Bad Vibes! Page 3
In order to keep things in focus, I will explore the effects of vibration on audio systems by examining how resonances interact with each of the major elements---floor, stands, platforms or shelves, components---and the various means of connecting them. Throughout our discussion I'll be referring to large torsional, or twisting, forces on structures, as well as significant displacements vs small movements. However, these are only relative comparisons. On an absolute scale, all of the vibration interactions in a typical audio system are very small (except for some room and speaker-cabinet resonances). Objectively, the degree of sonic changes from effective resonance control is subtle compared to that from switching from one model of speaker to another, yet its subjective impact clearly illustrates the adage that big results can spring from small events---the musical merit can be surprisingly significant.
The principles of "an ideal rigid body" and the use of compliant suspensions for isolation will be defined, as well as the key role played by damping in both concepts. Finally, I will compare the pros and cons of these techniques with a few practical examples that show how they can be combined to achieve superior all-around vibration control while highlighting the special qualities of pneumatic isolation. I've tried to make this fairly technical subject accessible to as many readers as possible by minimizing scientific terminology and formula-laden examples.
Any discussion of vibration control must start with the acknowledgment that you can never entirely eliminate the problem. Resonances in solid structures are global, insidious, and complex. Try examining your components with a stethoscope to hear how pervasive vibrations are in a stereo system---even without music playing. Yet a significant degree of damaging resonances from a multitude of sources cannot even be felt or heard under normal conditions.
The biggest offender is feedback from the music itself. High-amplitude, low-frequency sound from the speakers causes the most prominent audible disturbances, bombarding the entire system both acoustically and through mechanical coupling to the floor. However, it is the very-low-level, low-frequency vibrations (generally from 5Hz to around 100Hz) from passing trucks, elevators, machinery, and other sources, in addition to those from our speakers, that can make really effective control both challenging and costly. Since absolute isolation is impossible, realistic goals must be set based on a clear understanding of the forces at work. Otherwise you're likely to wind up out in left field after a lot of effort and expense, with results that are mixed at best.
While the physics of vibration control are intricate, mechanical vibrations can excite audio components from the floor via three common sources: stands, supporting platforms, or shelves. As you will see, common equipment-stand construction---even some heavy-duty "audiophile approved" models---can actually exacerbate floor-borne horizontal vibrations.
Acoustic pressure generated by the loudspeaker/room interface can couple directly to stands, platforms, and equipment enclosures, and then to signal-generating components. Internal vibrations arising from transformers, mechanical drive systems (including spindle motors or servos in CD players, and those in analog turntables), as well as electric current simply moving through wires and other components, can make effective solutions less straightforward---in particular because we not only must deal with external sources of vibration, but must minimize the effects of a component's self-inflicted resonances as well.
This last theory has also been proposed for the mechanism of sonic degradation arising from these internally generated resonances in nonmechanical, non-transducing electronic components such as preamps and amplifiers. For example, current-induced magnetic fields forming around transformers, wires, and other passive devices can cause these components to vibrate or move, however slightly, within their own fields, creating minute non-linear currents that may subtly alter the original signal. Another well-known process, familiar to tube aficionados yet also affecting solid-state devices, is microphonics, whereby mechanical vibrations are converted into electrical signals and then combine with the music as added distortion.
The Fundamentals: Rigid Coupling
For the sake of clarity and relevance to audio systems, I will explore the following principles first by looking at a typical supporting platform or shelf, and later through examining various suspension designs; however, the basic concepts apply to any solid structure.
In broad terms, the goal of vibration control is to minimize the relative motion between different elements that comprise a system. Ideally, this would be accomplished by connecting perfectly rigid elements to a structure that is also perfectly rigid so that the distance between any two points of the assembly would remain dynamically constant at all times, creating a system referred to as an "ideal rigid body." This means that the size and shape of the composite structure will not change, regardless of exposure to vibration, temperature fluctuation, or static load.
Of course, such a perfectly rigid system is impossible: all solid objects vibrate through subtle flexing and twisting forces. Nevertheless, this ideal defines the direction to be pursued. In practice, this goal is approached by starting with very stiff and uniform, yet well-damped, supporting platforms upon which the component(s) of interest is placed. (Damping refers to any process that diminishes the amplitude of vibrations through conversion into heat, friction, or other resistances.)