Importance of Cabinet Resonance?

[sparky77]

Not only is cabinet resonance a factor, but also internal cabinet reflections, this seems to be of particular importance with metalic cone drivers. The internal reflections, if not damped and absorbed, reflect off the back wall and into the driver causing the driver to resonate with a pinging noise, this is why a few on this site strongly advocate using damping/absortive materials such as rockwool and oc705 to line the cabinet to prevent reflections and add to the damping just as much as the cabinet bracing, and sorry, but polyfill only slows down the waves a little, it won't absorb any.

 

[D.R. Payne]

Internal reflections are a big deal, especially for larger speakers or towers tall enough to support a standing wave down into the midrange. It's expensive but this is one of the reasons (besides WAF) that some models include curved cabinets. My P-39F have boat-tail backs as well as a tapered top so there are no parallel surfaces.

 

[avaserfi]

Not only is cabinet resonance a factor, but also internal cabinet reflections, this seems to be of particular importance with metalic cone drivers. The internal reflections, if not damped and absorbed, reflect off the back wall and into the driver causing the driver to resonate with a pinging noise, this is why a few on this site strongly advocate using damping/absortive materials such as rockwool and oc705 to line the cabinet to prevent reflections and add to the damping just as much as the cabinet bracing, and sorry, but polyfill only slows down the waves a little, it won't absorb any.

Internal reflections are an issue with all drivers as long as the cabinet size is greater than one quarter the wavelength [one half creating a full modal resonance] of the passband of the driver. These reflections typically do not directly effect the driver, but rather their energy causes cabinet resonance.

Another method of decreasing resonance is via proper decoupling of the driver from the cabinet which can be done in a variety of methods.

As far as polyfill goes it has absolutely no use for such an application at all.

Internal reflections are a big deal, especially for larger speakers or towers tall enough to support a standing wave down into the midrange. It's expensive but this is one of the reasons (besides WAF) that some models include curved cabinets. My P-39F have boat-tail backs as well as a tapered top so there are no parallel surfaces.

While curved surfaces can help certain issues, they do not alleviate the energy created by these waves which cause cabinet resonance.

 

[Guiria]

As far as what resonance sounds like it is typically perceived as a 'smearing' of sound that reduces clarity.

I've always tried to put the sound of resonance to words. Smearing is the perfect word for it IMO.

 

[sparky77]

Probably the worst case scenario of cabinet resonace was a pair of KLH speakers a friend of mine had, at low to moderate volumes they sound ok, but if you turned them too lound you could actually watch the masinite type back pannel flex, and you could clearly hear it anywhere in the house. It was so bad that I actually cut out the back panel and glued in a peice of 3/4 inch mdf, and they sounded much better. Although better is being used sparing as a relative term, they were still squawkers.

 

[Guiria]

Another method of decreasing resonance is via proper decoupling of the driver from the cabinet which can be done in a variety of methods.

I'm going to do some searching but can you point out a couple of these decoupling methods? I've already got a method in the pocketbook from WmAx.

It's induced by both. You can not ignore either one. I have worked extensively with de-coupling drivers, and it does help substantially. But it takes more than a simple thin rubber gasket to optimize this technique. To get a high degree of isolation, and through a broad bandwidth, you have to create an effective suspension with a low Fs. This means using very thick, very soft closed cell materials, and having the driver mount with very low pressure, as to avoid stiffening the suspension.

The easiest method that is effective without going to more extreme mounting techniques, is to route an additional 0.5" into the driver recess hole. Use thick vinyl closed cell foam applied as one layer. Now cut a solid ring out to sit on this layer. You can use 1/8" metal, or use 1/4" hardwood ply and increase the recess depth to compensate for the ring. This will act as a stabilizer for the 2nd layer of foam. Now mount another layer of the closed cell vinyl foam. Mount the driver over this double layer of stabilized foam. Use a long screw and use metal washers, with some foam rings cut out to use as spacers between the screw/washers/driver. Tighten the screws enough just to hold the driver in place and get a good seal.

-Chris

I'm just interested in what else is out there? I'm building a set of speakers now and may incorporate the above idea into decoupling the driver.

 

[avaserfi]

That method is the easiest way to decouple a driver properly. The ideal method would involve building a cabinet within a cabinet and having the two decoupled while mounting the driver to the outer cabinet in such a way that no energy can be transferred between this point. Clearly this is very hard.

The method that is of medium complexity [relatively] would be B&W's method. The driver is installed from the rear of the front baffle with a ring of low Fs foam attached to it. A piece of threaded rod is connected to the rear of the driver and inserted into a hole in the rear baffle where foam is placed. This tension holds the driver up while ensuring it is decoupled.

Please let me know if you want a more in depth explanation of these methods as I just described overviews.

Properly decoupling drivers is not easy as you can see. The method Chris outlined is by far the easiest I know of.

 

[WmAx]

The ideal method would involve building a cabinet within a cabinet and having the two decoupled while mounting the driver to the outer cabinet in such a way that no energy can be transferred between this point. Clearly this is very hard.

The driver should be mounted to the internal cabinet in this example, less it is to excite the outer cabinet(which is to directly radiate into the room). Of course, these two cabinets must be mechanically de-coupled AND have air tight joints with no breaches(even a little breach will allow a large degree of SPL in the environment, thus negating the nested de-coupled cabinet system.

-Chrs

 

[jostenmeat]

I've always tried to put the sound of resonance to words. Smearing is the perfect word for it IMO.

You know, after reading about all this stuff over the last year, I always try to see if I can detect it. If and when I think might, I would call it "bloated".

But . . . how do I know that this lack of clarity is not in fact the resonance of the drivers themselves? Or room interaction? Comb filtering? Etc? I think I'm asking for more than I can chew on in this thread, with these folks, so neveeermiiiiiind . . .

 

[sparky77]

But . . . how do I know that this lack of clarity is not in fact the resonance of the drivers themselves? Or room interaction? Comb filtering? Etc? I think I'm asking for more than I can chew on in this thread, with these folks, so neveeermiiiiiind . . .

Hook up a speaker out in the middle of nowhere on a calm day, set the speaker square in some lever dirt, and listen to it at a variety of different volumes. Just one speaker mind you, and make sure its not pointing towards anything that could reflect sound besides a little dirt. This is just as good as a quasi anechoic chamber if its calm enough outside.

 

[davidtwotrees]

How about speakers that utilize resonance? Aren't there a whole slew of speakers derived from the BBC designs of the 1950's? I can't think of the exact model, but Spendor s3e comes to mind? Supposedly these small monitors had excellent sound and used resonance much the same was a violin or guitar would..........

 

[WmAx]

Hook up a speaker out in the middle of nowhere on a calm day, set the speaker square in some lever dirt, and listen to it at a variety of different volumes. Just one speaker mind you, and make sure its not pointing towards anything that could reflect sound besides a little dirt. This is just as good as a quasi anechoic chamber if its calm enough outside.

Under such a condition, resonance is minimized in audibility. The reflections/delays in an environment make the resonance far more audible.

The best way to 'casually' test for resonance is to compare two units, one being a known non-resonant reference, and band limit both to say, no higher than 1000Hz, so they both have near identical radiation patterns. Band limit response to above 100Hz, to avoid issues with bass respnose differences. Now use precision E.Q. to adjust the reference speaker to the same response as the other speaker. The majority of resonance is going be in the range of 100Hz-1000Hz, anyways, in almost any cabinet construction/design. Both should be as close to the same position as possible in a real acoustic environment. Blinded A/B comparison is the best method. If you stack the speakers(if they are bookshelf types this is feasible) vertically, it will be very difficult to get directional cues. Another possibility is to make a turntable and put the speakers back to back on it so it can be rotated so each speaker can be in the same physical space when listened to, and remove any associated variables with the speaker being in a slightly different position. Be sure to use cushion foam to set the speakers on the turn table, less the turntable be allowed to resonate and invalidate the test.

-Chris

 

[WmAx]

How about speakers that utilize resonance? Aren't there a whole slew of speakers derived from the BBC designs of the 1950's? I can't think of the exact model, but Spendor s3e comes to mind? Supposedly these small monitors had excellent sound and used resonance much the same was a violin or guitar would..........

That's just a way to justify the poor construction. It's a lot easier to make those claims rather than to correct the problem(very costly). The classic BBC designs (still sold by Harbeth, the original designer/manufacturer of many of them; some measurements are available from Stereophile) are still very resonant. These speakers typically have less cabinet bracing/dampening than even the average speaker, which is indeed, already pathetic. You can not design an excellent speaker in this manner. The coloration, while it can potentially be designed to work for a very narrow/limited range of recordings, will be destructive to all others, in sound quality. It is miscarry that the loudspeaker in the reproduction chain not add such critical coloration if sound quality is to be maximized.

-Chris