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Article:  Sorbothane® Vibration Isolation.

Sorbothane Isolation Hemispheres

Use of Sorbothane in domestic Hi-Fi.

       We live in a relatively noisy environment,something we come to get used to and accept..... However, intrusive sound / vibration whether from neighbours, corner sub-woofers, woofers, wooden floors, heavy traffic or whatever else can unfortunately impact on the enjoyment of musical experiences.

Introduction to Sorbothane

       Sorbothane is a relatively modern, synthetic, visco-elastic 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 used to calculate the performance of this material, moulded in certain simple geometric volumes, e.g. cubes, discs and strips but, at the time of writing, not 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.....

The nuts and bolts of it.

      A variety of geometric shapes can be cast in Sorbothane using simple moulds, eg sheets of varying thickness, strips and spherical based items, the most popular of the non-linear shapes being hemispherical. Other shapes can be cut from sheet materials using die cutters or more complex shapes with greater accuracy may be cut by CNC water jet.

      To remove cast shapes from their moulds, the mould interiors are coated / sprayed with a non-stick silicone release agent which prevents the Sorbothane from adhering to the mould surfaces. Prior to use the Sorbothane components pass through a thorough cleaning process. You can provide additional cleaning with warm water and a mild soap solution to remove any remaining contaminants. Do not however, soak the Sorbothane in water for any period of time. In addition to silicon, the material has free plasticizer present. This can be an issue for wooden based surfaces as the plasticizer will migrate into the woodwork grain / pores producing a visible stain. The severity of the stain depends upon the surface treatment previously applied to the wood surface. If this is important you should consider this aspect in more detail. 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.

       As an aside but nevertheless important, 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.

Sorbothane is a difficult material for most adhesive tapes to get to grips with. It's a high surface energy material which simply put means that adhesives generally wet the surface well. However, the free surface plasticizer makes it difficult for the tape to actually stick / grip with any real effect.

      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. These have not however been trialed. However, these options will undoubtedly cost far more than QTA's tape so.... Do not use rubber-based adhesives as these eventually break down and will fail with time.


      There are two orientations for mounting Hemispheres to components. Flat side DOWN or flat side UP. Which orientation is best, may well depend upon the source of vibration and its energy level. Our preferred / suggested method is to use flat side DOWN so that the component to be isolated is placed on top. This technique doesn't require adhesive versions as the Sorbothane is trapped under the item and stays put.
       If you require adhesive versions, QTA can supply, at small additional cost, sufficient double sided tape for your order. 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 the tape adhesive is a very fine thin film and requires a flat / non-rough surface to adhere well.

      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?

In order for a Sorbothane isolator to achieve a useable / optimum functionality it needs to be statically compressed to between (10 - 20)% of its free height. This works well for flat sheets, cubes and rectangles and simple regular shapes but is not so good for non-linear shapes like hemispheres. The recommended compression for hemispheres is (20 - 30)% of its free height. Note over or under compression 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.

Note: A selection of Hemispheres in both hardness and size are available from QTA.

So... now to the tables.

      It's worth noting that any number of hemispheres can be used to support an item but for best results the weight loading (compression) per hemisphere needs to be similar for all hemispheres used. Three is generally a very poor choice, poor on compression equality and poor on item stability, but 4, 5, 6 etc is OK. The data below is tabulated for varying Sorbothane diameters for a given Sorbothane hardness, Duro. The loading range is given per piece and per 4 pieces.

Weight Range for 30 Duro Parts
CODE         WT RANGE / 1     WT RANGE / 4    
11M30D (55 -140)gm (0.220 - 0.65)Kg
19M30D (170 - 420)gm (0.580 - 1.68)Kg
30M30D (350 - 850)gm (1.400 - 3.40)Kg
40M30D (0.65 - 1.55)Kg (2.600 - 6.20)Kg
50M30D (1.20 - 2.70)Kg (4.800 - 10.8)Kg

Weight Range for 50 Duro Parts
CODE         WT RANGE / 1     WT RANGE / 4    
11M50D (55 -140)gm (0.220 - 0.65)Kg
19M50D (170 - 420)gm (0.580 - 1.68)Kg
30M50D (350 - 850)gm (1.400 - 3.40)Kg
40M50D (0.65 - 1.55)Kg (2.600 - 6.20)Kg
50M50D (1.20 - 2.70)Kg (4.800 - 10.8)Kg

Weight Range for 70 Duro Parts
CODE         WT RANGE / 1     WT RANGE / 4    
11M70D (55 -140)gm (0.220 - 0.65)Kg
19M70D (170 - 420)gm (0.580 - 1.68)Kg
30M70D (350 - 850)gm (1.400 - 3.40)Kg
40M70D (0.65 - 1.55)Kg (2.600 - 6.20)Kg

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