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Speaker Cabinet Insulation

Jazzdogg

Jazzdogg

Less barking, more wagging!
Jul 29, 2006
9,679
8,502
San Diego, CA when we're not on the road RVing
#1
Since TalkBass is blessed with myriad experts, what is the effect of insulation when installed in a speaker cabinet? Does it change the imputed cabinet volume? Attenuate frequencies inside the box? Reduce cabinet resonance? Alter sensitivity? Provide a growing medium for carrots?

Why are some boxes insulated, while others are not? Does it matter whether the box is vented or unvented? What happens if you stuff a vent tube or slot with insulation?

What change in sound might one expect by adding insultation to an uninsulated box? Is it worth re-insulating a 40-year-old box if the insulation is missing or in poor condition? If it's worth doing, what's the best kind of insulation to use?

What are the "rules of thumb" for installing cabinet insulation?

Thanks in advance!
 
#2
Acoustic stuffing can be as simple as mattress topper material from Wal-Mart. It is cheap, and not as nasty on the skin as is fiberglass. The intended purpose in a full range cab is suppression of standing waves and reflections that color the sound through the cone.

For subs running below 100 Hz, none is necessary.

Stuffing vents and ducts is not recommended, as this restricted air volume is calculated without the viscous effect of the stuffing. Do not cover vents with stuffing. The driver itself must be kept free of the stuffing. Nylon netting is an easy way to accomplish this.

An inch or so is entirely adequate thickness. Empirical testing has shown that completely stuffing a sealed box will effectively make the box acoustically larger by changing the box operating mode from adiabatic to isothermal. In one of his books, David Weems says the practical limit to this is about a 20% apparent increase. The stuffing rate is typically 1 pound per cubic foot of internal box volume.
 
#3
bgavin said:
For subs running below 100 Hz, none is necessary.
Click to expand...
In theory, no, but in practice subwoofers generate harmonics that must be damped, lest they reflect back to the cone or out a port. The exception is folded horn subs, which filter the harmonics out in the bends of the horn.
Empirical testing has shown that completely stuffing a sealed box will effectively make the box acoustically larger by changing the box operating mode from adiabatic to isothermal. In one of his books, David Weems says the practical limit to this is about a 20% apparent increase. The stuffing rate is typically 1 pound per cubic foot of internal box volume.
Click to expand...
Weems and the other proponents of this theory have been proven to be in error. Stuffing a sealed box increases the resistive impedance within, lowering box Q. This can result in the taming of a response peak caused by a box that's too small, with the resulting flatter response emulating that of a larger box. But fb and broadband sensitivity remain the same, so the effect isn't the same as using a larger box. As to the heat, the higher friction of damping fibers rubbing against each other rather than just air molecules does result in the creation of more heat, but it's anecdotal and is a symptom of what's occuring, not a cause.
 
#4
billfitzmaurice said:
In theory, no, but in practice subwoofers generate harmonics that must be damped, lest they reflect back to the cone or out a port. The exception is folded horn subs, which filter the harmonics out in the bends of the horn.
Weems and the other proponents of this theory have been proven to be in error. Stuffing a sealed box increases the resistive impedance within, lowering box Q. This can result in the taming of a response peak caused by a box that's too small, with the resulting flatter response emulating that of a larger box. But fb and broadband sensitivity remain the same, so the effect isn't the same as using a larger box. As to the heat, the higher friction of damping fibers rubbing against each other rather than just air molecules does result in the creation of more heat, but it's anecdotal and is a symptom of what's occuring, not a cause.
Click to expand...

huh? :eek:
 
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#5
I'm interested in this topic. Highly technical stuff aside, is there an advantage to adding insulation to a cabinet that has little or none? Example, I have a 212 that has some thin batting on the back and top surfaces. Would the performance be noticably improved by adding some material to all surfaces? Is the makeup of the material important?
 
#7
Jazzdogg said:
Is it logical to infer from your comments that adding insulation to attenuate standing waves will be more worthwhile in a multi-driver 3-way design?
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Only in the sense that if you are going to the trouble to get something that's supposed to be more attentitive to midrange quality than you should make sure it in fact doesn't skip an important step.

Damping materials should line any direct radiator cab.
 
#8
Scotius2001 said:
I'm interested in this topic. Highly technical stuff aside, is there an advantage to adding insulation to a cabinet that has little or none?
Click to expand...
Yes. A cabinet that's not fully lined is defective IMO.
Do insulating practices differ for sealed and vented cabs?
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Yes. Sealed cabs can benefit from stuffing rather than simply lining, vented do not.

Is it logical to infer from your comments that adding insulation to attenuate standing waves will be more worthwhile in a multi-driver 3-way design?
Click to expand...
No. With the notable exception of folded horn subwoofers all cabinets should be fully lined.
 
#9
billfitzmaurice said:
Weems and the other proponents of this theory have been proven to be in error.
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Some time back, I found a web page where the guy detailed his experiments in this vein. He measured the T/S characteristics as he progressed, and he was able to properly demonstrate and document his results. I cannot find this site anymore, pity.

If you have valid refutation, please send it my way. I'd like to see the arguments that disprove the concept.

http://www.lungster.com/l/speakers/BassListArchive.shtml#t1

This link is a short, but interesting read on box lining.
 
#13
bgavin said:
Some time back, I found a web page where the guy detailed his experiments in this vein. He measured the T/S characteristics as he progressed, and he was able to properly demonstrate and document his results. I cannot find this site anymore, pity.
Click to expand...
The effect of stuffing has been measured. Box Q goes down, Fb and sensitivity remain the same. It can be modeled in programs like Horn Resp and Akabak, which have provision for lining/stuffing thickness and density.
What kind of material, density and thickness is best?
Click to expand...
I use Walmart mattress toppers.
 
#20
Bill, from memory Vance Dickason also reported that stuffing a box can result in an apparent increase in box volume. I only have his cookboook, so that would be my source.

Am I mis-remebering, or was his work on this also proven incorrect. Perhaps he was publishing/referencing the same Weems findings you mentioned?
 

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