Use Sorbothane for Vibration Control in Speaker Cabinets
Use of Sorbothane in domestic Hi-Fi
Typical Sorbothane Hemispheres
We all live in a relatively noisy environment, often not all of our own making but.... errant vibration whether from neighbours, corner sub-woofers, woofers, wooden floors, heavy traffic or whatever else can impact on the enjoyment of your musical experience.
1. How to Use Sorbothane Hemispheres
Sorbothane is a relatively modern, synthetic, viscoelastic polymer which has the ability to absorb vibrational energy and release it as heat. It does this at a molecular level, so no good for warming the hands then. The material has, in part, physical properties which mimic both solids and liquids. This makes the mathematical analysis of performance both difficult and somewhat imprecise. Empirical analysis, (do the experiments and try to fit some form of maths to it), is the usual approach. However, whatever the maths, experimental trials indicate this is one of the best, if not the best material for vibrational isolation and it has an improved performance over other often-used materials such as Rubber, Neoprene and EPDM.
A Software program is available to calculate the performance of this material, moulded in certain simple geometric volumes, e.g. cubes, discs and strips but sadly it doesn't handle hemispherical shapes. Whether you agree with the use of such materials in domestic Hi-Fi is a matter of personal preference and taste. Can you tell / hear the difference? Well, everyone has their own circumstances, and one size doesn't always fit all, so.....
2. The Nuts and Bolts of it
Sorbothane shapes are cast in moulds, over simplistically, like jelly moulds. To extract the shapes from the moulds, unlike jelly, where hot water does the trick, the mould interiors are previously sprayed with a non-stick, silicon spray. This needs to be adequately cleaned from the sorbothane surfaces before further processing / use. Manufacturers do a good job here but additional cleaning can be done with warm water and mild soap solution. Do not soak the material for any period of time in water. In addition to silicon, the material has free plasticiser present. This can migrate into wood and wood-based surface preparations, leading to surface staining marks. If this is important to you, take note. The staining issue is not large but it does exist. It is dependant upon the surface treatment of the host material.
As an aside, the smaller sizes of hemispheres can look very much like sweets to young / small children.
Do NOT let them play with the material, nor try to eat it.
If you intend to fix / glue the shapes to a surface, then adhesive pads can be applied to the bottoms of the Sorbothane. Note, the rounded profile faces 'upwards', you stick the hemispheres to the 'product support' NOT the product. Sorbothane has a high surface energy which implies that adhesives generally wet the surface well but the free plasticiser makes it difficult to source an adhesive which sticks / bonds well. Do not use rubber-based adhesives, as the adhesive breaks down and will fail with time.
Acrylic-backed adhesive tapes work well as do soft-setting Cyanoacrylates, but not the hard-setting versions as these will crack under flexure.
If you wish to use alternative products then 528 Evostick, Bostic 2402, Loctite 4851 or 4861 can be investigated. However, these options will undoubtedly cost far more than QTA's tape so....
Placing hemispheres under your equipment will reduce the effective footprint of your item. If you have, for example, long thin speakers, then you should check the stability of the resulting structure. Is it child / dog proof?
QTA can provide a double-sided double backed acrylic transfer tape, this transfers only the adhesive, (not available in your usual local store), as an optional extra. This can be cut and trimmed to size and attached to the hemisphere bottoms as required. Acrylic adhesives can take up to 72 hours to develop to full bond strength, so don't immediately pull it off to test it. Note: This also circumvents the staining issue, well, the issue with the hemisphere bottom anyway. The top is another issue.
In order to further deter free plasticiser migration and hence staining, the surfaces may be given a spray coat of Urethane. If you go this route, you can trial the results / experiment on the cheaper, smaller size options, hence minimising outlay.
For 'simple' symmetric volume shapes, (discs, rectangles, squares), the static compression, ie the downward force required to achieve usable / optimum functionality as an isolator is estimated to lie between (10 - 20)% of it's free height. For a Hemisphere, a non-linear shape, this figure changes to (20 - 30)% of it's free height. Under-compression or over-compression of the material will not achieve optimum results.
The hardness of the material is measured on the Shore 00 Scale in Duros, typically ranging from (30 to 80) Duro. The larger the number, (Duro), the firmer / harder the material and hence the higher the load it can support. If you're a volume user you can, at the manufacturing stage, specify the exact value of Duro / hardness you require. However, for normal domestic applications the Duro hardness generally available is either 30D, 50D, or 70D. For hemispherical shapes a selection of diameters is available, notionally between (10 and 50)mm.
3. So now to the tables
The tables indicate some basic data about the material by loading the hemispheres and measuring the resulting compression. The larger diameters require heavier loads to provide the required percentage compression. The lower and upper load limits are those loadings which produce the (20 to 30)% reduction in height. This is shown as the Loading Range per piece in gms. The Mid loading figure gives an average load figure for 4 pieces. The tables show the load increases as the material hardness, i.e. Duro, increases. The 4 up loading therefore gives a suggested loading for the various diameters and hardness.
Table 4 provides additional information regarding product selection. There is obviously considerable overlap in the data regarding loading and this gives alternative choices for material selection. As we're using the material, (somewhat like blotting paper, as an adsorber), the more material we can use, (within the compression constraints), the better. So review tables 1 to 3 in conjunction with tables 4 & 5.
Now, we've not said anything about frequency of damping etc. This would be a minefield and unless you are blessed with accelerometers and vibration analysis software, and able to do your own system analysis, we're going to leave it here. Do the evaluation, give it a try and see what you think.
4. Tabulated Sorbothane Loading information
|Order Code||Spec. 30 Duro||Loading Range each pc gms||Mid Loading /4 pcs|
|11M30D||11mm 30 Duro||55 - 140||390 gms|
|19M30D||19mm 30 Duro||170 - 420||1180 gms|
|30M30D||30mm 30 Duro||350 - 850||2400 gms|
|40M30D||40mm 30 Duro||650 - 1550||4400 gms|
|50M30D||50mm 30 Duro||1200 - 2700||7800 gms|
|Order Code||Spec. 50 Duro||Loading Range each pc gms||Mid Loading /4 pcs|
|11M50D||11mm 50 Duro||260 - 500||1.5 Kgms|
|19M50D||19mm 50 Duro||550 - 1250||3.6 Kgms|
|30M50D||30mm 50 Duro||1600 - 3700||10.6 Kgms|
|40M50D||40mm 50 Duro||3100 - 7400||21.0 Kgms|
|50M50D||50mm 50 Duro||5800 - 12600||36.8 Kgms|
|Order Code||Spec. 70 Duro||Loading Range each pc gms||Mid Loading /4 pcs|
|11M70DA||11mm 70 Duro||750 - 1700||4.9 Kgms|
|19M70DA||19mm 70 Duro||1400 - 3150||9.1 Kgms|
|30M70DA||30mm 70 Duro||4000 - 9000||26.1 Kgms|
|40M70DA||40mm 70 Duro||5700 - 12500||36.4 Kgms|
|Your TOTAL Item Weight Kgms||Usable||Better||Best|
|Item Weight Range (0 - 2) Kgms|
|(0 to 0.5) Kg||11M30D||****||****|
|(0.5 to 1.0) Kgms||11M30D||19M30D||****|
|(1.0 to 1.5) Kgms||11M50D||19M30D||****|
|(1.5 to 2.0) Kgms||11M50D||40M30D||****|
|Item Weight Range (2.0 - 5.0) Kgms|
|(2.0 to 2.5) Kgms;||19M50D||30M30D||****|
|(2.5 to 3.0) Kgms;||19M50D||40M30D||****|
|(3.0 to 3.5) Kgms;||19M50D||40M30D||****|
|(3.5 to 4.0) Kgms||19M50D||40M30D||****|
|(4.0 to 4.5) Kgms||19M50D||40M30D||****|
|(4.5 to 5.0) Kgms||19M50D||40M30D||50M30D|
|Item Weight Range (5.0 - 10.0) Kgms|
|(5.0 to 6.0) Kgms||11M70D||50M30D||****|
|(6.0 to 7.0) Kgms||19M70D||30M50D||50M30D|
|(7.0 to 8.0) Kgms||19M70D||30M50D||****|
|(8.0 to 10.0) Kgms||19M70D||30M50D||50M30D|
|Your TOTAL Item Weight Kgms||Usable||Better||Best|
|Item Weight Range (10.0 - 18.0) Kgms|
|(10.0 to 12.0) Kgms||19M70D||30M50D||****|
|(12.0 to 14.0) Kgms||30M50D||40M50D||****|
|(14.0 to 16.0) Kgms||30M50D||40M50D||****|
|(16.0 to 18.0) Kgms||30M70D||40M50D||****|
|Item Weight Range (18.0 - 40.0) Kgms|
|(18.0 to 20.0) Kgms||30M70D||40M50D||****|
|(20.0 to 26.0) Kgms||30M70D||40M50D||****|
|26.0 to 40.0) Kgms||40M70D||50M50D||****|