The world of hifi is full of discussion about the effects of acoustic pollution, both airborne and through materials. And when it comes to the hardware, we casually use terms such as mechanical vibration, resonance, coupling, grounding, damping, isolation and so-on without really understanding how it all works. The truth is at the basic level it’s a very simple subject – nothing more difficult than a high school physics lesson. But why bother about it? Because it’s a fundamental of the Systematic Approach and it becomes relevant when the time comes to really push the boundaries of system performance.
So here are the basics:
Different types of solid materials conduct sound waves at vastly different speeds and efficiencies. Compared to the speed of sound in air, soft materials such as rubber conduct sound very slowly, about 1/4 the speed of sound in air. Very hard materials such as steel and granite conduct sound extremely quickly, about 16 to 18 times the speed of sound in air. Materials such as copper and silver (the metals used for conductors), which you could say are moderately hard, conduct sound at about 10 to 12 times the speed of sound in air. Commensurate with their speed, these materials 'lossy' characteristics follow a similar pattern - but note that whilst soft rubbers are very lossy, even moderately hard materials such as hard plastics and solid wood are only fractionally lossy compared to air, and very hard materials lose hardly anything in comparison to air. So for instance, a steel bar will conduct acoustic energy at very high velocities over a very long way, and so too does copper, silver, aluminium and wood.
When you place 2 similar-characteristic materials into firm contact with each other, say steel against aluminium, sound energy in one will readily pass from one through to the other. We say there is a low acoustic impedance at the junction. When you place 2 dissimilar-characteristic materials into firm contact with each other, say steel against rubber, sound energy in one will not readily pass into the other, it will bounce off the junction and be reflected back. There is a high acoustic impedance at the junction. And we should be clear that we use the terms coupling or grounding for a low impedance junction, and de-coupling or Isolation for a high impedance junction. And remember, crucially, that vibration continues through a low impedance junction and is reflected at a high impedance junction.
Acoustic conduction and acoustic resonance are two completely different things. Resonance is a mechanical function. Take a steel rule and clamp it firmly at one end, flex and release the other end and it will spring back and forth at 3 or 4 Hz say. Place a piece of plasticine on the end and the resonant frequency reduces. The characteristics of this resonance are set by the length of the rule, the springiness of the steel, the mass on the end of the rule and the air's damping effect. So in similar terms, the sheet metal on a component case might flex or resonate at a few tens of Hertz. But take the steel rule again and place its end firmly against something of similar hardness (low impedance junction) that is vibrating and the vibration will travel from one and all the way down to the end of the other at approximately 12,000mph (speed of sound in air at sea level x the factor for steel = 768mph x 16 = 12,288mph), and with extremely low losses. This is acoustic conduction. And don’t assume this is limited to the audio band because it goes right up into ultrasonic frequencies.
Armed with this knowledge we can now see how our mechanical interfaces will behave. We should think about sources of vibration, how and where it can travel and what parts of the system are sensitive to vibration. Soft feet provide a high impedance interface so they decouple – they prevent vibration passing through but also reflect back all the vibration that’s present on either side of the foot. So you might prevent vibration from passing up from a shelf into a component, but you will also lock in any vibration the is being generated within the component. Hard feet or spikes provide a low impedance junction so they couple – they allow vibration to pass through without any significant reflection at the foot. This type of foot will drain some of the vibration generated within a component into the shelf unit that it is stood on, but equally it will pass vibration from the other components and the floor back into the component.
Now both of these foot techniques have an effect on sound quality, but along with the advantage of either type, both also have a significant disadvantage. And so, both are very variable in their results and very dependent on the type of system and rack that are being used in (and the cables too).
So this information is one of the core principles for system building, and component design too. But I'll leave it there for now, and pick up again in my next Systematic Approach Blog.