I was recently chatting with some people about the best ways to build speaker cabinets and why that is. When I built my speakers, the cabinets were designed with a great deal of effort to address two issues that had been shown to be audibly important, inexpensive to correct, but commonly not addressed properly in other speakers. I'll also note, I am not the one who designed my speakers not came up with these solutions and what follows in this post is nothing original. What I've noticed however is that these "concerns" which remain easy to fix, still are not readily addressed in a lot of DIY efforts.
The first is edge diffraction, and to be honest, that isn't why I wanted to post this. None the less, I'm often amazed more efforts aren't made to reduce both edge diffraction on speaker cabinets as well as avoiding diffraction surfaces near to the speakers (i.e. the screen frame, screen wall, etc.).
The other is cabinet panel re-radiation. Basically a speaker cabinet is not a sound proof box. The sound produced by the driver is re-radiated through the cabinet. In addition, as much as 50+ db's of the rear wave is reflected off the rear wall and back through the cone of the speaker. These have a measurable and audible negative effect on the sound of a speaker. Research into the causes of reradiated sound both through the panels and through the cone have found some things that I never understood to be true with regard to the cabinet reradiation problem. Had you asked me about this 5-10 years ago, I would have said that I think very little sound re-radiates through the cabinet wall, has little audible effect other than at resonant frequencies, and that acoustic stuffing in the cabinet absorbs more than enough of the rear wave. Turns out that is wrong. Understanding and addressing this is difficult, it isn't that well studied, their are conflicting opinions, and unfortunately, even some conflicting research papers. Some of the most intensive research really has been done by manufacturers, not independent scientists, and much of it has never been published.
This was a "white paper" by Albert Von Schweikert, and while some of his claims start to verge on voodoo, most of this is scientifically valid and was researched in a believable way.
These guys have investigated and analyzed different cabinet wall construction and landed on a proprietary approach that reduces the problem. What is more interesting is their graphs showing the reduction in re-radiated sound from the panels.
"Mark" wrote a nice two part article on reducing re-radiation and in part 2 discusses the benefit of damping.
As for the significance of the problem,
I think this is a good article. it's important conclusion, speaker walls re-radiate a lot of sound, sometimes more sound than the driver itself, and....bracing doesn't necessarily make it better. What they show is that relying on wall cabinet measurements alone to show that a stiffer and better braced wall cabinet does not necessarily mean lower total radiated sound from the cabinet. That is really important.
Here is where we get to my point. In the second Audioholics article Mark makes mention of two modes of damping a speaker cabinet. Extensional and Shearing. Extensional is what most people do, if they do anything. That is applying a damping material to the inside surface of the cabinet. The other is Shearing, and this is what we know as Constrained Layer Damping. It is by far the most effective means of damping panels and dissipating energy. It is the approach we use in soundproofing theater walls, reducing noise and vibration infiltration in our rocket ships, reducing the distortions in microscope images, you name it.
Some researchers have also talked about another mode of sound transmission through a cabinet, through the walls themselves. Instead of the sound emanating from the speaker inside the cabinet and then transferring through the cabinet wall (and in some cases exciting a resonances such that it transfers out of the walls at a greater total amplitude than the initial amplitude level from the front of the speaker) it actually transfers through the frame of the driver to the front baffle, through the front baffle to all other surfaces, which then act as a re-radiating surface.
For a DIYer the fixes aren't all that hard and require the use of materials we have access to, but I just don't see it done very often. I don't understand why?
CLD baffles are the most important, if nothing else is CLD, the baffle should be. This can be accomplished by using two layers of material with a damping layer between. Green glue seems an obvious DIY choice, but for what its worth, my speakers were made with urethane rubber as the CLD material. It has a durometer of 40 and slight amount of additives to increase its damping slightly (talc I believe). For all practical purposes the cabinet is held together with sorbathane, in case people are wondering what this stuff is. Sorbathane is Urethane.
I think the take away from these articles is that bracing is really important and you need to rigidly brace a box so that you tie opposing panels together and break up their unbound surfaces. This raises the resonant frequency and often raises its Q as well. What I think is the other important take away is that this can't be done without damping and that CLD damping is probably best where possible. Bracing doesn't fix the re-radiation problem, it just makes it easier to fix. The fix is still dissipating that energy before it exits the box.
Another important take away is that since the driver is directly exciting the box vibrations and the sound is traveling through the entire cabinet, we somehow need to break up these direct connections. This is where decoupling becomes important. This means things like mounting the speaker onto a ring that is itself decoupled and damped. My own speakers have the speaker driver mounted onto a cast urethane mounting ring that is internally composed of two layers of MDF (one is 3/4" and the other is a piece of 3/4" that was recessed for the driver .5", leaving .25" of MDF. This means the layers of CLD for the driver mount are driver, urethane, .25" mdf, urethane, .75" MDF. The sidewalls and rear panel can also be decoupled to help break this up. The side panels can be mounted to the inner "shell" of the cabinet so that the driver and outer-shell of the baffle is decoupled through CLD from the sidewalls and thus also the rear panel. If the rear panel is CLD, same thing, the outer shell of the cabinet can be decoupled from the side walls, further breaking up the transfer of vibrations.
Without CLD the bracing would actually contribute to the re-radiation problem considerably. However, if you have CLD panels and tie the walls together so that the brace is touching only the inner shell, and it is separated from the outer-shell by damping, then re-radiation through the bracing is minimized.
In terms of acoustic damping inside the cabinet, it is important to absorb the rear wave, but its important to remember that just like fiberglass insulation doesn't soundproof a wall, batting or fiberglass inside a speaker cabinet doesn't soundproof the cabinet wall. Sound waves reflect when they hit an abrupt change in acoustic impedance and do so anti-phase to the original source wave. I like the idea of creating a change in the acoustic impedance as you approach the barrier surface. Something like loose batting in the middle with fiberglass or cotton dense insulation along the wall.
Finally I'll just mention none of this applies to subwoofer cabinets in the least. They need to be pretty rigid and dense, but their resonance frequency is well above their operating range, meaning we don't need to dampen it.
I'd really love to see people start incorporating these principles into their DIY designs. We often talk about building DIY speakers that best manufactured speakers. These principles are common in well engineered commercial offerings and they have been shown to make a measurable and audible impact, so why not incorporate them.