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Do room modes even matter? #1647339
03/28/04 08:55 PM
03/28/04 08:55 PM
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New Milford, CT, USA
Ethan Winer Offline OP
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Ethan Winer  Offline OP
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Yesterday Doug Ferrara and I ran a series of static low frequency response measurements in my living room to determine the effect of moving the listening position. This room is 25 feet long by 16 feet wide, with a vaulted ceiling that varies in height from eight feet at the front and rear walls to 11 feet in the center. The room also has a large number of broadband bass traps - 19 in total - and all but five are in corners. I used only the two main loudspeakers for this test, intentionally turning off my subwoofer to avoid additional variables. The ETF program from http://www.acoustisoft.com was used to measure the response at each location. The results of these tests are described in detail following, with graphs, and they confirm what I believe is the primary cause of the peaks and deep nulls that occur in all small rooms.

The first graph below shows the change in frequency response when moving from the front toward the back of the room. In all cases the measuring microphone was centered left and right. We started about one foot forward of where I usually sit, then moved farther and farther back in increments of about one foot. The graph's line colors are set to become progressively darker, which shows the trend more clearly than using blue, red, yellow, and so forth:



So what does this graph tell us? Most important, of all the peaks and deep nulls only one of them is related to a room mode! All of the others are directly related to the 1/4 wavelength distance from the rear wall, as annotated on the graph. The peak at 45 Hz is modal, and is due to the 25 foot room length - its frequency does not change with distance, though its level does. All subsequent peaks and nulls shift upward in frequency in direct relation to moving the microphone closer to the rear wall. A tape measure and calculator confirmed the relationship between the frequencies and distances.

Note also in the graph above that the modal peak at 45 Hz is not much of a problem near the front of the room. At least not in a room that has a serious amount of acoustic treatment and bass trapping. Some day I hope to run tests like these again in another room, both before and after installing treatment.

This next graph was taken near the usual listening position, and shows the change in response when moving from two feet left of center through two feet right of center. As you can see, when moving left to right the major peaks and nulls do not change frequency, only level, which further confirms that the distance from the rear wall is the primary influence on low frequency response:



The last graph shows the result of changing height at the usual listening position, as when moving from sitting to a standing position. Although there is some change in the center frequencies, they again remain related closely to the distance from the rear wall:



The most important information I get from these tests is that a room's modal behavior is the secondary mechanism that defines its behavior. The primary mechanism is the 1/4 wavelength effect - boundary-induced comb filtering - that occurs at a predictable distance from the rear wall. Why the rear wall? Because that's the main direction the wavefront travels as it leaves the loudspeaker.

The notion that boundary-induced comb filtering is even more important than modal response makes a lot of sense once you think about it. The only difference between modal and non-modal frequencies is that modal frequencies fit exactly between the walls, and so have the potential to ring longer when excited. So it's clear to me, anyway, that the primary cause of peaks and nulls at all low frequencies is the 1/4 wavelength distance from the rear wall.

One other thing this data shows is that frequency-specific absorption and even EQ can indeed be useful at very low modal frequencies. Since the single mode-induced 45 Hz peak remains at that frequency no matter how the listening position is moved, that means it can be targeted successfully with narrow band absorption and/or EQ.

All comments are most welcome.

--Ethan

Re: Do room modes even matter? #1647340
03/30/04 04:33 AM
03/30/04 04:33 AM
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philbo_Tangent Offline
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Ethan,

This is most interesting...

I think it would be an idea to repeat this experiment in an untreated room. Or perhaps, more to the point, in a room that is untreated, then in the same room after being treated.

Phil

Re: Do room modes even matter? #1647341
03/30/04 05:33 AM
03/30/04 05:33 AM
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kylen Offline
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Thanks for sharing your findings & thoughts. This would seem to be a pretty important development in the scheme of things.

Re: Do room modes even matter? #1647342
03/30/04 06:03 PM
03/30/04 06:03 PM
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Ethan Winer Offline OP
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Phil and Kylen,

> a room that is untreated, then in the same room after being treated. <

I agree, and I'll do that the next time I'm involved personally in an installation.

Also, I should have made this clearer in my original post: I'm not really arguing that modes are unimportant. But it seems clear to me that they're a subset of the more general case of acoustic interference.

--Ethan

Re: Do room modes even matter? #1647343
03/30/04 07:40 PM
03/30/04 07:40 PM
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caseyLA Offline
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Well this is great info. Thanks for sharing.

I'd like to make my own contribution to this. Construction on my studio space will be complete in about a week, and I'm in the process of building many insulation traps to be used in accordance with Ethan's suggestions.

I'd like to make similar front-to-back and side-to-side measurements before and after acoustic treatments. I just need to get up to speed with the ETF software.

Initially, I was just going to make a few RT60 measurements after installation to see if things were cool, and get on with producing the album. But if a more in-depth analysis would benefit our knowledge of room modes, I'd be happy to take the time and do things more scientifically.

Any thoughts? I may have a few questions for you Ethan, in the next few weeks.

-casey

Re: Do room modes even matter? #1647344
03/30/04 09:42 PM
03/30/04 09:42 PM
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New Milford, CT, USA
Ethan Winer Offline OP
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Ethan Winer  Offline OP
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Casey,

Great - I encourage you to measure before and after treatment.

One great thing about ETF is you can measure the room at many locations very easily, then analyze the data in different ways later. As long as you measure using broadband sweep or MLS, rather than low frequencies only, you'll be able to later view impulse response, wideband and LF frequency response, and reverb time - all from the same saved data file.

If you need help with ETF ask here. I'm still a beginner too, but I know how to run and interpret half the tests, anyway. \:D

--Ethan

Re: Do room modes even matter? #1647345
04/01/04 10:31 PM
04/01/04 10:31 PM
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caseyLA Offline
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So, if the majority of these peaks and nulls are related to 1/4 wavelengths and not room modes, what are the practical consequences of this in small rooms?

I suppose trying to "tune" a room with many narrow band slat type resonators based on room mode calculations would be a waste of time. Also, I suppose this means that one should always test the mix at different positions along the length of the room, and try to sit away from the back wall as far as possible.

Any other practical implications?

-casey

Re: Do room modes even matter? #1647346
04/01/04 11:10 PM
04/01/04 11:10 PM
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Ethan Winer Offline OP
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Casey,

> if the majority of these peaks and nulls are related to 1/4 wavelengths and not room modes, what are the practical consequences of this in small rooms? <

The practical consequence is, as you observed, that this shows why absorption that targets specific frequencies is inadequate. It also shows why trying to fix the peaks and deep nulls with loudspeaker EQ is futile.

--Ethan

Re: Do room modes even matter? #1647347
04/06/04 08:29 PM
04/06/04 08:29 PM
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Stefan M. Offline
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Hi

Completely new to this fora, b

Re: Do room modes even matter? #1647348
04/06/04 08:49 PM
04/06/04 08:49 PM
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Stefan M. Offline
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Quote:
Originally posted by Stefan M.:
Hi

Completely new to this fora, b
Hmmm...not a prefect start but what I wanted to share was a couple of measurements done at different places in an 10x10 cm grid. Some of the peaks/dips donīt move in frequency (only in magnitude) so my interpretation is that these are room modes (32 Hz, 63 Hz). Other move and I guess the ones affected by side movement is comb filtered by side reflections. Same principle for depth movement. Anyway I wanted to incude the graphs but didnīt find a simple way. Do I need to upload to a server???

Iīm planning to build a number of bass traps especially an Helmholtz for the 63 Hz: 12 MDF-board (9.36 kg/m2) with 95 mm spacing. A Natural place is behind the curtains in my home theatre and I can make them 200x120 cm each side of the screen. I then got the idea that I should check where the pressure is highest so I feed my sub with a steady state 63 Hz sinus and moved around with an SPL meter. I then found maximas in the center of the room along the side wall but not in the corner I was planning to put the base absorber... Advice anyone?

RT-60 is 0.2 - 0.3 above 125 Hz and increases to 0.8 at 63 Hz.

Re: Do room modes even matter? #1647349
04/06/04 10:49 PM
04/06/04 10:49 PM
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New Milford, CT, USA
Ethan Winer Offline OP
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Ethan Winer  Offline OP
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Stefan,

> my interpretation is that these are room modes (32 Hz, 63 Hz) <

Yes, not only because they don't move, but also because they're related to each other by a 2:1 ratio.

> I wanted to incude the graphs <

This web site can't host images, so you have to post them somewhere else. Then you can include a link in your post to make them appear here.

> I then found maximas in the center of the room along the side wall but not in the corner <

With all acoustic treatment you put it where the reflections occur - the corners and walls - not necessarily where the problems show up. I admit I don't have a lot of experience with Helmholtz traps, so I'm not sure what to suggest. I can tell you that the most damaging problems are between around 80 and 300 Hz. So whether or not you try to fix the really low frequency peaks, you still need plenty of broadband absorption to target the mid and upper bass range.

--Ethan

Re: Do room modes even matter? #1647350
04/23/04 06:57 PM
04/23/04 06:57 PM
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blairl Offline
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Based on these results would you recommend lining the entire back wall with broadband low frequency absorbers?

Re: Do room modes even matter? #1647351
04/23/04 08:57 PM
04/23/04 08:57 PM
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jamesway Offline
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Go for it, that's what I'm going to do after reading this post. It seems pretty simple logically also, you'd naturally think the wall the speakers face would be producing the most problems, as long as speakers were very close to the opposite wall.

Re: Do room modes even matter? #1647352
04/24/04 06:16 PM
04/24/04 06:16 PM
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New Milford, CT, USA
Ethan Winer Offline OP
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Blair,

> would you recommend lining the entire back wall with broadband low frequency absorbers? <

Sure, focusing especially on the rear wall-wall and wall-ceiling corners.

--Ethan

Re: Do room modes even matter? #1647353
04/25/04 05:45 PM
04/25/04 05:45 PM
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Newtown, CT
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Hiltonius Offline
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Quote:
Originally posted by Ethan Winer:
All comments are most welcome.

--Ethan
Fantastic post Ethan. As usual, a vivid and precise explanation of your observations.

Nice ad, btw. 8^)

best,
Rich

Re: Do room modes even matter? #1647354
04/25/04 08:16 PM
04/25/04 08:16 PM
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New Milford, CT, USA
Ethan Winer Offline OP
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Hey Rich,

Holy cow, imagine seeing you here!

Thanks. And see the follow-up I just posted earlier today.

> Nice ad <

Yeah, and thanks for making that happen. Your smiling face is all over our web site.

--Ethan


 
   
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