The fact that the support and interface we place under a speaker perceptively changes the sound clearly illustrates the insidious nature of vibration as it relates to an audio system. Almost every aspect of sound reproduction - tonality, spatiality, dynamics, coherence, etc. - is compromised by unwanted vibrational energy. That is the result of a disturbance in the relationship between frequency, amplitude and phase of the original signal that the audio system is reproducing.
As the speaker drivers are radiating acoustic energy into the room (the energy that we want - music) they are also sending energy into the speaker cabinet because of air pressure from the inward motion of the cone and by conducted energy through the frame and mounting flange which becomes unwanted stored energy (USE). This USE causes the cabinet walls, the crossover components, the connected speaker cable, etc. to vibrate (the drivers are also subject to compromise by their own vibrational energy that they've sent into the cabinet that is then reflected back towards them after a delay in time). The cabinet vibration has the most consequence towards corrupting the speaker's performance. If we were able to quiet the driver's acoustic output into the room and just hear the result of the cabinet walls vibrating I think we would be shocked as to just how much audible acoustic energy the cabinet would be radiating! THIS version of the audio signal would have a different frequency balance than the driver's output (it would sound muffled being dominated by the primary and secondary resonance frequencies of the cabinet) it would be lower in amplitude (but not uniformly lower because of the non-linear nature of the cabinet materials) and would be delayed in time (the amount of time it would take for the energy to leave the drivers, be absorbed by the cabinet and then be released into the air) thus affecting phase integrity. If we think about this 'smeared' version of the signal (which contains corrupted frequency, amplitude and phase) being mixed back into the original signal it is no wonder that USE significantly affects the reproduction and that altering the USE has an audible effect!
Now, just imagine if we make a significant reduction of USE in the speaker cabinet. The amount of audible difference would be profound. Placing high mass on top of the speaker cabinet will significantly increase the resonance frequency (a good thing) and decrease the amplitude (also a good thing) of the top panel. The weight load will then be translated onto the side panels with a related change in their resonance frequencies. Furthermore, the added mass will more effectively couple the speaker bottom to the top plate of the speaker support, the floor or more suitably to a high-mass high-absorption platform so the USE can be drained from the speaker cabinet.
If we used laser infrarometry to measure the displacement of the speaker panel we would see a noticeable reduction in displacement (vibration) when high mass is set on top. In addition, if a high-mass high-absorption platform is placed under the speaker, this multi-stage vibration control system forces the speaker to be more effective in its main task of reproducing music - the drivers do not waste their energy in making the cabinet or internal parts vibrate because the cabinet is far more resistant to displacement. The drivers have no choice but to use their energy more efficiently in creating music.
The most effective method for supporting a speaker would be a high mass element on top of the cabinet, a high-mass high-absorption platform directly under the speaker (on top of a rigid and strong stand for a mini-monitor) and a pneumatic base on the floor to decouple the speaker's energy from entering the floor and being transmitted to the equipment rack. This configuration is highly successful in eliciting the peak performance from the speaker without a redesign of the cabinet or the component parts.
The other components in an audio system will also benefit by a reduction of USE in their chassis. Besides speakers, turntables exhibit the largest degree of improvement by proper vibration control. Since they are electro-mechanical devices it is almost intuitive to us that this be so but the purely electronic devices also benefit: tubes are microphonic, the master and sub-clocks (which are based on oscillating crystals) in digital devices are affected, a spinning disc inside a digital player will exhibit non-linear movement, all component parts (transistors, ICs, capacitors, resistors, wire, diodes, etc.) that process the signal become microphonic, motors, fans and buzzing transformers induce vibration into surrounding parts, and the list goes on.
What are the sonic results of vibration contamination? As we discussed, frequency, amplitude and phase are corrupted. Frequency balance is skewed: one portion of the spectrum is highlighted or diminished as compared with another. Brightness may increase, midrange may become too forward, bass may bloat and become ill defined. Amplitude of the signal is changed: the dynamic range of an instrument and indeed the dynamic relationships between the instruments are altered. Phase integrity of the signal is deteriorated: the spatial relationship of the instrument with its environment and the spatial relationships between the instruments are altered. In fact, frequency, amplitude and phase are interrelated and changing any one affects the other two. If all three are affected at the same time (by the presence of unwanted vibration) the resulting cacophony significantly reduces the ability of the system to convey what is actually contained in the recording - and that's what audio is all about. Not just what sounds pleasing because it makes someone feel fuzzy all over but what is musically and emotionally fulfilling because it reflects the actual sound of the instruments as they have been captured in the recording.
When we eliminate the sonic results of vibration contamination we more accurately hear what the individual components in a system are doing. It is possible that these results might be misinterpreted by some individuals. For example: if a speaker is providing excess out-of-phase elements the size of the soundfield might INCREASE beyond what is actually in the recording. Bigger is not always. This individual will have adjusted speaker placement and acoustic room treatment based on this exaggerated sonic view of the soundfield. Once the out-of-phase elements are properly controlled by reducing vibration the size of the soundfield may become smaller in this incorrect set-up and the listener may say, "Oh, this is not as good as it was before because things are not as large." What they should be doing is reevaluating speaker position and room treatment to optimize the now correctly operating speaker. Once that is accomplished they will find that not only is the soundfield as large, if not larger than before, but the instruments are in proper relationship with one another and ambience is cohesive instead of exaggerated. Frequency changes can also be misinterpreted: in a vibration plagued system a too forward midrange during transients is a typical symptom. Some people might misinterpret this as the system exhibiting "good presence" or a forward brightness region is often described erroneously by some listeners as "good detail". The removal of the vibration will eliminate these effects. Some may feel at first, that the removal of these exaggerated artifacts is a step backward in reproduction, but what they are hearing in the now vibration free system are the possible cumulative effects of previous tweaking and/or component choices made with a vibration drenched palette. Once the problems caused by USE are removed some system choices may need to be reevaluated.
Best Regards,
Barry Kohan
President
Bright Star Audio
Disclaimer: I am a manufacturer of vibration control products.
