Principles of Mechanical Isolation and Mass-Coupling To Control Vibration In A/V Systems
Cones, dots, blocks of maple, blocks of marble, magnets, ball bearings. Do they isolate? Do they couple? Do they help? Are they a waste of time and money?
It is up to us as A/V consumers to use our noggins and apply the information seen in ads and promotions intelligently. Vibration CAN be a problem, under some circumstances, if not treated.
Where can vibration be a problem?
With components that are sensitive:
Speakers are a different animal. They are made to vibrate, so they are in a class by themselves and are handled differently.
In my experience, other than turntables, no well-designed product should be assumed to be a vibration problem unless it is found to be. A simple test can tell you a lot. With no music playing and with system volume turned up for a loud volume level, tap on the component in question, slap it a few times (gently, don't go crazy here). With a CD / DVD / Bluray player, tap it gently while playing. Does it skip or fuzz out? Does it hesitate, indicating excessive error correction? Any change from the norm indicates a possible vibration sensitivity.
How to treat a vibration-sensitive system element:
Understand how the vibration controller works! They may claim seemingly magical characteristics, but there are two main ways of controlling vibration:
Speakers:
If there is a lot of vibration in the house structure, from traffic or equipment elsewhere, then vibration getting into the speaker can be a problem. It is usually LF vibration, between 1 Hz and 20 Hz, and can affect imaging negatively. Isolating can be difficult. You are looking for something that allows a degree of "float" at those frequencies. And you need to consider all directions of isolation: "reverse cones" might help in the left-right and front-back directions only, IF AT ALL (ask yourself, what is being isolated and what is being coupled and HOW???? it is usually not that hard to figure out), but NOT in the up-down direction of vibration.
Some high-end speakers go to crazy lengths to absorb and eliminate all vibration except for the vibration of the speaker cones themselves - anything else can be a disruption to image clarity. There are speaker enclosures that weigh many hundreds of pounds using materials and/or internal bracing to eliminate unwanted vibration. Here are two areas of possible benefit that I have found:
Common strategies:
How much is enough?
That is really hard to judge without measurements. A good strategy is to try something on your own that is cheap and simple. Ball bearings on heavy ceramic concave dishes - this you can try for a few bucks. Do a listening test for image clarity. Measure at a pre-out with REW's RTA function, focusing on the LF noise floor (I like the RTA 1/48 octave, 65536 FFT Length, 4 averages setting).
Cones, dots, blocks of maple, blocks of marble, magnets, ball bearings. Do they isolate? Do they couple? Do they help? Are they a waste of time and money?
It is up to us as A/V consumers to use our noggins and apply the information seen in ads and promotions intelligently. Vibration CAN be a problem, under some circumstances, if not treated.
Where can vibration be a problem?
With components that are sensitive:
- Turntables - the most common vibration-sensitive A/V component.
- CD / DVD / Bluray drives - sometimes, it depends on how well they are designed. Some can get knocked around and never give a single bit-error of an iota of jitter, others are more sensitive.
- Components that use vacuum tubes, especially preamps - microphonics are to blame, vibration in the tubes can be converted directly to audio noise.
- Components where low-level signals are treated or amplified (preamps) - microphonics again.
Speakers are a different animal. They are made to vibrate, so they are in a class by themselves and are handled differently.
In my experience, other than turntables, no well-designed product should be assumed to be a vibration problem unless it is found to be. A simple test can tell you a lot. With no music playing and with system volume turned up for a loud volume level, tap on the component in question, slap it a few times (gently, don't go crazy here). With a CD / DVD / Bluray player, tap it gently while playing. Does it skip or fuzz out? Does it hesitate, indicating excessive error correction? Any change from the norm indicates a possible vibration sensitivity.
How to treat a vibration-sensitive system element:
Understand how the vibration controller works! They may claim seemingly magical characteristics, but there are two main ways of controlling vibration:
- Isolation - some isolating element makes the component in question "float" independently of the vibration source.
- Mass-Coupling - a large mass is used to absorb the energy of the vibration so it is not troublesome.
Speakers:
If there is a lot of vibration in the house structure, from traffic or equipment elsewhere, then vibration getting into the speaker can be a problem. It is usually LF vibration, between 1 Hz and 20 Hz, and can affect imaging negatively. Isolating can be difficult. You are looking for something that allows a degree of "float" at those frequencies. And you need to consider all directions of isolation: "reverse cones" might help in the left-right and front-back directions only, IF AT ALL (ask yourself, what is being isolated and what is being coupled and HOW???? it is usually not that hard to figure out), but NOT in the up-down direction of vibration.
Some high-end speakers go to crazy lengths to absorb and eliminate all vibration except for the vibration of the speaker cones themselves - anything else can be a disruption to image clarity. There are speaker enclosures that weigh many hundreds of pounds using materials and/or internal bracing to eliminate unwanted vibration. Here are two areas of possible benefit that I have found:
- If the sweet spot is narrow and hard to set up, then imaging can be a negatively affected by enclosure vibration. Use spikes to couple the enclosure to the house structure. Or use poster putty to couple them to a marble block or stone floor, for instance. Imaging can be dramatically improved.
- If bass seems soft or mushy, coupling (as above) can be beneficial to hold the enclosure in place at lower frequencies (Newton's second law - equal and opposite reaction) and bass "tightness" can improve dramatically.
Common strategies:
- A cone tip against a hard, smooth surface isolates in two directions by allowing sliding action. It does nothing of benefit in the direction that the cone points toward ("chattering" is even possible, and is BAD).
- A cone tip against a softer surface couples in all directions by digging into the softer material so they must move together.
- A cone tip against a hard surface with an indentation couples in two directions by being locked together at the cone/indentation interface. It does nothing of benefit in the direction that the cone points toward ("chattering" is even possible, and is BAD).
- "Dots" and "pads" are completely dependent on the characteristics of the material, and upon whether they have adhesive in use to stick them to the surfaces involved, and are hard to predict. At frequencies where the material is stiff, coupling is possible if adhesives are used. At frequencies where they are soft, some isolation is possible.
- Poster putty (blue tack) is popular as a coupling element. It is sticky yet easily removed. It might provide an additional benefit of some isolation at very low frequencies.
- A rolling element (a ball bearing on a concave surface) isolates in two directions by allowing sliding action. It does nothing of benefit in the up/down direction ("chattering" is even possible, and is BAD).
- Magnetic or air-flow levitation isolates in all directions. Air flow will usually introduce some vibration of its own. An electromagnet with a noisy DC driving source can couple noise into a system magnetically.
- Wood or metal or stone is usually a mass to be coupled to for absorbing vibration. All materials have resonant frequencies depending on size, shape, and material. Resonant frequencies in the audio range are usually problematic. Greater mass generally means lower resonant frequency and better absorption of audio frequency energy.
- Layers of wood or metal or stone or some combination, glued together with an isolating air pocket in between. There are elements of coupling and of isolation involved. A specific design would have to be scrutinized individually. Remember that resonant frequencies of isolators and couplers in a given design are important, and can work together to make an effective design or can work together to make a disastrous design.
- There are complex suspension products recently on the market for speakers. They provide a combination of coupling and isolating protection, by using materials in a configuration that, generally speaking, isolates at subsonic frequencies and couples at audio frequencies. Some manufacturers build such a suspension into the base of their speakers. In general, I believe the approach to be beneficial, depending on the speaker design and on the presence of subsonic noise in the area from heavy equipment or traffic.
How much is enough?
That is really hard to judge without measurements. A good strategy is to try something on your own that is cheap and simple. Ball bearings on heavy ceramic concave dishes - this you can try for a few bucks. Do a listening test for image clarity. Measure at a pre-out with REW's RTA function, focusing on the LF noise floor (I like the RTA 1/48 octave, 65536 FFT Length, 4 averages setting).