5
$\begingroup$

Once my friend told me that he was listening some background noise while his earphones were not plugged in any device. I didn't believe him but 10 minutes ago I experimented the same. I can certainly say that I heard music. I guess it was radio.

Here are the things that I remember I did:

  • The earphones were plugged to my laptop but I wasn't listening anything.
  • I heard some kind of music and checked that there was not any video-audio program running.
  • I unplugged the earphones and during nearly one minute I could hear the music and finally it stopped.
  • I've plugged the earphone to the laptop again but the music had gone definitely.

The earphones are the ones that you receive in trains, something like this.

Maybe what I was listening to some AM frequency radio stations. So what explanation do you have?

$\endgroup$
7
$\begingroup$

Interesting. In your title you say "background noise." So I was going to suggest the sea shell effect - putting a sea shell to your ear you think you hear the sea, even if it is far away.

If you did think you heard music, that could have been an illusion. Your brain is wired to look for patterns - see animal shapes in the clouds, hear people talking (or music playing) when the vacuum cleaner is on, find co-incidences in the events of life, etc. From among external noises resonating in the earphone and funnelled into your ear, your brain picks out familiar sounds of the kind it expects to hear.

But if it was a recognisable, continuous tune which stops after a predictable time and does not come back, there must be some other explanation.

http://science.howstuffworks.com/question556.htm


An alternative explanation which I am less sure about, but which is mentioned quite often with speakers, is that part of the earphone (possibly the wire lead) is acting as an antenna, picking up radio waves.

The earphone contains a few capacitors which get charged up during use. When you disconnect power the charge on the capacitors slowly drains away at a rate depending on what resistors they drains through. (Recall that RC is the time constant.) The leaking capacitors could be powering the amplifier in your earphone, amplifying the pick-up of radio waves, until they have fully discharged.

During normal use the pick-up noise ("radio interference") might still be there but insignificant compared with the music you are listening to.

This explains better why it sounds like a radio station (because it is a radio station) and why it persists for only about 1 minute after disconnecting the power (because that is how long it takes the capacitors to discharge).

http://www.alectrosystems.com/tips/audio_tips/audio_tip_RFI.htm

$\endgroup$
  • 2
    $\begingroup$ One can test the antenna idea by rolling up the cable. If the signal goes away, it was that, if it doesn't, I would go with the seashell. :-) $\endgroup$ – CuriousOne Jul 8 '16 at 19:26
  • 1
    $\begingroup$ Yes, I recommend the broad paper cockle for news, the horse conch for sport, and the smooth duck clam for light entertainment. $\endgroup$ – sammy gerbil Jul 8 '16 at 19:35
  • 1
    $\begingroup$ It is not enough to have capacitance to create a primitive AM receiver; you need a rectifier. Luckily, a bad solder joint (not uncommon in cheap earphones) may be able act as a diode because of a small layer of oxide on dissimilar metals - a phenomenon used in for example the foxhole radio. I doubt that "headphones you get on trains" have batteries and amplifiers built in... $\endgroup$ – Floris Jul 8 '16 at 21:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.