat resonator or panel overhead? generally this calculation is harder without all the room dimensions to get the real room averages.

thanks glenn, we were looking at something similar to your suggestion. i appreciate everyone's help, it's great having a community like this. chris

Let me jump in the discussion, to share with you my last "feeling" about reaching low frequencies, less than about 150hz with a slap type Helmoltz . I think that the thickness of the slats that you need, to tune down Helmoltz under certain freqs, makes it heavier to respond and come to audible action. Then , the yield of such slats/depth might be smaller than with tighter ones, at same size & amount. (please, dont see my bolt writings as affirmations , :oops: but as a way to ease-catch-reading. ) I resume: The relation in size proportion might have to be equal to the difference of size of the frequencies themselves to work as well in low and high frk: You need X% larger amount of them for X% lower freqs ! So, maybe that's when the hinge would be a more efficient technic for low low lows, like direct pressure/spring of an "Ethan's- BBC A9 " plywood membrane bass trap, or the ultimate "John-SAE"'s hanger set.. ....................................................................... tell me if all that is silly, as it's all just based on 'feelings" .................................... Cheers

I think that the thickness of the slats that you need, to tune down Helmoltz under certain freqs, makes it heavier to respond and come to audible action.

Sorry, that was wrong: It's not the slats who get in motion in the case of a Helmholtz resonator, but the air in the cavities. But still, if the air is what is excited by sound pressure, I feel there is a need for a wider range of cavities corresponding to low frequencies, for the level of absorbsion to be significant.