The Active Phase-Noise/Distortion Artifacts of Electrostriction & Magnetostriction
We have all heard the sound of a transformer "hum" before - right? In most cases they are being powered by a 60 Hz AC utility power line (wall outlet, etc.), but what you usually hear coming from them sure doesn't sound like a clean, pure 60 Hz tone. There ya go - audible sound being generated by an electrical component directly into the air without the need for any loudspeaker. Hmm… interesting.
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In this example the noise and distortion are directly related to the 60 Hz signal feeding or "driving" the transformer, so we know the two are related,... but the resulting spectral content of the noise it produces is anything but an accurate reproduction of the original 60 Hz signal. Hold that thought.
Then there is the occasional “sizzle & buzz” produced by overhead high tension power lines when the humidity is high. That too sounds faintly like a 60 Hz signal but with a lot of higher frequencies added. Actually, such power lines are carrying “3-phase” AC power, so the higher pitches are partially due to an additional frequency content being comprised of multiples of 60 Hz as well. That makes sense, but the bigger question is; “How is it that we can hear electrons flowing through wires in the first place?”
Well, there is the Electrostriction Effect, which is a type of mechanical vibration that is caused by a difference in AC VOLTAGE (like music signals, 60 Hz AC Power, test tones, etc.) having been applied across two electrical conductors and/or the terminals of an electronic component.
As an example, when AC voltage is applied to the terminals of a typical electrical capacitor it will cause the capacitor plates to be attracted to each other with every 1/2-cycle of the applied signal. As one can imagine, if the AC voltage happens to be an audio signal, then the plates of the capacitor will tend to vibrate in synchrony with it. At some frequencies the plates may “resonate” with the applied signal very strongly like a tuning fork will resonate with a certain tone of an instrument or human voice. Then at other frequencies the plates may be well damped and not vibrate very much at all. The variations in such behavior are unique to every capacitor based on its type, size and construction.
As one might suspect, such behavior as described above is not good if we are trying to achieve the purest of audio fidelity. The reason such mechanical vibrations are bad is that there is an “almost” simultaneous reverse force that results called the "Piezoelectric Effect," which causes the same capacitor to generate AC voltage signals that are directly associated with the mechanical vibrations of its plates and inject them back into the electrical circuit that the capacitor is connected to.
As bad as that may seem, things get even worse because now the frequency of this newly generated signal will have doubled and be twice that of the applied signal and also DELAYED IN TIME with respect to the original music signal that stimulated the whole process.
Therefore, the capacitor has effectively become a generator of phase-shifted 2nd Harmonic Distortion, and possibly several multiples thereof. In fact, due to the capacitor's complex physical geometry, materials and construction, it's more likely that the distortion produced will be an exceedingly complex amalgamation of both harmonic and a-harmonic signals of varying magnitude and phase. So now you know why so many audiophiles “fuss” over the types of capacitors used in the manufacture of their beloved audio components.
OK, now extrapolate the above effect by multiplying it by however many capacitors and other susceptible components and conductors there are in the circuitry of a given audio component, and then again by however many audio components and cable there are in a given audio system. THEN consider that Electrostriction represent only about 50% of the problem because there is also the Magnetostriction Effect to contend with as well.
In it’s simplest form, Magnetostriction is similar to the effect of two magnetized objects (such as permanent magnets) being attracted to or repulsed by each other. Seeing that a magnetic field is produced externally around a conductor whenever electrical CURRENT passes THROUGH it, if two wires or other types of conductors happen to be arranged parallel to each other they will tend to physically attract or repel each other. If the current is a result of an audio signal, then the wires will do so in a similar fashion to that of the capacitor plates in the example above.
Furthermore, the physical shape of a Ferromagnetic material (such as are used in the cores of transformers and coils) can be changed simply due to a change in the state of its latent (at rest) magnetization. In the case of circuitry components, a change in the state of their latent magnetization will again occur due to the magnetic field developed from an applied electrical current passing them them.
This physical change in the shape of ferromagnetic materials is different and a bit more complex in nature as compared to that of the Piezoelectric Effect of Electrostriction, and is called the "Villari Effect." That being the case, due to both simple reverse-Magnetostriction and the Vallari Effect, small currents are generated and injected back into the circuitry of a given audio component. Therefore, these currents represent yet another source of extreme low-level distortion that we are concerned with herein.