Does external noise (not electrical noise) affect system voltage? If so how does a source of sound energy distort a voltage waveform?


What exactly is happening in this lecture at 46:50 when the professor increases the noise of the chain saw?

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    \$\begingroup\$ Microphonic effect; either magnetically induced or electrically induced as per capacitor microphonics. \$\endgroup\$ – Andy aka Nov 12 '19 at 8:03
  • \$\begingroup\$ It’s hard to tell exactly what kind of trick the professor might be playing. Notice it is an electric chain saw, so it’s possible it’s electromagnetic noise from the saw’s motor (brushed motors are notoriously noisy, electrically speaking) being coupled to the circuit somehow. \$\endgroup\$ – joribama Dec 2 '19 at 23:49

In principle, acoustic waves do not generate electricity on metal wires directly.

However, special devices can be built to transform acoustic energy into electric energy and vice-versa. In dynamic microphones, for instance, acoustic waves move a thin membrane attached to a wire coil in the presence of magnetic field created by a magnet. The perceived change of the magnetic field by the coil generates electricity. A similar effect happens in a speaker, this time in the opposite direction: an electric signal will make the coil move, what will move a membrane and create an acoustic wave.

Another practical effect for the acoustic/electrical conversion is the piezoelectric effect. Certain materials can generate electricity when subject to mechanical stress, and vice-versa, they can generate mechanical movement in presence of electrical signals. Certain buzzers make use of this effect, like the one responsible for the "beep" sound from a microwave oven. These types of devices can also be used in the opposite direction, as well, like in guitar pick-ups. As a curiosity, the piezoelectric effect is also used in filters in cell phones, but at very very high ultrasonic frequencies.

You may also have certain components in a circuit that unitentionally produce sound from electricity. Transformers, for example, can hum due to the vibration of their cores. Ceramic capacitors can produce very soft clicks due to the piezoelectric effect. These same components can theoretically work the other way and produce electric signals out of acoustic noises. However, in practice the electric signal that may be created is negligible in most cases.

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    \$\begingroup\$ A vibrating wire R in a steady magnetic fleld may generate a changing voltage in the wire. \$\endgroup\$ – analogsystemsrf Nov 12 '19 at 10:03

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