Recently I came across the issue of "Effect of Damping Material in the Enclosure" in John M. Eargle's (JBL) Loudspeaker Handbook, Chapman & Hall, 1997, on pages 62-64, in the section dealing with "Sealed Low-frequency Systems Analysis". I think that stuff is of interest here, and some of it may also be relevant for Vented boxes.
"At relatively high frequencies, standing waves may exist in the enclosure. Placing damping material on the inner walls of the enclosure will damp these out, resulting in smoother response. At low frequencies there are additional effects that derive from certain thermodynamic action."
"If an enclosure is filled with damping material, ... this has the effect of increasing the volume of the enclosure by a significant factor. Previously it was thought that the maximum possible increase in effective volume was in the range of 1.4, or 40%, but Leach (1989) provided a more accurate analysis indicating that the maximum ratio is 1.31. In normal practice most loudspeaker engineers observe an increase of perhaps 1.2." Or 20%. This is only half of the stated theoretical maximum value of many years ago...
"As a practical matter, many engineers think that the isothermal volume increase of normal amounts of damping material is roughly equal to the internal volume displaced by the driver and normal bracing in the construction of the enclosure, and they may make their initial volume calculations accordingly."
"The damping material should be chosen for relatively low mass... The amount of material is usually determined empirically; too much material, tightly packed, will of course diminish the effective volume in the enclosure."
Filling an enclosure with damping material normally applies to closed boxes. Lining cabinet interior walls with same is usually the proper procedure for vented boxes. The above quote also brings the issue of "virtual volume" seen by the low-frequency driver. In a vented box, this is a seldom occurrence but may still happen in a pinch, when a little more volume is really needed to save a box. Then lining thicker damping material in such a box may avoid a re-build. A minor vent re-tuning may be appropriate re "larger" volume.
Low mass in the present context means low density. You don't want damping material to be near solid or rigid and act like it otherwise it will reduce effective volume. Something softer and "fluffy", like polyester batting or fiberglass wool, would be more appropriate.
In the closed box LF systems section of his "The Loudspeaker Design Cookbook", 5 th Ed., 1995, Vance Dickason mentions "Practical equivalent volume increases of 15 % - 25 % are quite attainable". (p. 22-23) Because the quantity of material required to create virtual volume is not well determined for Fiberglass wool, Dickason proceeded with a computer simulation to see the influence of fiberglass fill (standard 1lb./ft^3 R-19 household type about 3" thick). The effect on Low Frequency response at zero stuffing, 50% stuffing and 100% can be seen: the shape of the curves is comparable to that of a small volume increase, and just above cut-off point a 0.5 db or so decrease in amplitude at 50 % fill and a - 1 db or so at 100% fill can be seen.
With regards to Vented box LF systems, Dickason also made a simulation for effect on LF response of the same material in a QB3 alignment enclosure with 0 % fiberglass, 10 % (lining one of each opposite side with 1" fiberglass) and 50 % (lining all four sides and rear wall with 3" thick R-19 fiberglass). (p. 61) Although he describes the results as "minor response changes", there's next to no difference between the 0% and 10% curves (as one would expect from lining only one of each opposite side with 1"). The curves difference between 0% and 50% damping is only about - 0.25 db above cut-off point but about (+0.5 db) at -3 db point and a bit deeper bass. Again, the effect is comparable to a slight volume increase, or to a bit lower tuning Fb. This example shows R-19 like a minimum, and that R-25 may be preferable for even more volume "added".
"JBL uses a 25 mm (1 in) padding of 1/2-pound density fiberglass stapled to the enclosure interior on all surfaces except the baffle." JBL, FAQ speaker building, q & a, # 19, Half-pound density per which unit? Cubic foot?
"Any fiberglass will do, but if you use R-19 or R-25 insulation type fiberglass, you can ignore the volume of the bracing in the box because thick fiberglass adds virtual volume. If you must use dacron or felt instead of fiberglass, subtract its volume from the box (make the box bigger)." JBL Pro, Enclosure guide, Page2. This would mean that Dacron and Felt are too dense for absorption/virtual volume, effectively reducing net cabinet volume. Also, paper backing, if any, must be removed from fiberglass wool before installing it.
BTW, JBL does not recommended the use of Rock wool (no reason given) (E-series, Instruction Manual, 2-81, page 5). I mention it since some members of this site do use that as loudspeaker cabinet damping material, probably more in Europe, as with Felt.
"Q: Does Fiberglass significantly affect enclosure tuning?
A: No, not unless the enclosure is stuffed full of fiberglass, in which case the apparent volume of the enclosure increases by 12% to 20% as seen from the point of view of the bass driver. Stuffing the enclosure full with fiberglass is not recommended because it introduces system losses { Qa, absorption}, is expensive and interferes with port operation. The exception to this would be a sealed "air suspension" type system enclosure where more virtual volume is needed and actual volume is not available, and/or where box dimensions which are multiples of each other can't be avoided and the fiberglass stuffing will help absorb the internal sound reflections." JBL, FAQ speaker building, q & a, # 20.
I think the above exception of more virtual volume needed also extends to Vented boxes, using thicker fiberglass wool to line (not stuff) usual interior surfaces, while being careful to avoid any reduction of the vent's performance (free air flow, at the inner end, above, below, and on both sides of the duct).
Finally, a word of caution about Fiberglass wool. Large vented box woofers do move a lot of air inside/out, specially at high power. This may create some "wind" inside the enclosure carrying around fiberglass fibers that can not only exit by the speaker port, but worst also make their way inside the driver's back vent and on to the voice coil gap, if they are not contained at the source (e.g. with a very light coat of spray paint or cotton cheesecloth wrapping), or blocked at the driver's back vent entry (e.g. metal grill, speaker grill cloth glued on or a foam).
Richard
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Originally Posted by Robh3606
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