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Using sound absorption to reduce standing waves
Using sound absorption to reduce standing waves
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Old 15th June 2020, 10:50 PM   #21
oldspkrguy is offline oldspkrguy  United States
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Yes, there is merit in all you have said. I have used test tones, sweeps, etc. already; just not with a microphone or measuring tools. Having said that; as a former musician; I have a very keen ear for what sounds right. Many of my family and close friends are also musicians; practicing even now. I am also a retired Engineer and Technician so I know and fully understand the value of valid testing and measuring. I asked my brother to listen critically to several test tracks while I made changes to my woofer X/O. He isn't technical at all so had no clue what I was up to. He is one of the current musicians I mention though and he and I both agreed on the exact same set-up as sounding the "most real" and "natural".

It has been 20+ years ago but I once had a calibrated microphone and access to very high quality, laboratory grade electronic test and measurement equipment. What I am JUST NOW getting into is all of the free and in-expensive software packages and tools available "these days".

I will do some "blind A/B/X" testing with these musicians; I will know what is going on but they will not. This takes MY personal bias out of the equation entirely!

Cheers!
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Old 16th June 2020, 01:01 AM   #22
AllenB is offline AllenB  Australia
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Using sound absorption to reduce standing waves
Quote:
Originally Posted by 33Polkhigh
iirc when a sound wave hits a soft surface like foam there is an impedance change that creates a backwave.
We know doing it the other way has problems. Technically nothing 'hits' the stuffing because the driver is already up against it. Its effect would be continuous, simply a different load.
Quote:
Originally Posted by 33Polkhigh View Post
Centered maybe the best spot since placing a driver at the end of a pipe will give the most standing waves.
This is going to give you two half length pipes. The full length mode won't eventuate. There will still be modes like before, they will be pushed up in frequency.
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Old 16th June 2020, 02:23 AM   #23
bentoronto is offline bentoronto  Canada
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Using sound absorption to reduce standing waves
Quote:
Originally Posted by oldspkrguy View Post
...I will do some "blind A/B/X" testing with these musicians; I will know what is going on but they will not. This takes MY personal bias out of the equation entirely!
Sobering to have a show of hands when younger guests are by for who can hear tones at 10kHz, 11 kHz, and so on.

Or try this excellent website (results nicely match* my Can. Hearing Society audiology tests):

Online Hearing Test & Audiogram Printout

B.
* "nicely match" doesn't mean "pleasantly revealing"
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Old 16th June 2020, 02:30 AM   #24
oldspkrguy is offline oldspkrguy  United States
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Quote:
Originally Posted by AllenB View Post
We know doing it the other way has problems. Technically nothing 'hits' the stuffing because the driver is already up against it. Its effect would be continuous, simply a different load.

This is going to give you two half length pipes. The full length mode won't eventuate. There will still be modes like before, they will be pushed up in frequency.

If anyone is interested; check out my latest project "3 1/2 way with sub...". My woofer chamber is NOT filled with stuffing on purpose. Felt on the walls...YES; large foam pyramids...YES; stuffing/fill is only in the lower chambers and accounts for about 50% total. Again, mic and interface on the way; I am convinced I am 85% to over 90% fine tuned to the best possible "compromise" but will keep going...retired with nothing better to do has it's MANY benefits...Speaker builder, musician and Technician OVER 50 years; Engineer over 45 years...did NOT just fall into this crazy hobby by accident or yesterday!
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Old 16th June 2020, 02:58 AM   #25
AllenB is offline AllenB  Australia
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Using sound absorption to reduce standing waves
Quote:
Originally Posted by oldspkrguy
the best possible "compromise"
I notice you mentioned tones and sweeps, and I can see the benefit of having a generator frequency control at hand when trying to hear through these things.

When you say compromise, is the result minimum phase and all coming through the cone?
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Old 16th June 2020, 06:58 AM   #26
ScottG is offline ScottG  United States
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Quote:
Originally Posted by 33Polkhigh View Post
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've been advocating this sort of structure or rather structures for a while now (..I even did a design for a member quite some time ago for a sand-damped subwoofer box that had not only the center absorber, but also two rear corner absorbers). Principally you need to keep absorption OFF of the interior walls.

Most of what's subjectively *disliked has nothing to do with standing waves and their reduction, rather it's the restrictive change in the driver's mechanical compliance under normal use (..not simply mechanical compliance as a singular value, but rather a far more complex interaction with driver motion and the box's interior that is of course transient with dynamic changes with reproduced music).

Timely, Veritasium has a recent video on Turbulence. Note that "fluid" in this case is the air inside the box:

YouTube

The center of the box has much higher air velocity than at the cabinet's interior panels. The compliance of the driver however primarily couples to those walls - it "spreads-out", and you can see from the video what is occurring.

Here is a discussion I had with another member with his fullrange driver in-pipe, stuffing and no stuffing:

My Fostex FE 108EZ project, Part 2

Note that not all drivers have such different subjective results with stuffing (on cabinet interior walls or not). It depends a lot on the driver's mechanical nature and particularly its surround in relation to the driver's excursion under operation.

This is particularly critical to the subjective result of tweeters. (..lower excursion under typical use, the more critical it is). (..and the other area is the profile of the diaphragm - trying to decrease damping while still maintaining a good result.)


*reflections going through the cone diaphragm are another source of dislike, mostly as freq.s increase. So here in particular it's good to have that absorber behind the driver, though again away from interior surfaces including the driver. Suspending a "sheet" of ultratouch insulation in the cabinet does a great job (..as an alternative to using a "dowel"/cardboard-tube and wrapping it like a paper-towel/toilet paper roll). Again of course, keeping the stuffing for the absorber off of interior surfaces.
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Last edited by ScottG; 16th June 2020 at 07:09 AM.
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Old 16th June 2020, 07:39 AM   #27
Legis is offline Legis  Finland
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Quote:
Originally Posted by 33Polkhigh View Post
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.

Here's the impedance of a 4x15" line array consisting of two separate modules.

Click the image to open in full size.

Click the image to open in full size.

The 400Hz bump is driver's own upper surround/cone edge resonance, not cabinet's. Above picture is a simulation of standing waves inside the cabinet using Vituixcad (no modes near 400Hz).

You can see there is very small bump at the 160Hz which is the lowest cabinet resonance. If I connect only one of the woofers the lowest ~160Hz mode is greatly amplified. But with dual drivers it smoothens out almost completely because their acoustic center is at the pressure minimum of said mode and driver's don't excite it.

The cabinet is lined with only 2cm thich acoustic felt on top/bottom/back and sides have only one layer/1cm, so it doens't do anything really for the lowest mode. If damping material were to be used to tame the 160Hz resonance, it would need to be VERY thick like 20cm at least. It would greatly affect the box's air spring also.

With only lining the box the box's air spring and compliance stays "non-lossy" ie. you get high impedance peak at the system resonance (which I want). Choosing a driver with high Qms/low Rms (low friction) would be in vain if the lossy air spring's friction would dampen it anyway.

Minimum damping for the maximum effect, just enought to adequately kill the box but leave the air spring as non-lossy as possible if one wishes to get that tactile, dynamic, easy to breathe and "sensitive" at low spl bass quality imo.
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Last edited by Legis; 16th June 2020 at 07:42 AM.
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Old 16th June 2020, 11:38 AM   #28
oldspkrguy is offline oldspkrguy  United States
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Great inputs here from everyone! Take a look at my photos; not the best but then you will get a better understanding of what I'm trying to describe. As I said, maybe elsewhere?, blocking my Aperiodic "flow resistor" vent seems to muddy the bass; making it less defined. Having the mic and measuring software will allow me to confirm or disprove what my ears are telling me. Again. maybe elsewhere; the felt damping is only glued down on some woofer chamber walls and the removable (side) panel. All other felt, stuffing/fill and foam pyramids are just shoved into place and held by friction and compression. This gives me many options for box treatment. This is the main reason I wanted a removable panel. I never mentioned this but the photos show it; I used 2 inch wide rubber strips; 1/8 inch thick as a sealing gasket on the removable panel. There are about 18 screws holding this in place; I seriously doubt I have any flex or movement here because I added cleats or wood blocks for these screws so I am not affixing the panel to the MDF itself. Should it become necessary; I can always increase the screw size for a tighter fit.

My thread is "3 1/2 way with sub..." I have done horns, vented (we called it bass reflex growing up) and closed box designs plus a few hybrids. I have never done a TL of any kind. I used to do mostly large BR designs with low tuning; these days it's mostly smaller closed boxes. My 3 1/2 way is the first time at using an Aperiodic vent.

I know from acoustics studies we sometimes need to do short bursts, chirps, gated measurements, etc. Continuous tones can be misleading; same with slow seeps.

WHAT DO YOU GUYS RECOMMEND FOR BEST TESTING FOR ROOM MODES; STANDING WAVES, PEAKS, DIPS, ETC.??? Is REW sophisticated enough to "sniff" these things out? I have never used it before; looking forward to new discoveries here!

Cheers!
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Old 16th June 2020, 11:42 AM   #29
celef is offline celef
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I guess the only way to preserve a driver charactar is to use a infinite baffle, driver rear output freely guided to another room as far as i know. 2nd best, response wise would be a very large closed box with the right amount acoustic damping inside, i guess it would be a necity to use a large amount, preferable mineral wool of a quite a low density.
Anything else would alter the driver natural response and introduce resonances
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Old 16th June 2020, 11:47 AM   #30
oldspkrguy is offline oldspkrguy  United States
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I read something in "Speaker Builder" magazine literally decades ago. Someone used a fireplace chimney as the "enclosure" with a large woofer. I don't remember the details; I don't think the chimney was considered a tuned vent; I think it was considered an infinite baffle.
 

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