In my last blog I talked about the propagation of vibration through different materials and what happens at the boundaries of different materials. But this is of no interest if there is no consequence to it all. There is of course – almost all components in an electronic circuit are to some degree 'microphonic' - that is, if they are vibrated, they will turn some of that vibration into a small unwanted electrical signal, which is literally added to the actual signal we want to process. If we allow increasing levels of vibration to pass into our circuit components we get increasing amounts of damage, ranging from subtle loss of precision and detail, right up to gross blurring and harshness. The damage also increases as the electrical signal level increases, so the problem sounds worse at higher volumes, and usable headroom is curtailed and important timing cues from percussion peaks are lost.
Let’s look at two types of devices:
Capacitors are basically two large metal foils tightly wound together with a layer of insulation between them. The electrostatic repulsion of electrons gives such device some very useful electrical properties. But there are mechanical properties too. That repulsion of electrons is a force and we use exactly that type of force in an electrostatic speaker, where we deliberately allow one of the foils to move as we apply a changing signal. Or reverse the process, apply a force to a foil and you get a signal out – you’ve made a simple microphone. In a capacitor we have all those forces as a signal passes through it, but we try not to let the foils move else we will add a sort of echo to the signal. If a capacitor exhibits this effect we say it is self microphonic. There are many capacitors on the market that have been developed to lower this effect such as the Sanyo Oscon and the Elna silmic. The other problem is letting external vibration into the capacitor, which you might think would come from airborne vibration, but no, it’s fed primarily into the capacitor from structural vibration. It propagates into the capacitor mostly straight up the feed wires!
Wound devices are used in many places throughout electronic circuits. Transformers, chokes and inductors. These are again a mechanical transfer device where a changing current in a coiled conductor makes a changing magnetic field, which then produces a current (or back EMF) in another part of the coil. These forces are allowed to give movement in an electric motor, or if you apply movement and you get a current you have a generator. The wound devices in our hifi are static motors – the forces are still there, they are just not allowed to move. But again, like the capacitor, where there is a force transfer function present we suffer from exactly the same problems of self microphony and propagation microphony. At Quiescent we have our transformers built for low self microphony – extra tight winding on oversize cores whilst paying careful attention to the layup of the wires and uniformity of resin.
So defeating microphony requires two parts to the strategy – the use of low self-microphonic components wherever possible, and the control of structural borne vibration to minimise propagated vibration into our sensitive components. Hopefully the discussion about structural acoustics in the earlier blogs are now starting to make sense – it’s precisely because of propagated microphony that we have to take care with the systematic setup of our systems.