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Speakers plug magnetic field

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Hello everyone


I discovered something amazing: physics in daily life; more specific: electromagnetism.


So I took the plug of my pc speakers (10V, 0.5A, 50/60Hz), put the volume on the maximum, and experienced first of all that the speakers emitted a sound when the plug was touching my hand. If I took my hand very close to the keyboard of my laptop, another sound was emitted. If I put my hand on my microscope (plugged in, 5V, 1A), another sound. My desk lamp of 11W, 230V caused the loudest sound. When I touch the lamp, but not the plug, another, very weak sound, gets louder when I close in to the plug with my other hand.


This is rather logic, to me: B is proportional to I, so it's logic that the lamp causes a louder sound.


Now this, I find strange:


The sound is louder when I touch the plug and bring my hand near the lamp, than when I directly bring the plug close to the lamp.


Can someone tell me why it makes sound when I just touch the plug? Is it because I'm sitting in some sort of magnetic field?

Can someone also explain to me the thing I find strange?

Does this actually have anything to do with magnetic field?





Edited by Function

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Speakers are made of magnet and electromagnet.

Electromagnet is attached to membrane that's free to move back and forth in one axis.

Moving membrane causes air to move in uniform way.

When electrons flow in one direction through electromagnet is repelled from magnet, and membrane moves outside.

If they flow in opposite direction, electromagnet is attracting with magnet, and membrane moves to inside.


If you have raw speaker, connect it to f.e. 1.5 V battery and hold, and you will see movement of membrane.

Swap ends of wires, and you will see movement of membrane in opposite direction.

Start connecting and disconnecting wires very fast, and you will hear sound/crackles.

The faster you will do it, the more it will sound like normal sound.

There is needed 20-20000 Hz for human to record it as normal sound.


Magnet is pretty visible in raw speaker on the left:



Using two transistors NPN (f.e. 2N3904), two electrolytic capacitors and couple resistors and variable resistors, you can build sinus (or sort of) wave generator with controlled frequency.

After connecting it to speaker you will hear sound and see movement of membrane (with very low frequency).


Here is circuit




Can someone tell me why it makes sound when I just touch the plug?


Your PC speakers have probably built-in amplifier.

It won't work with raw speaker without amplifier.

Edited by Sensei

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Before the loudspeakers, you have amplifiers, whose input at the signal connector has a rather high impedance. This makes them sensitive to electric fields rather than magnetic fields. he loudspeakers alone give zero noise under these conditions.


Any conductor pick up electric noise created by the environment, the 50Hz/60Hz mains being one common source, radio and TV emissions an other one. Against that, the signal cable to the amplifier is (too poorly) shielded, but the unplugged connector is not, so it catches ambient fields.


A human body catches fields better than the connector does because it's bigger. With one hand near a lamp you catch the field, with the other you inject it (or the voltage, and so on) in the amplifier.


Electric noise pickup is a basic annoyance of any analogue electronic design, and digital as well. Combating it is non-trivial, both from electromagnetism and experimental knowledge; this know-how should have been part of the formation of every electronics designer, but it's not, alas.

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