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Inexpensive Vibration isolation products group test

Part I: General Introduction

[Italian version]

I've always had an interest in vibration control techniques...
"Why is that?" quoth plebs chorus, stage left "Is it because you saw some laser-holography experiments during your photography degree, which required vibration control down to wavelengths & frequencies of light?"
...well not really...
"is it because you've seen how Harley Davidson try to control the ridiculous levels of vibration from a big 45° V-twin using squidgy rubber engine mounts and yet Ducati try to control 90° V-twin vibes with rigid mounting?" they ask,
...not really that either...
"Is it because you worked in a recording studio near a main road where the traffic rumble was transmitted through the ground?"
...not just that either...
"or is it because you once worked on sound systems in public areas?" they continue
...well that too...
"and lived in a Victorian house with bouncy wooden floors"
...that as well.

So can we get on with the reviews now?
Silence comes the stern reply.

I was asked by Lucio to review inexpensive vibration control accessories in a comparative review. Accessory reviews often describe the effects of one product compared to no-product-at-all, but rarely have the opportunity to compare the products of several manufacturers using different philosophies to arrive at very different solutions.

There are 2 kinds of vibration that may raise the noise floor or upset the equilibrium of audio equipment:

1. Airborne, or acoustic vibration (soundfield between source & recipient)
2. Structural, or impact vibration (solid materials between source & recipient)

I have observed that vibration control in electronics casework is as important as choice of passive components inside. As the early Naim pre-amps evolved I noticed the addition of what looks like a block of Sorbothane under the circuit board of the 42.5 update of the NAC42. David Holgate in his report on his 8th October 2003 Naim visit describes how Naim have taken this much further in their latest products: "Attention to detail is sometimes mystical and sometimes practical. For example, a very heavy solid brass plate lowers the resonant frequency of the suspended boards in the NAC552 below audible levels."

Until very recently products designed as supports for hi-fi equipment have fallen into 4 generic types:

1. Light but rigid, with spikes. Designed to minimise the mass that may be excited by structural & airborne vibration, and arguably either decoupled from it or rigidly coupled to it by spikes (depending whose theory you read).
2. Heavy and rigid, with spikes or soft decoupling. Designed to "mass-damp" vibration on the argument that massy things move less with a given energy input.
3. Heavy and lossy components. Use of sand filling, for example, to absorb vibration between source & recipient.
4. Flexible elastic decoupling. Use of soft materials or springs or magnetic or air suspension.

Vibration isolation for scrooges
As my old boss used to say "You can't shine sh*t". So none of these products is going to rescue a poor system set up in the wrong place. None will solve the problems created by placing any of your audio components on heavy furniture like wooden sideboards or cupboards. Any enclosed cabinet with large surfaces is likely to sound worse than simply placing your hi-fi on the floor, even a suspended wooden floor, because the cabinet simply adds to the existing wood-floor disaster.

If you have a concrete floor then that is the best place to support any stands or audio tables. If you have a suspended wooden floor but masonry walls (brick or stone or block-work) then the walls might be a better place to support your audio shelves (properly bolted frames to the masonry, with decoupling between frame & shelf). If you have suspended wooden floors and drywall, plasterboard or timber-frame walls you should consider

1. Moving house
2. Rebuilding your existing house
3. Finding another hobby

Room corners or the area between stereo speakers are the worst places for airborne & impact noise. A room-corner on the plane of the speakers (maximum constructive-interference antinodes) is absolutely the worst place to site any audio component, but this is where so many end up, often in an alcove by a fireplace just to exacerbate the situation. For this test this is where these products will be tested in order to exaggerate their effects, but it is not where your hi-fi should be.

Taking a stand
There is a wide variety of shelf-table-stand products offered to the hi-fi enthusiast at a wide range of prices. The more expensive ones already incorporate suspension or absorption devices and adding extra ones may well impair the performance. Some of these fancy coffee-tables cost more than €1500. With careful positioning and a few modifications, cheaper or older types will perform almost as well. Buy used support equipment cheaply, because accessories do not hold their value like main components. Then tweak with the products reviewed here.

The first fundamental question: Is it better to choose a heavy soggy material or one with a distinct narrow high-Q ring and support it on a mechanical filter?

Turntable suspensions are such filters, usually mass & spring conforming to Hooke's law at least 2 octaves below the audio spectrum. Perhaps it would be better to damp the narrow hi-Q resonant frequency? Barlow's BBC experiments with loudspeaker wall materials in the 60s & 70s came to this conclusion & my own experience supports this.

Since the late ‘70s there have been numerous steel frame tables & shelves made by many manufacturers, mainstream & independent. They all comprise a steel frame (box-section or angle) and chipboard or mdf shelves perched on tabs or spikes. If you hold the frame up and strike it, what do you hear? It rings like a bell. I have tried various methods to damp this ring, including lead shot, dry sand, silicone, expanding formaldehyde-foam-filler, car-repair resin, automotive underseal and various combinations. Dry sand works best, the fine washed kind sold for children's play seems best of all.

Shelf Materials
Toughened glass (as required for CE approved furniture) also rings like a bell. It can be used as layers of a sandwich filled with bubble-wrap packing material, but this defeats the aesthetic reasons for choosing glass, and the bubble-wrap soon sags.

Thick (10mm) laminated glass does work though, which is a sandwich of two layers of glass with a bonding material in between. It has the high speed of sound associated with glass (a peculiar supercooled-liquid structure), but every part of it is held and constrained by the flexible middle layer. I find it works well under every kind of component and is only bettered by other materials in particular applications.

Material combinations often outperform each material alone. The early 90s Rivelin Eclipse turntable, designed in Sheffield by Chris Orchard, comprised a stainless-steel & sorbothane sandwich solid-plinth. On a proper turntable support it seemed as quiet & immune to vibration (airborne & impact noise) as the Linn Sondek & Michell Gyrodec suspended-subchassis turntables that I compared.

Hardwood looks good, and if damped sounds better than any chipboard or mdf. The chosen damping method depends on the component it will support. Chipboard & mdf are aesthetically naff and some concerns have been expressed about their formaldehyde content. So much is made from them because they are really cheap & easy to manufacture, but I have always found better sounding materials in every experiment.

Part II will try stuff with turntables to hear what effects they have while Part III will deal with CD players and amps.

Mark Wheeler, wondering how I'll ever test all the combinations

[Fast forward to Part II] - [Fast Forward to Part III]

Copyright 2004 Mark Wheeler - www.tnt-audio.com

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