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Using sound absorption to reduce standing waves
Using sound absorption to reduce standing waves
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Old 13th June 2020, 01:01 PM   #11
keithj01 is online now keithj01  United Kingdom
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Do note that the presence of stuffing material will usually increase the effective box volume, as seen by the driver. This is because the material tends to act as a heat sink, such that changes in pressure resulting from cone movement are approaching isothermal (as opposed to the adiabatic changes in a box containing only air).
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Old 13th June 2020, 01:17 PM   #12
oldspkrguy is offline oldspkrguy  United States
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Originally Posted by Pano View Post
Yes for sure on the Dayton mic. I have one, and the Behringer version. Very handy indeed and the Dayton comes with a calibration file. Nor sure about the M-Audio. Does it have phantom power? I have owned 3 M-Audio cards, the little one with a single XLR does not have phantom power. Currently I use a Behringer USB card and the M-Audio are on the self. The Focusrite Scarlett is popular in many circles.

EDIT: It does have phantom power, so it will work with the measurement mic.
The M-Audio DOES have phantom power; is very popular because suppliers have trouble keeping it in stock. It is 24 bit, 192 KHz and the pre-amps are supposed to be very high quality and low noise. I ordered one just now; we'll see how it works out. On sale at about $109 US. Thanks, I'll get the Dayton mic also. I really wanted much higher quality but the price goes way up and I can't justify that.
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Old 13th June 2020, 02:21 PM   #13
oldspkrguy is offline oldspkrguy  United States
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Originally Posted by Legis View Post
Also lining the box with 1-2cm thick acoustic felt is usually enough. This is usual damping for reflex cabinets but works with closed systems also and do not give "over damped"/"hits with a cotton ball" feeling that full stuffing does (imo). Maximum effect with least amount of damping material is the best solution sonically for my ears.

Woofers in the middle (or their acoustical center, if many woofers share the same air space) of any dimension sit at the pressure minimum of that dimension's primary mode (the lowest mode) and do not excite it much, which is good because the lowest mode is usally the only one that needs any extra damping after lining the box with felt.

Wave breakers, non-parallel walls etc. also work very good.

If you go to my thread on "3 1/2 way with sub..." I posted some photos. Essentially; I did something similar what you have said above. I located the woofer near the top on purpose. The Aperiodic vent is in the same chamber with the woofer but it is offset from center X,Y, and Z axis. This gives me an asymmetrical surface to volume "ratio" for "wave-breaking" as you mention. The idea was to not have everything at even multiples of dimensions as a further attempt to "break-up" standing waves (why we sometimes use the golden ratio; avoiding even multiples, etc). The woofer chamber has 2 thicknesses of felt on the largest internal surface areas; single layers in the smaller areas. The 4 inch foam pyramids do not completely cover the back or sides of that woofer chamber but approximately 1/3 rd so this is NOT a fill attempt in this region. Again; I have an Aperiodic vent so that path is supposed to remain clear of any damping, stuffing, etc. The 2 lower chambers DO have fill; poly' stuffing. This puts the stuffing at maybe 50%. Without measuring; I am getting a fairly flat in-room response from about 37 Hz to my X/O of about 300 Hz. I am not detecting any major peaks or dips. I literally just now ordered the microphone, interface and stand, etc. I will try what others have suggested including 2 channel loopbacks, impedance, etc. using REW.

Thanks for the ideas, experiences and inputs; that's what we're all here for...ideas and suggestions!
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Old 13th June 2020, 03:05 PM   #14
AllenB is offline AllenB  Australia
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Using sound absorption to reduce standing waves
Polk, some good information there. Modes in a closed box are independent of the bass resonance.
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Originally Posted by 33Polkhigh View Post
the sound reflecting back from the wall of stuffing due to an impedance change.
I can't see it changing, not primarily anyhow.
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Old 13th June 2020, 05:26 PM   #15
Rallyfinnen is offline Rallyfinnen  Sweden
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I tried to absorb the lowest frequency in towers in a similar way (standing wave top to bottom), but it did not work so well. It seemed to require too much stuffing to be effective, almost blocking the passage, and the woofer could not 'breathe' into the whole volume of the box, and the bass got 'constipated'.
The impedance curve showed a really smooth damped resonance, and the resonance frequency was not increased, but still the bass was not good. Putting the damping material in the ends of the tower seems to objectively give better results for the top/bottom standing wave IMHO.
However, a 'burrito' behind the woofer would probably take good care of the higher frequency standing waves between the side walls, and the reflection back to the cone from the rear wall, and very little felt would probably be needed on the back wall. Burrito sounds like a good approach since the damping should be most effective where the velocity is highest, and that should be in the center of the box.
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Old 13th June 2020, 06:41 PM   #16
oldspkrguy is offline oldspkrguy  United States
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My original test box was on the small side so the Q was too high. When I put the first tower together (larger size, different shape, etc.); I went ahead and installed the Aperiodic vent. I did this because I realized it would be easier to block it off after the fact (if need be) than to disassemble, remove felt, stuffing, etc. to install it later. As it turns out; the bass is better behaved in my room using the vent. My calculated box size gives a Qtc of almost exactly 0.707 (without vent, damping, stuffing, etc.). so it may not just be the lowering of the overall Q; it could be because the resonant impedance peak is reduced; the woofer can "breathe" better even if it means f3 increases and the roll-off goes from 2nd to 3rd order. Whatever the reason or reasons; blocking the vent makes the bass "muddier" and less defined. My room gain may actually be the biggest contributing factor here. The felt in the woofer chamber and removable panel is glued into place. The felt elsewhere, stuffing and foam pyramids are all just shoved in and held in place by compression and friction. I did so that I could easily add or delete as needed. Once I get my measurement set all in place; I can fine tune damping, stuffing, blocking, etc. My biggest issue is the room placement options are very limited so I have to make the woofer boxes behave as best I can where they fit easiest in the room.
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Old 15th June 2020, 05:21 PM   #17
33Polkhigh is offline 33Polkhigh  United States
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Quote:
Originally Posted by Legis View Post
Also lining the box with 1-2cm thick acoustic felt is usually enough. This is usual damping for reflex cabinets but works with closed systems also and do not give "over damped"/"hits with a cotton ball" feeling that full stuffing does (imo). Maximum effect with least amount of damping material is the best solution sonically for my ears.

Woofers in the middle (or their acoustical center, if many woofers share the same air space) of any dimension sit at the pressure minimum of that dimension's primary mode (the lowest mode) and do not excite it much, which is good because the lowest mode is usally the only one that needs any extra damping after lining the box with felt.

Wave breakers, non-parallel walls etc. also work very good.
Lining the walls helps to damp panel resonances. But one of the problems with lining is that it still presents a flat surface which gives the subjectively more hollow box sound. Lining the walls is still probably the most common approach.
I've been curious how woofer position affects the internal sound. Centered maybe the best spot since placing a driver at the end of a pipe will give the most standing waves.

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Originally Posted by Pano View Post
Well yes, but it's maybe not all that bad. If you use very good stuffing like wool or cotton, the absorption is fairly even across the audio spectrum, dropping off in the bass of course. Wool and cotton do better in the bass than do fiberglass or rock wool.

I do like the acoustic burrito idea. Years ago we hung a wool felt curtain in the middle of the cabinet and it made a remarkable improvement. I've used that trick many times.
I definitely like the sound of wool stuffing. It adds body to the sound and is solid sounding. Fiberglass is cheap and has more of a crisp sound. One could also stuff a wool sock with fiberglass to get real fancy.

Quote:
Originally Posted by AllenB View Post
Polk, some good information there. Modes in a closed box are independent of the bass resonance.

I can't see it changing, not primarily anyhow.
iirc when a sound wave hits a soft surface like foam there is an impedance change that creates a backwave. So this is one of the reasons to use an odd or rounded shape, at least in theory. I've found that an object in the middle gives the speaker a more solid sound without sounding "stuffed". I can't say I know for sure what causes the stuffed sound though.
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Last edited by 33Polkhigh; 15th June 2020 at 05:31 PM.
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Old 15th June 2020, 05:27 PM   #18
33Polkhigh is offline 33Polkhigh  United States
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Quote:
Originally Posted by Rallyfinnen View Post
I tried to absorb the lowest frequency in towers in a similar way (standing wave top to bottom), but it did not work so well. It seemed to require too much stuffing to be effective, almost blocking the passage, and the woofer could not 'breathe' into the whole volume of the box, and the bass got 'constipated'.
The impedance curve showed a really smooth damped resonance, and the resonance frequency was not increased, but still the bass was not good. Putting the damping material in the ends of the tower seems to objectively give better results for the top/bottom standing wave IMHO.
However, a 'burrito' behind the woofer would probably take good care of the higher frequency standing waves between the side walls, and the reflection back to the cone from the rear wall, and very little felt would probably be needed on the back wall. Burrito sounds like a good approach since the damping should be most effective where the velocity is highest, and that should be in the center of the box.
For a standing wave that's a long wavelength (like in long skinny apeaker) I wouldn't think that a sound absorber would be big enough. Stuffing the length might be the only way to deal with it. I have a subwoofer that I've tried a few different stuffing materials and pattern in and nothing made much of a difference.
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Last edited by 33Polkhigh; 15th June 2020 at 05:36 PM.
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Old 15th June 2020, 08:39 PM   #19
Pano is offline Pano  United States
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Using sound absorption to reduce standing waves
Quote:
Originally Posted by 33Polkhigh View Post
One could also stuff a wool sock with fiberglass to get real fancy.
Ha ha. Yes, very fancy.
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Old 15th June 2020, 09:17 PM   #20
bentoronto is offline bentoronto  Canada
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Using sound absorption to reduce standing waves
Quote:
Originally Posted by oldspkrguy View Post
... I am not detecting any major peaks or dips. I literally just now ordered the microphone, interface and stand, etc. I will try what others have suggested including 2 channel loopbacks, impedance, etc. using REW.

Thanks for the ideas, experiences and inputs; that's what we're all here for...ideas and suggestions!
You seem like an interesting and inquisitive fellow, a good new member. But sadly, you seem unaware of a galaxy worth of reasons why testing has to be blind and not to aggrandize the value of your ears, matters of taste excepted.

So as you start working with REW, how can you work so as to best promote your education? For example, you could take a blank sheet of paper and draw a line across representing your opinion of the frequency response at your seat of your speakers.

And then measure with your new mic. If you are brave.*

I'd give that advice to all the folks in this thread who think their ears are data. But only the brave would take my advice given the risk to their egos.

B.
* and if you are really brave, start a new thread and post, and ask others to do the same exercise
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Last edited by bentoronto; 15th June 2020 at 09:22 PM.
 

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