[csconner58]

Hello to all the GR research and AC folks out there.

This question is about addressing the cabinet resonances that Danny often highlights when he measures customer supplied speakers.

How does a person best asses / measure the resonances of a cabinet?

I know using a tool such as DATS V3 or Pocket Clio one can see resonance in the impedance sweep when the drivers are mounted in the cabinet. What about something like an accelerometer? Are there other methods to determine if your cabinet has resonances that are adding “bloom, or other coloration of the original signal”?

If a person can determine that the cabinets are exhibiting these resonances how does one determine if just damping the cabinet walls (aka with a product such as NoRes) is sufficient or if the cabinet needs to be stiffened by mechanical means?

If a cabinet needs to be stiffened mechanically what is the best method?

I see speaker builds where builders use small 1” x 2” wood bracing front to back and side to side within the cabinet, I also see some DIY'ers cut out wood (most times it appears to be MDF sheets) that is shaped to attach to the sides and back of the cabinets with contours cut out for the drivers. I’ve also seen one or two references to using diagonal bracing from back to side or front/top to side walls.  I’ve even seen one gentlemen that cut MDF slabs that were then glued onto the side walls of the cabinets.

When does adding mechanical bracing begin to negatively impact the internal volume of the cabinet and affect the performance of the speaker?

Finally, is it advisable to use acoustic stuffing such as polyfill in a ported cabinet? Or is that primarily used only in sealed cabinets?

Notes;

I’m primarily focused on improving production built "vintage" speakers from the 70’s and 80’s, even the 90's. I guess this could apply to custom builds as well. I also assume in high end, custom builds this is addressed in design/build.

Also, I’ve watched the majority of Danny’s YouTube videos especially all of the Tech Talk Tuesday episodes, and I’ve read his “I can make your speakers sound better threads” plus many of the other forums, videos and blogs out here.. In addition to speaker building articles, and few of the more highly recommended books.. Even after all of that research I don’t feel these questions are really addressed in a succinct manner.

Thank you

Craig

[Tyson]

Check out my Klipsch Forte upgrade thread, here on the GR Research circle for IME best practices about bracing and NoRez:

https://www.audiocircle.com/index.php?topic=166996.0

I actually drove from Denver to Danny's in Texas (near Dallas) and we did it together.  Here's my tldr:

Re: bracing - angled braces connecting to edges (like front to side, top to side and side to back) are the best way to stiffen a panel while taking up the least airspace.

Re: damping material - NoRez is the best but you can do something similar with Dynamat.  Really you just want something that will mass load the panels and not take up much airspace.  I like NoRez the best because it has a mass loading layer in addition to a foam layer which helps dampen the cabinet as well as absorb driver rearwave acoustic energy.

Re: cabinet stuffing.  Most speakers will be better with a bit of stuffing.  But be careful, it's very easy to over-stuff the cabinet.  How can you tell if you've overstuffed it?  The speaker will sound muffled.  If/when that happens, remove some of the stuffing and listen again.  At a certain point you will end up with the best balance.  Also I don't like fiberglass or rockwool that stuff makes my skin itch like crazy.  Much easier to deal with is stuffing made of recycled denim.  Oh and one last thing - the stuffing must be fluffy, not dense.  So use your hands to pull it apart as much as possible so it's very stringy and light. 

[WGH]

I’m primarily focused on improving production built "vintage" speakers from the 70’s and 80’s, even the 90's.

Cabinet volumes in the '70's, '80's and '90's are not as critical as they are today, speakers were probably tuned by ear because computers and software did not exist, designers had to be good with a slide rule. The advantages of a well braced, non-resonant cabinet far surpasses any loss in volume. I modified both my JBL L100 speakers and the Von Schweikert VR2 speakers.

JBL L100
https://www.audiocircle.com/index.php?topic=189072.msg1978878#msg1978878

Von Schweikert VR2 speaker bracing modifications
https://www.audiocircle.com/index.php?topic=44748.msg400398#msg400398

Von Schweikert Triple Wall Laminate Mod in the VR2 Speaker
https://www.audiocircle.com/index.php?topic=72808.msg683847#msg683847

"The Audibility Of Cabinet Panel Resonances and Pat. Pend. Method Of Reduction Of Audible Coloration" by Albert Von Schweikert, Chief Design Engineer, VSA Corp.
https://www.audiocircle.com/index.php?topic=70291.0





Testimonials from Indiansprings:
I have heard Wayne's VR2's. I was very impressed. When properly braced, as Wayne has done, they become a speaker that is at a completely different price-point. They are detailed, fast, have wonderful imaging, hit hard and low. This was my chance to have a speaker that is completely out of my price range. That is how good they sound. I asked Wayne if he would make mine sound like his for free. He said no problem Well not quite, but he does incredible work for what he charges. I cant wait to get them home and try them all out. I will keep you all posted.


First let me say this. I would like to extend a huge THANK YOU to Wayne. He went out of his way to get everything finished for me within a very limited window of time. He also took time out of his busy schedule to cut me 5 CD's of great music.
I got everything home and installed back into my theater. There is a huge improvement in every aspect. I have several hours of listening into them now. The VR2's are much more DYNAMIC after these mods. The entire sound-stage is greatly improved. The smallest details are clear. I am excited when I fire my system up again. I will post more after I watch some movies and get a little more seat time. I would recommend this upgrade to anyone that has VR2's.
https://www.audiocircle.com/index.php?topic=79773.0

[planet10]

How does a person best asses / measure the resonances of a cabinet?

The most powerful tool ihave used is the simple Mehanics stethoscope. You can quickly assess where the box is moving. Qualitative, not quantitative, but ifigure a mic instead of an ear or an accelerometre.

one can see resonance in the impedance sweep when the drivers are mounted in the cabinet.

It will tell yout hat some resonances exist, and at what frequency. Could be box or driver.

If a person can determine that the cabinets are exhibiting these resonances how does one determine if just damping the cabinet walls (aka with a product such as NoRes) is sufficient or if the cabinet needs to be stiffened by mechanical means?

Adding mass without stiffening is counterproductive., Someone gave me quite a bit of NoRes. After trying it i gave it all away.

If a cabinet needs to be stiffened mechanically what is the best method?

Braces. Much harder to retrofit into an existing cabinet than if you are DIYing the box. In a retrofit your only acessisthru the driver cutout, which puts significant restrictions on what youcam do. Youwill alsobesubtracting net box volume.

Here is a 135 litre Monster miniOnken for a TAD 16” to show the efforts imake for a quiet box:



The research Tappan did (summed up in this image) is very important to bracing implentation, i all to often see it ignored. A good example is the horizontal brace between woofer and tweeter between baffle and back.



BTW: the implementation of the above turned out very well, and illustrates use of better material than MDF for the box (bad for speakers IMO), these used stranded bamboo plywood … the best flat nsheet speaker building materials have found, and given it needs no veneer, despite high initial cost, means no material or labour veneering the [quality] plywood)

is it advisable to use acoustic stuffing such as polyfill in a ported cabinet? Or is that primarily used only in sealed cabinets?

Usual is to use lighti damping on the walls. Minimal impact on performance.  It will reduce the relex action but (ideally kills higher frequency air-space resoances. In a retrofit of a reflex more than usual danming can reduce the hump often seen when slimming the volume of a reflex.

dave

[WGH]

Braces. Much harder to retrofit into an existing cabinet than if you are DIYing the box. In a retrofit your only acessisthru the driver cutout, which puts significant restrictions on what youcam do.

My retrofit braces were tapered because most vibrations are in the center of the panels, non-parallel sides break up internal reflections and tapers reduced the brace size. I used hot melt glue along with Grizzly Xpress polyurethane glue. The hot melt glue held the brace in place until the Xpress expanded and made a permanent bond. The cross pieces were installed after the Xpress dried using more Xpress.
https://www.grizzlyglue.com/

Notice some braces taper to a point, these were installed in-line with the drivers. An easy install if you take your time, measure twice, install once and keep your focus (notches face up). The wood is white oak.

[planet10]

non-parallel sides break up internal reflections

You do the math, and it probably less benefit than you think.

Some of my boxes have tapered edgs but their prime purpose is to reduce the diffration signature of the box. The big chafmers on the front are prt of it.

Those look interesting for retrofit purposes. Not a good idea to place  the brace dead centre. More complex if one brace is insufficient.

Another trick — which i credit to GM — is to run rods from panels and put parallel panels under stress.

dave

[WGH]

You do the math, and it probably less benefit than you think.

Those look interesting for retrofit purposes. Not a good idea to place  the brace dead centre.

Nice illustrations, good info for someone thinking of retrofitting some bracing.

The Von Schweikert VR2's are long gone. The tapered design evolved because if the braces were straight the drivers wouldn't fit, it evolved to an industrial design choice that nobody would see but I knew was there. I think the taper had minimal, if any, effect on the sound. Off setting the 8 braces probably would not have made a big (or little) difference with the VR2's. The (relatively) slow drivers (compared to SEAS and RAAL) were ultimately the weak link in the design. The mods reached their logical end point.