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Moving membranes or piezoelectric materials obviously produce sound waves, but how can "purely" electrical circuits such as transformers or DCDC choppers (and others) often have an audible noise? Is the material microscopically expanding and shrinking with the current?

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Everything is a speaker, or a microphone, or both. Most things just don't do it intentionally and usually aren't that efficient at it :) – hobbs Aug 26 '14 at 4:02
"coil noise" / "coil whine" – user11153 Aug 26 '14 at 9:12
i think when they explode, they will produce sound – user52044 Aug 26 '14 at 10:49
Search in Wikipedia for the Barkhausen effect or Barkhausen noise. – user52108 Aug 27 '14 at 7:56
I was not aware there were any "purely" electrical circuits. They all have to exist in a physical universe. – Connor Wolf Aug 27 '14 at 11:30
up vote 66 down vote accepted

What you are really asking is how can electrical circuits cause small motions. After all, sound is motion of the air.

The answer is that there are various ways electric fields or electric currents can cause forces or motions. These effects are harnessed in the design of various transducers, which exist to deliberately cause or sense small motions. However, the laws of physics that allow these transducers to function don't stop outside the transducer case. They exist everywhere, so many things are unintended transducers. The difference is that usually the effect is rather weak without it being deliberately designed for as in a transducer.

Some of these effects are:

  1. Electrostatic force. Two objects at a different voltage will have a force between them. The force is proportional to the voltage and inversely proportional to distance. This is the same force that allows a balloon to stick to your hair after rubbing it against a cat or something. For ordinary circuits, this force is very weak, and conductors are held in place much more strongly than it. Still, you can sometimes get audible sound from this with high voltage circuits.

  2. Electrodynamic force. A moving charge creates a circular magnetic field around it. The magnetic field is proportional to the current, and can be made quite strong by looping the wire into a coil. This magnetic field can be made to move things, and is the basis for how solenoids, motors, and loudspeakers work.

    Moving charges likewise experience a force if flowing thru a magnetic field of the right orientation. Most loudspeakers actually work on this principle; they are made so that a strong permanent magnet is fixed and the coil moves, which in turn moves the center of the speaker cone. The same thing happens in any inductor. Each piece of wire with current thru it experiences some force due to the overall magnetic field. Some of the buzzing you hear from transformers is individual pieces of wire moving a little bit as a result.

  3. Piezoelectric effect. Some materials, like quartz for example, will change their size or shape slightly as a function of applied electric field. Some small earphones work on this principle. There are also "crystal" microphones that work on this principle in reverse, meaning applying force to the crystal causes it to create a voltage. Common barbecue grill ignitors work on this principle by whacking a quartz crystal hard and suddenly enough to create a high enough voltage to cause a spark.

    Some capacitor materials exhibit enough of this effect that when rigidly mounted on a circuit board can cause audible sound. I had to respin a board once and replace a ceramic cap with a electrolytic just because the ceramic was causing annoying audible whine.

  4. Magnetostrictive effect. This is the magnetic analog of the piezoelectric effect. Some materials change shape or size depending on the applied magnetic field, and this effect works in reverse too. I have worked on magnetic sensors that exploited this effect.

    Materials in transformers and inductors are chosen to not have this effect, but a small amount is there anyway. The core of a inductor actually changes size very slightly as the magnetic field changes. This can cause audible sound, especially if the inductor is mechanically coupled to something that presents a greater area to the air, like a circuit board.

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+1 Now I'll have to find a cat to rub a balloon against it and test the eletrostatic force! :) – woliveirajr Aug 25 '14 at 16:42
Well organised, thorough and overall excellent answer as usual. Thanks for contributing so much on SE Olin. – Mister Mystère Aug 25 '14 at 22:45
@Mister: Thanks, yet someone thinks this answer is wrong, misleading, or badly written since it receive a downvote. WHOEVER DOWNVOTED THIS: Please explain what exactly you are objecting to. – Olin Lathrop Aug 27 '14 at 13:17
@OlinLathrop haters are just gonna hate. have my +1. – Vladimir Cravero Aug 27 '14 at 14:52

An ideal inductor or transformer might be a purely electronic component, but a real inductor or transformer produces a (rapidly changing) magnetic field. It is a design aim of such a component to keep that magnetic field within the component (for instance inside the ferromagnetic core), but that won't be achieved for 100%. The 'leaking' magnetic field will cause things to move (vibrate), and these things will make the air around them move likewise. Presto: a (unwanted) electromagnetic speaker.

A similar effect can probably be had in high-voltage capacitors, where the conducting plates attract each other depending on the voltage. This corresponds to an electrostatic speaker :)

A third effect is (unwanted) piezoelectric effects in components. I am not sure if this is actually the case in an observable level.

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I don't think it has anything to do with "The 'leaking' magnetic field...". The wires wrapped around a core of an inductor or transformer core are exerting a force due to the electromagnetic field. That field is changing (or the transformer or inductor aren't much use!), so the components of the part are moving, causing sound to move through air. Simples! – gbulmer Aug 25 '14 at 20:46
Thanks for your answer, it is particularly interesting to complete/illustrate Olin's answer. – Mister Mystère Aug 25 '14 at 22:46

It isn't expanding or contracting the material, that emits the sound in transformer or inductor-based circuits. However the parts are moving.

Transformers are subject to significant mechanical forces caused by the alternatine electromagnetc fields. That causes wires and laminations to move, and hence emit sound. DC-DC converters often have wound inductors, which also move for the same reason.

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Here is one more

Sound by changing properties of surrounding plasma or gas due to exposure of an electrical field and/or electrical discharge

Based upon the "Singing Arc" which was discovered around 1900 by William Duddell, the Ionophone or as it is mostly called plasma speaker/tweeter (is actually used in speakers) produces sound waves by charging plasma to change the size of the plasma within a usually narrow field between electrodes. Due to the very low mass that has to be moved this speaker can produce a very accurate reproduction of the waves feed to the electrodes, especially good for high frequencies.

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Moving membranes or piezoelectric materials obviously produce sound waves, but how can "purely" electrical circuits such as transformers or DC/DC choppers (and others) often have an audible noise? Is the material microscopically expanding and shrinking with the current?

While others have explained the part about the material moving nicely, one key point is that Audible noise requires movement in the human audible range. Typically that means 20 Hz to 20 kHz, but can be slightly lower or higher, as well as accounting for age/hearing loss. Anything oscillating above or below that range (Infrasonic or Ultrasonic) will not normally be heard. As luck would have it though, that range is the typical used in many circuits, from DC/DC choppers, transformers, EL Panel Inverters, PWM for Light Circuits, so its often a byproduct.

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Of course, audible electronic noise in higher ranges might not be heard by humans/adults, but animals sure won't like it either. – Passerby Aug 27 '14 at 8:18

Another effect not yet touched upon is wire straightening under load - wires do tend to straighten when current is passed through them, whether microscopically or visibly. The wire within a power transformer's windings tries to straighten very slightly 100 to 120 times per second (depending upon the frequency of municipal power).

This phenomenon can be very readily observed when "jump starting" a vehicle with smallish jumper cables, especially if the vehicle being started has a badly depleted battery. When the starter is engaged, it's often easy to see the jumper cables "jump" and stiffen as they straighten slightly under heavy loading.

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I am willing to believe the phenomena of 'straightening wires'. However, if that is electromagnetic force, this is evidence, but not an alternative explanation. A straight conductor would be a minimum configuration for electromagnetic force through a wire. So are you offering this answer as evidence? Or is there a non-electromagnetic force explanation? – gbulmer Aug 25 '14 at 14:54
@gbulmer, I'm not sure I have a really good explanation for the phenomenon, whether it's magnetic in nature or more simply a manifestation of the fact that electrons and holes have a slight preference for straight-line travel. Certainly I don't think my answer is "the only correct answer", but then I don't think this question has any single answer - I think there are many causes. – TDHofstetter Aug 25 '14 at 15:21
I always assumed it was resistive heating in the in the jumper cables, causing expansion. – bitsmack Aug 25 '14 at 15:59
The current demanded by the starter motor is fairly heavy - wouldn't a magnetic effect be the most likely explanation? – peterG Aug 25 '14 at 17:19
@TDHofstetter - okay, I think I understand. Newton said everything has a preference for straight line travel, so I don't think that tells me anything. I think electromagnetism is an understandable, measurable, and well understood explanation. IIRC electron drift is only a few 10's of centimetres/second, and compared to the bulk of copper in the cable, their mass is small (copper atomic mass 63.5, electron mass 5.5e-4, i.e. 10e-5 smaller) so that seems several orders of magnitude too small to explain your observation. Maybe someone can disprove that. – gbulmer Aug 25 '14 at 20:42

There's been a lot of theory here. In practice, usually loose wires of inductors are involved. Tapping around on the coils (not!!!! with anything magnetic like a screw driver: trying that on coils in CRT flyback circuitry is something you don't do more than once) may help locating the culprit, and suitable warm glue or nail polish may help getting it under control.

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The question was more about why it happens, not how to fix it. – Eric Aug 26 '14 at 12:10

From my experience, most of the time a transformer makes noise, is due to a loose lamination or a loose mounting. A mechanical chopper makes noise because the reed that "chops" the current moves/vibrates. Obviously anything that moves, makes a sound. A transformer usually produces a 60 Hz hum, while a chopper depends on the frequency for which it was designed (typically 400 Hz).

I don't believe that the material is microscopically expanding and contracting, but if it were, the frequency would be so high it would be inaudible. In addition, it might not be loud enough.

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The only purely non-mechanical circuits that can produce sounds are microwave transmitters. But they will cook your brain.

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I think the OP was looking for things that make sound waves in air, not other effects that are perceived as sound. – Dave Tweed Mar 30 at 10:50

protected by Olin Lathrop Aug 27 '14 at 14:39

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