helmholtz and room modes

hi! my rir is about to be finished, due to immense time pressure i unfortunately wasnt able to document the construction as thorough as i would have liked... well, i couldnt document it at all... :( my question for today :) after all the reading about helmholtz resonators and room modes i'm a lil bit confused... do i arrange the timber slats in such a way that i treat standing waves (for example 70 hz, 105, 140 hz etc) or do i wanna build a broadband low mid absorber? ... carpet on the floor is considered a felony ? greetings from the couch! chris

or do i wanna build a broadband low mid absorber?

Yes! But better make it bass traps. Superchunks. In as many corners as you can. Then measure the room again with REW, and post the results here so we can see them.

... carpet on the floor is considered a felony ?

Yes! You will be taken out and shot by firing squad if you do that! :) - Stuart -

Hi! before screwing the slats to the wall i uploaded some pictures. so, what i do is take the slats calculate the absorption frequency via the helmholtz calculator and make my way round the room, right? the left side of "b1" is further away from the concrete wall behind it than the right side of b2. how do i calculate the absorption frequency in this case? thanks for your quick and competent answers! greetings from the couch... chris

Just follow John's standard dimensions and it will work out fine. - Stuart -

:!: Remember that the slat resonators must be sealed! Also, if you're planning to tune them for specific frequencies, or registers then you'll also have to compartmentalize, otherwise you're resonators will average out.

if you're planning to tune them for specific frequencies, or registers then you'll also have to compartmentalize,

Actually, that isn't necessary: it is the slug of air in the slot that resonates, not the air in the space behind. The air behind is the spring, but the actual resonating mass is the chunk of air in the slot itself. The resonant frequency is governed by the dimensions of the slot and by the depth of the cavity, but not by the width of the cavity. If you want to broaden the Q then you should damp the cavity with insulation, and if you want to make your treatment broadband then you can angle the slats away from the wall, so that at each point the depth is slightly different. And the cavity must be sealed, as you correctly said, or it won't work at all. But it isn't necessary to place partitions between the slots. - Stuart -

Hi Stuart, That is true if you were only aiming for broadband treatment though isn't it? My point is in regard to narrowing the Q as it were, to 'tune' them to specific problematic frequencies. Possibly you misunderstood what I mean't by compartmentalizing - I didn't mean in terms of slot width, but rather partitioning the cavity behind. I'm basing my comment on information I've read here in the forum. e.g. viewtopic.php?f=2&t=8205&p=60994&hilit=slat+resonators#p60994 viewtopic.php?f=1&t=9101&p=64266&hilit=slat+resonators#p64266

That is true if you were only aiming for broadband treatment though isn't it? My point is in regard to narrowing the Q as it were, to 'tune' them to specific problematic frequencies. Possibly you misunderstood what I mean't by compartmentalizing - I didn't mean in terms of slot width, but rather partitioning the cavity behind.

I did understand what you were getting at, and it seems to be a common misconception that you need to divide the cavity between slots that are tuned to different frequencies. But it isn't necessary, and in fact if you look at the equation for calculating the resonance, the width of the cavity just does not even come into it: the only relevant factor of the cavity is the depth. Take a look at page 217 of the Master Handbook of Acoustics. Everest explains it well: "It is just a bit harder to realize that partitions between each segment can be removed without greatly affecting the Helmholtz action." He's talking about perforated panel resonators there, but in the next paragraph he extends the same principle to slot resonators: "It is but a short step to a slot-type resonator. The separating walls in the air cavity can also be eliminated without destroying the resonator action." He then goes on to say that the only reason you might want to keep the subdivisions in, is to reduce unwanted spurious modes of vibration. He gives one form of the equation on page 223/224: f=216(SQRT(P / (d) (D))) Where P is the percentage of perforation, D is the depth of the cavity, and d is the thickness of the slat. There is nothing in the equation regarding width of the cavity. In the second thread you linked to, what Glenn is talking about was dividers to separate entire octaves, and that does make sense: it goes along with the "unwanted spurious modes of vibration" that Everest was talking about. In that specific case, Glenn was showing a wall that deals with three different, octaves and he suggest placing a divider between the octaves, not between individual cells. In reality, cell dividers are not necessary in order to tune each slot to its own frequency. - Stuart -

Thanks for clearing that up Stuart. The octave division is what I had in mind when I referred to registers. This is what I've been considering for my own build. The side panels divided into Bass -(lower third of Panel), mid (middle section of panel possibly with an additional thin plastic foil) & highs (the upper third of the panel). I assume the targeted frequencies would primarily be those representing the axial modes of the rooms width (as given by physical sounding-tests e.g. REW).

I assume the targeted frequencies would primarily be those representing the axial modes of the rooms width

Probably, but not necessarily. You might have major tangential or obliques that are giving you problems too.

(as given by physical sounding-tests e.g. REW)

Exactly. REW or ETF or similar things will give you a pretty good indication of where your problems are, then you can treat those. But I don't know if I'd go to all the trouble of trying to tune your slots to specific frequencies. Rather, I'd go for a broadband approach that covers the entire bottom end. The issue isn't so much about the individual modal frequencies, as about the evenness of their spread across the spectrum. You want them evenly spaced (which is what ratios are all about), and you want the energy spread around too. If you tune to specific frequencies, then you run the risk of not treating nearby issues. You also have the problem of making sure that you put the right treatment in the right place, for maximum effect. Since each mode has peaks and troughs at different points in the room, it makes no sense to put your (for example) 118 Hz slot at a location in the room where there is a null at 118 Hz! But if you take the broadband approach, angling your slots to cover a wide range of frequencies at a wide range of positions, then you cover all of your bases quite well. If you really are intent on tuning, and are going to do compartmentalized sections, then maybe a good approach would be to measure the room and do one section first, based on the largest issues, then measure how the room reacts after that first section is installed, adjust the bands that you are covering with the second section, install that and measure again, then do a final set of adjustments for the frequencies and bandwidths of the third section. - Stuart -

Stuart, Your advice regarding the broadband approach is probably more likely, especially with a small room as I have. As for the tuning, we'll have to see, then I'll take your advice. :wink: My apologies to the OP, it was not my intent to waylay your thread, but hopefully the info was useful to you too.

sorry for the delay - dividing the cavity helps with efficiency. the large volume of space behind the slats becomes "softer" as a spring especially if you vary the slot openings. your smaller slots become less effective due to the more open ones. so adding the dividers allows you to more efficiently partition the slats/slots - i prefer more smaller slats wider slots in the middle for MF and HF and larger slats smaller slots top and bottom for LF and low MF. net-net, if you don't vary slot sizes, then you're more likely to be close to the actual calculations but then you aren't really getting a wider band of impact either... then again, the main controller is the cloth behind the slats in terms of Q and the slat-slot ratio dictates your perforation ratio. one other caveat - your top and bottom slats should be at least 50% of the largest slats used to ensure the cavity space spring effect on either end is balanced (something Eric Desart pointed out a number of years ago). for sure, this is an inexact science at best but the effects are notable...

gullfo wrote:

- your top and bottom slats should be at least 50% of the largest slats used to ensure the cavity space spring effect on either end is balanced ...

Hi Glenn, Is that at least 50% of the width of the widest slat per compartment or of the entire panel -I'm assumng the former? I think the thread I linked to in my post above is the one with Eric you're referring to.

hi there! so, here come the measurements.... reverb time is currently .743 ms which is a lil bit long..... so, what advise will the oracle of the magic forum of john sayer offer? the humble student chris

BriHar wrote:

Is that at least 50% of the width of the widest slat per compartment or of the entire panel -I'm assumng the former?

the former.