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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, 08:50 AM   #1
33Polkhigh is offline 33Polkhigh  United States
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Default Using sound absorption to reduce standing waves

This subject has been talked about for a long time, but is still not well understood by even experienced speaker builders. What exactly goes on inside a cabinet is part of the art of speaker design.

One aspect is cavity resonances. These are different from panel or material resonances and are related to the size and shape of the internal air space of the cabinet.

The bottom line is that cavity resonances (which is what standing waves are) are generally not good unless they being used to generate extra bass.

One way to get rid of standing waves is to stuff the speaker. I have never liked the sound of a "stuffed" speaker. I believe what I am hearing is the sound reflecting back from the wall of stuffing due to an impedance change. Also the stuffing selectively absorbs some frequencies over others.

What's better is to put a sound absorbing structure in the center of the cabinet. I did this on a recent build and was pleased with the results. What this is is essentially a sound absorbing burrito. Its poly batting with fiberglass in the middle. Then its rolled up like a burrito and stapled together. It then sits in the middle of the cabinet behind the drivers. It sits between several braces and is also held in place by string so it won't move.

I noticed a very clean sound from using it. I don't think it significantly affects cabinet volume either.

Also here is an excellent tool for calculating standing waves of a typical square shaped cabinet

HiFi Loudspeaker Design
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Last edited by 33Polkhigh; 13th June 2020 at 08:55 AM.
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Old 13th June 2020, 09:39 AM   #2
oldspkrguy is offline oldspkrguy  United States
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I actually wanted to do the exact same thing but my knock down cabinets wouldn't allow for this due to the structure of the bracing. My thread is the "3 1/2 way ..." if you are interested. I used felt in places, stuffing in places and large foam pyramids in places. The woofer chamber doesn't have any stuffing, just felt and foam pyramids. The main reason for this is because I am using an Aperiodic "flow resistor" vent. I am getting good results. I would love to have an impedance measuring set up but can't really justify the cost. Just curious; are you or anyone else reading this aware of any relatively inexpensive measuring hardware and software tools for impedance? Back ages ago; I had access to very expensive test and measuring equipment and tools; I also had a shop big enough, and plenty of tools to build my own enclosures from scratch. Almost EVERY box I built was designed to avoid standing waves; slanted sides and top, 5 sided boxes, etc. ZERO parallel surfaces exterior walls and even including all internal bracing, stiffeners, etc.
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Old 13th June 2020, 09:45 AM   #3
wesayso is offline wesayso  Netherlands
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Impedance Measurement

Rather inexpensive but works like a charm. The impedance plot can tell you a lot of what's going on inside the cabinet. Compare the impedance plot to the driver in free air vs in the enclosure to find resonances.
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Old 13th June 2020, 09:48 AM   #4
33Polkhigh is offline 33Polkhigh  United States
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Quote:
Originally Posted by oldspkrguy View Post
I actually wanted to do the exact same thing but my knock down cabinets wouldn't allow for this due to the structure of the bracing. My thread is the "3 1/2 way ..." if you are interested. I used felt in places, stuffing in places and large foam pyramids in places. The woofer chamber doesn't have any stuffing, just felt and foam pyramids. The main reason for this is because I am using an Aperiodic "flow resistor" vent. I am getting good results. I would love to have an impedance measuring set up but can't really justify the cost. Just curious; are you or anyone else reading this aware of any relatively inexpensive measuring hardware and software tools for impedance? Back ages ago; I had access to very expensive test and measuring equipment and tools; I also had a shop big enough, and plenty of tools to build my own enclosures from scratch. Almost EVERY box I built was designed to avoid standing waves; slanted sides and top, 5 sided boxes, etc. ZERO parallel surfaces exterior walls and even including all internal bracing, stiffeners, etc.
Its almost embarrassing how few measurement tools I have. I have trained my ears over time to hear details and diagnose problems, but I will be the first to admit that even trained ears will benefit from good measurements.

If I was going all out I might look into complex cabinet shapes. One of the nice things about an absorber is that by sitting in the middle it prevents parallel sides from resonating. So its easier to implement. The burrito shape only took about 15 mins to make and I even weighed the amount of fiberglass.
One issue with using a sound absorber is that if it gets large it will begin to affect cabinet volume since the waves don't reach the center.
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Old 13th June 2020, 09:58 AM   #5
oldspkrguy is offline oldspkrguy  United States
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Quote:
Originally Posted by wesayso View Post
Impedance Measurement

Rather inexpensive but works like a charm. The impedance plot can tell you a lot of what's going on inside the cabinet. Compare the impedance plot to the driver in free air vs in the enclosure to find resonances.

THANKS! I just installed REW, haven't tried it yet though. Next up on my list is a microphone, etc. While we're on that topic; do people have good experiences with the following? Again; I can't justify big bucks for high end microphones, etc.

Dayton Audio - EMM-6 Electret Measurement Microphone


used with this interface

M-Audio


The M audio has 2 channels so I could do loopbacks; someone else clued me in on that as the best way to do measurements.
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Old 13th June 2020, 10:02 AM   #6
oldspkrguy is offline oldspkrguy  United States
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Quote:
Originally Posted by 33Polkhigh View Post
Its almost embarrassing how few measurement tools I have. I have trained my ears over time to hear details and diagnose problems, but I will be the first to admit that even trained ears will benefit from good measurements.

If I was going all out I might look into complex cabinet shapes. One of the nice things about an absorber is that by sitting in the middle it prevents parallel sides from resonating. So its easier to implement. The burrito shape only took about 15 mins to make and I even weighed the amount of fiberglass.
One issue with using a sound absorber is that if it gets large it will begin to affect cabinet volume since the waves don't reach the center.

I am a former musician and have many musician friends and family. In extended and critical listening tests; most agree I have things just about as good as I can get them. I would like to do some measuring though just for the final tweaks! There is a LOT to be said for "trained ears"; I tend to notice very subtle changes other people don't recognize...
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Old 13th June 2020, 11:12 AM   #7
Legis is offline Legis  Finland
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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.
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Last edited by Legis; 13th June 2020 at 11:15 AM.
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Old 13th June 2020, 12:21 PM   #8
celef is offline celef
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I think this topic is interesting, it is also interesting that we all seems to be sensitive to different things, a speaker box without acoustic dampening inside, stuffing, sounds horrible to me, it is like listening to distortion of very high magnitude, and this sounds natural to some
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Old 13th June 2020, 12:34 PM   #9
Pano is offline Pano  United States
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Using sound absorption to reduce standing waves
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.
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Old 13th June 2020, 12:41 PM   #10
Pano is offline Pano  United States
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Using sound absorption to reduce standing waves
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Originally Posted by 33Polkhigh View Post
Also the stuffing selectively absorbs some frequencies over others.[/url]
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.
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