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Author Topic: Importance of non-axial modes?  (Read 2975 times)

Ethan Winer

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Importance of non-axial modes?
« on: October 13, 2006, 04:34:46 pm »

Fran,

I'm wondering what your opinion is about the importance of non-axial room modes. It seems to me they are less important than axials, and maybe not even important at all. But what do you think? Do you even consider non-axials when designing a room? If you do consider them, do you give them less "weight" than axial modes?

Thanks.

--Ethan

Etch-A-Sketch

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Re: Importance of non-axial modes?
« Reply #1 on: October 13, 2006, 05:38:51 pm »

this also brings up another question I had regarding room modes... Having non parallel surfaces is supposed to help against room modes, correct?  But I've never really seen a difinitive answer as to how much it helps. For example, if you don't have parallel walls, will the axial, tangential, and oblique modes be non-existent, or just not quite as strong as they would with parallel walls?
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franman

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Re: Importance of non-axial modes?
« Reply #2 on: October 13, 2006, 09:45:53 pm »

Ethan:

We use the Bonello criteria which is purely Axial based, and also consider oblique and tangential modes when doing simulations.  Typically (in the texts) tangential modes are weighed at 50% significance (I believe) compared to Axial and oblique at 25%....

In practice, we have found that once you get past the first two axial modes, the build up of "other" modes becomes very helpful in filling in the energy. They certainly are significant in the transitional zone above 125Hz or so and below 300....

that's how we look at it..

Chris:

ALL rooms have modes, boxes, trapezoids, eight sided, curved, etc.. They all have modes. They are just much more difficult to predict in non-rectangular environments which is one of the reasons we often start with rectangular shells and build the treatments "inside" the box to look more like studio geometry.....

Of course, we also design many hard shells with non-parallel geometry when it best serves the design requirements such as layout, sight lines or arrangment of adjacent spaces... or when the roof is angled in the host structure (and all of the above).. then we do more intensive simulations studying min, max and average dimensions....I've also developed a bit of a feel for these non-parallel designs through hundreds of "experiments" ... oops I mean projects... so I know what has worked and can kinda reverse engineer why. Yes, you still have modes and these rooms still need (maybe even more) broadband bass traps... okay?
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franman

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Re: Importance of non-axial modes?
« Reply #3 on: October 13, 2006, 09:48:59 pm »

Lars... feel free to chime in on the mode weighting issue please!! (he wrote some very useful simulations that we use all the time.. no, sorry we aren't gonna publish them!!)
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jimmyjazz

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Re: Importance of non-axial modes?
« Reply #4 on: October 13, 2006, 10:30:39 pm »

franman wrote on Fri, 13 October 2006 21:45

ALL rooms have modes, boxes, trapezoids, eight sided, curved, etc.. They all have modes. They are just much more difficult to predict in non-rectangular environments which is one of the reasons we often start with rectangular shells and build the treatments "inside" the box to look more like studio geometry.....


Exactly right.  We're talking about resonance.  An acoustic mode, regardless of room shape, is analogous to the resonance which occurs between a mass and a spring or an inductor and a capacitor.  It's all related to the "sloshing" of kinetic and potential energy.

Changing room dimensions, or even room shape, merely changes the frequencies at which these resonances occur, as well as the distribution of the peaks and nulls in the room at those frequencies.  Sure, it's a multi-dimensional system, so it's more complicated than a single mass on a single spring, but the analogy still holds.

Does a round drum resonate?  Yes?  What about a rectangular drum?  What about a trapezoidal drum?
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Ethan Winer

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Re: Importance of non-axial modes?
« Reply #5 on: October 14, 2006, 02:17:05 pm »

Fran,

Excellent, thanks very much!

My only follow-up question relates to this:

> Typically (in the texts) tangential modes are weighed at 50% significance (I believe) compared to Axial and oblique at 25% <

I know this is what "the texts" say, but I wonder if this is true in practice. Has anyone actually tested it? For example, it seems to me the really important mode - the one that accounts for the worst peaks and nulls if not the worst resonances - is the length mode front to back.

One of the reasons I'm skeptical, for lack of a better word, is because I've seen predicted modes vary as much as 20 percent from what's actually measured. So when I view a "full" mode list showing all the axial and non-axial modes, there are so many modes, and they're so close together, I'm not sure it's even possible to sort them out and know which is really contributing to the particular frequencies measured. That is, a length mode might be predicted at 35 Hz but when measured it's really 30 Hz. So when you have a large number of modes, and you measure a peak or null or resonance, how can one even be sure it's caused by one mode versus another?

--Ethan

franman

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Re: Importance of non-axial modes?
« Reply #6 on: October 14, 2006, 02:56:17 pm »

Ethan,

I have seen many rooms (including our demo room Smile where the up-down mode had the biggest detrimental affect on the speaker low frequency response... fix: move the speaker down by 6" or so and voila, no more problem... the speaker (woofer) had been sitting right in the middle of the height dimension and it obviously over excited some of those modes..

The whole modal thing is also closely tied to speaker boundary interference, in other words where you put the woofers in relation to the boundaries and how this sets the room modes into action. This is one of the nice things RPG room optimizer does: It allows you to place independent and dependent woofers in a room and run simulations on the variations in not only the room dimensions but the speaker locations to really optimize... this program has shown me some very accurate results compared to actual field measurements in a few instances... so we tend to trust it... It just that it starts with random starting points (within your limits) so you have to run it over and over and wait for "optimal" results and keep track as you go....
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jfrigo

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Re: Importance of non-axial modes?
« Reply #7 on: October 15, 2006, 03:42:23 am »

franman wrote on Sat, 14 October 2006 11:56

I have seen many rooms (including our demo room Smile where the up-down mode had the biggest detrimental affect on the speaker low frequency response... fix: move the speaker down by 6" or so and voila, no more problem... the speaker (woofer) had been sitting right in the middle of the height dimension and it obviously over excited some of those modes.


I'm just tweaking a room with this same issue. The front-back mode certainly is not one you want to ignore, but the up-down is surprisingly prominent in some room setups. Altering speaker height or adding a trap can help. This may be an occasion to use a tuned trap (depending on what's already there) as you can reconcile the predicted mode with the measured mode and precisely knock out the offender.
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Ethan Winer

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Re: Importance of non-axial modes?
« Reply #8 on: October 15, 2006, 12:29:54 pm »

Fran (and Jay),

> move the speaker down by 6" or so and voila, no more problem <

Excellent point, and likewise for SBIR. Though SBIR peaks will not be accompanied by ringing so it's easy to separate those out by viewing a waterfall plot.

Thanks.

--Ethan
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