"a thin layer of Dynamat, if you have it, might be a good idea on the inside rear behind each woofer. And maybe apply some along the sub-enclosure sides as well? "
On a related but different topic: padding for woofers - I've heard elsewhere the same thing, that some sound dampening material could improve the sound quality from some speakers.
Is there a specific type - foam vs rubber; thick vs thin; and where do you stick it - on the back behind the woofer, or back+sides? Does it matter what speakers you have: wood/plastic; and with/without ports?
Does it improve sound quality or just reducing rattle?
edit: after a quick google search, this is what I found recommended for open speakers: bubblewrap
and for closed speakers: fibre fill
So, I never heard of bubblewrap as a damping material. Personally, I don't see the purpose. bubblewrap is relatively thick, due to the inflated air bags, or bubbles, which will take up a lot of internal air volume, which may choke the driver snd severely limit its low end bass extension. imo, bubblewrap is a terrible idea. Not to mention the bubblewrap will deflate over time, becoming useless.
From my personal experience, using a closed-cell foam or rubber damping material may slightly help when applied to flimsier materials, such as the plastic construction of boomboxes or plastic speaker enclosures. The idea is to suppress the resonance of the plastic. And also to reduce reflection of the interior sound waves back into the woofer cone (intermodulation distortion).
What has to be understood, is it is a band-aid effect. You cannot really 'fix' a bad design, or a cabinet designed of inadequate material.
Also, there is much misconception as to the effect of even costly materials, such as Dynamat.
The fact is, bass frequencies will not be blocked by any internal damping. Consider a vehicle loaded with large subs, driving past your home, in the evening. The loud obnoxious booming bass thumps can be heard all through the house, as the vehicle drives past. Those bass freqs vibrate the walls, windows and then radiate right into your living room or bedroom. So, how can a thin layer of any material stop that bass?
Any such material will slightly reduce, or more accurately slightly lower the resonant freq of the plastic cabinet. That will help a little. It may reduce the hollow sound created by the higher resonating plastic reflecting and reacting to internal bass waves. However, it is useful in reducing vibrating sounds.
Open cab, or bass-reflex types are more susceptible to such hollow sound because the reflected waves exit the cab and we hear them. Sealed cabs do not share this problem, and may sound smoother, to some extent.
Materials like felt and heavier foam, such as old fashioned carpet padding, work pretty well, from my experience.
Keep in mind, adding any layer of rubber, foam or felt damping will slightly reduce the amount of internal volume or air space within the enclosure, via displacement, which may sometimes have negative effects, especially if the internal cab volume is limited, in the case of many boomboxes.
Polyester fiberfil or, as in the old days, fiberglass, serve a very different purpose. The material may slightly soften the sound, in the case of a woofer (not subwoofer), which reproduces mids, and even some treble, The mid and higher freqs are weaker and slightly more likely to be somewhat damped, internally, reducing internal reflections back into the cone.
However, the main effect is to change the internal cab into a thermal-accoustic effect (I can't recall, off hand, the actual correct term). The reason fiberfill is added to cabs is as follows:
A given driver will create a certain amount of pressure within the cab as it moves back and forth. The force of movement causes air molecules to collide, causing friction. The driver acts much like an electric motor and just as a motor, it creates heat. The friction caused by molecules colliding also creates heat. Heat causes more energy released within the cab. More energy increases air molecule movement, thus increasing pressure, especially in sealed cabs where there is no release of pressure. The more internal pressure, the stronger the push-back against the driver, reducing driver efficiency and raising the cab's system resonance, which ultimately affects bass response.
Adding fiberfill to the sides and rear of cabinet change the internal interface of the driver to the cab. Now, you have a layer of light fiber material which readily allows air molecules to pass through. As the internal air passes through the fiberfill, the individual fibers absorb some of the created internal heat (a heat sink effect). By reducing the heat, or more accurately limiting its rise, internal air pressure does not rise as quickly, thereby maintaining a better , more stable bass response. This is why it is said that adding some fiberfil damping will 'trick' the woofer into thinking it is in a larger box lol. The same driver in a larger box, will not affect the internal air volume as much, because the relative size of the driver is smaller compared to the amount of air in the larger cab. So, not as much heat and friction is created and less air pressure is the result, in a larger cab. Similar to the results of a smaller cab loaded with fiberfil.
I hope this explanation isn't confusing lol.
But, from my experience, adding fiberfil is trial and error. Too little won't hurt, but too much will begin to reduce, or more accurately displace internal air, thus negatively affecting the driver performance and bass response.
Ultimately, the lower the resonance of the cab material itself, the better, due to the material being closer to inert and less likely to 'color' the sound. That's why MDF and particle board are good choices. they are heavy and dense.
Plastic is light and stiff and resonates at much higher freqs and 'colors' the sound. As the plastic resonates, it generates sound waves which we hear as annoying colored sound. That's why a bass-reflex plastic cab will sound hollow. You are hearing the effect of the bass reflecting off the plastic and causing the plastic cab itself to vibrate and generate unwanted sound waves.
In the old days, I recall seeing a cool floor speaker cab design. The cab consisted of two layers of heavy wood or MDF. There was an inner layer and then an outer layer, with a small space in-between, maybe a fraction of an inch.
The space is then filled with sand!
Such a cab will nearly deaden any resonation, thus allowing the woofer to more accurately reproduce sound without color.
But, just the thought of trying to lift or move such a cab hurts me lol.
wikiboombox.com
