When starting to research the various methods of vibration control, one immediately realizes that there are differing opinions. Some believe that active isolation is the only way to properly control vibration, while others believe in mass spring damping, also known as passive isolation. Still more talk about direct coupling versus decoupling as well as attenuation, tuning and absortion. This page will try to decipher and debunk some of the misinformation and marketing hype that is out there.
Active isolation systems attempt to cancel out vibration by vibrating a platform 180 degrees out of phase with a measured vibration stimulus. A typical system has three main parts; a passive base, a measurement unit, and 6 axis voice coils/ rotational motors. The passive base attempts to limit the transmission of higher frequency vibration (>200 Hz) through passive means such as air bladders, springs or viscoelastic material. The “active” components are usually only active below 200 Hz. The measurement unit accurately, and hopefully quickly measures the actual vibration of the platform on which a component rests. The measuring unit sends this information to the 6 axis voice coils / motors. The voice coils then vibrate 180 degrees out of phase with the measured vibration. That is, if the platform is moving one direction, they move the opposite way to cancel out the motion. The measuring unit continuously senses the motion of the platform and thus forms a feedback loop with the voice coils / rotational motors. Generally, active isolation systems do a good job at isolating a component from vibrations in the structure it is sitting on. They are used extensively in the electron microscope industry and work particularly well with relatively large amplitude vibration sources such as footfall. They have two significant drawbacks however. First, they do less well at dealing with airborne vibration (such as loudspeaker output) and machine generated vibration (such as motors/transport mechanisms. The reason is the sensor that detects the vibration is in the platform, not in the component. Typically the amplitude of the vibration in a component case is reduced by the time it reaches a sensor leading to erroneous inputs (or no input) to the motors. Second, latency causes unwanted voice coil / rotational motor movement.
The system feedback is deigned to operate quickly, but by definition, the system can’t respond instantaneously. Thus the voice coils will still be moving even after the original vibration has stopped. This ring-down effect slows down the settling time of the system.
- Attenuates even very low frequency (1-2 Hz) component vibration while providing a stable base.
- High cost, large size, extreme complexity (pneumatics, etc).
- Measurements take place in the platform – not the component, therefore ineffective at attenuating vibration that does not reach the sensor.
- Latency and use of servos is source of noise and ring down effect. Voice coils still moving after vibration has stopped.
Carbon fiber and / or epoxy composites have been used in several products in the audiophile market. This is in part a case of “if its good for fighter planes and race cars then its good for audio components”. Carbon fiber composites do have remarkable structural properties; most notably extreme lightweight and stiffness. Unfortunately these properties make it a good resonator and sound transmitter. Also, It is expensive and difficult to work with.
- Lightweight, stiff, good tensile strength
- Excellent resonator
- Poor internal damping
- Often misused
Ceramic Bearings and Cones
There are many products designed around very hard ball bearing interfaces where the theory is that the single point of contact on the bearing provides an efficient path to allow vibration to flow out of a component or speaker into the supporting structure or floor. If the impedance from the component to footer is mismatched and there is no damping in the system then vibration is reflected back into the component and can cause sonic coloration. If there is an impedance mismatch but there is damping at work, then the vibration energy can be dissipated due to the damping. Hard bearing systems attempt to match the impedances sufficiently to allow vibration to move relatively freely both from the component to the platform and from the platform to the component.
- Rigidity – Less “over-damping” effect
- Allows energy to cross the footer instead of being reflecting back into the component or speaker.
- Energy reflected back into a unit can cause unwanted coloration.
- Does a better job dealing with air-born and machine generated vibration. Allows this energy to be transmitted and dissipated.
- To use an electrical analogy, these bearings and spike devices attempt to “ground” the component or speaker. That is, they provide a pathway for vibration to flow into a mechanical ground in the same way that a electrical circuit can be grounded by hammering an electrode into the earth. The assumption is that the floor and structure are mechanically neutral in the same way that the earth is electrically neutral. Most of the time this is not the case as floors and structures have many resonant modes that can be excited by the flow of vibration.
- Efficient 2-way vibration path from floor or platform to the component or speaker, Vibration from the stand or floor will travel into the component or speaker.
- No self-damping. The system will not inherently dissipate much energy.
There are a variety of anti-vibration coatings used in the high-end audio industry. Coatings are at times applied to cases, stands, and the inside of speakers. The coatings can be effective at damping high frequency vibration but generally due a poor job with low frequency vibration. This discrepancy can lead to muted top end and an overemphasis on bass.
- Helps with case or enclosure ringing
- Space efficient technique
- Cost effective
- Poor low frequency performance
- Difficult or impossible to use as a retrofit
- Performance can vary dramatically with temperature
Compliant Material Decoupling
Many products rely on soft or rubbery materials like sorbothane to attempt to de-couple the motion in a platform from the component sitting atop the soft footers. The dominant mode of vibration isolation is careful tuning of the resonant frequency of the overall system. This tuning relies on manipulation of the mass-spring-damper system variables to achieve a low system resonant frequency. If this resonant frequency can be tuned low enough (<10Hz or so), then frequencies in the 20Hz + range are significantly attenuated as they pass the footers.
- Simple and inexpensive to implement.
- Difficult to get resonant frequency low enough. If resonance occurs in the audible range, say 20 Hz, then any time a 20 Hz signal reaches the footer it actually is amplified by the footer.
- Difficult to make structurally strong. Typically a spring or sorbothane system capable of achieving a very low resonance ends up being very flexible and unstable.
- Slow settling times. An Impulse takes a long time to settle due to the very flexible nature of the mass- spring-damper system.
- “Over-damping”. Measurements reveal that most very compliant systems tend to remove high frequency components from the audio signal resulting in a dead or over-damped sound.
Mechanical System Decoupling
Some products attempt to create an entire platform with a low natural frequency. In this case the platform could be rigid but it is decoupled from the floor through a series of elastomers, counterweights, springs, or air bladders. These systems can do a good job at isolating a component from structure born vibration, and are less prone to the over-damping described above, but they do nothing to attenuate machine generated vibration and air-born vibration. They are also normally quite complex and heavy.
- Passive – no electronics
- Can achieve good isolation from structure born vibration
- Does not significantly attenuate machine generated vibration
- Does not significantly attenuate airborne vibration
- Can be complicated (compressors, springs, counterweights).
- Must be tuned to the weight of the component for proper operation.
Sorbothane, a brand name for ultra-soft poly-urethane rubber, has found widespread use in the vibration damping field due to its good internal damping characteristics. Properly designed sorbothane footers can achieve good vibration isolation but at a cost. In order to attenuate very low frequencies they end up being very mechanically compliant and allow a lot of movement in the component. Listening tests almost always label the sonics as “over-damped”, “Lifeless”, or “stifled”. One theory on this is that due to the viscoelastic nature of the material, after an initial displacement or compression it takes time to “spring-back” to its original form. Subsequent compressions that occur before it is back in its original state are damped with a different tan delta or loss factor. Thus over the short amount of time the footer is exposed to vibration its damping characteristics actually change.
- Good damping characteristics
- Relatively low initial cost
- Sonically dead
- Can wear out requiring replacement
- Mechanically unstable – components can shift and move
A10-U8 Component Control System:
- Rapid settling time – at least as fast as active systems.
- Does not over-damp
- Impedance mismatch prevents flow from structure to component
- Damping at component side dissipates machine generated vibration and air-born vibration transmitted to case.
- Quiescent state is very rigid due to hexagonal close pack of spherical bearing array
2NS Loudspeaker Interface System:
- Reduces signal to floor – reduces unpredictable floor resonances.
- Cleans up the signal going to the floor – materials sequence filters signal from speakers and insures a very sonically flat frequency response.