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I bought a new speaker and it hums even with volume knob turned to 0. After opening it up, I noticed it was the iron transformer oscillating (and screwing it tighter does reduce the noise).

How is it possible to produce a 100Hz noise out of 50Hz mains ac? Is there any method to reduce this oscillation besides mechanical locking?

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    \$\begingroup\$ Are you sure that it is 1 kHz? The base frequency of auditable noise of a 50 Hz transformer should be 100 Hz. Additionally the sinus of the mains might be distorted and the transformer also makes these harmonics auditable. \$\endgroup\$ – Stefan Feb 9 '16 at 8:36
  • \$\begingroup\$ You mean the transformer was vibrating, transformers cannot oscillate. How did you determine that the 1 kHz is 1 kHz ? Perhaps there was a different mechanical resonance at a higher frequency which was excited by the 50 Hz vibration. A non-sinusoidal 50 Hz signal will contain harmonics at multiples of 50 Hz so 1 kHz is also present. \$\endgroup\$ – Bimpelrekkie Feb 9 '16 at 8:36
  • \$\begingroup\$ [Edited] My fault. I measured it with an Android app oscilloscope (pick up from microphone). I re-calibrate the settings and found it to be 100Hz (confirmed with a 100Hz sine wave). \$\endgroup\$ – user2513881 Feb 9 '16 at 8:41
  • \$\begingroup\$ Buy a better made transformer, that has been submerged in a epoxy, this glues laminations together, the core can be also welded together at each corner. I think that you bought a too chep transformer. \$\endgroup\$ – Marko Buršič Feb 9 '16 at 9:13
  • \$\begingroup\$ The solution to your problem is to use a Ferrite core digikey.com/products/en/… \$\endgroup\$ – saad May 15 '17 at 20:28
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The base frequency of auditable noise of a 50 Hz transformer should be 100 Hz since the two half waves (negativ and positive) are giving the same movement in physical space on the moving part of the transformer.

Mechanical fixation and/or sound insulation is the way to go. Apart from screwing the transformer down, use some liquid glue (e.g. potting compound, protective lacquer) to cover coils and iron sheets. Be careful to use non-flamant, temperature resitive materials for potting. Also try to insulate the sound propagation to the housing by mounting the transformer on a foam pad.

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  • \$\begingroup\$ Why do the two half waves give the same physical displacement? \$\endgroup\$ – user2513881 Feb 9 '16 at 16:51
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    \$\begingroup\$ @user2513881 the magnetic field causes the core to stretch/contract which causes it to vibrate. The amount it stretches or contracts is a function of the magnitude of the magnitude field. In a sine wave the magnitude peaks twice each period, hence the hum is twice the frequency. \$\endgroup\$ – Tom Carpenter Apr 24 '17 at 13:00

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