Why does my cell phone change the volume on my iPod?

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Debbie asked the Naked Scientists:
First I want to say that I've been listening to your show as a podcast for
several years now, and I enjoy it very much.  I even want to try to come to Cambridge for a postdoc once I graduate, partially because I've become such an anglophile!

Unusual as it is, I have kept using my original iPod even after I got an iPhone, which means I often carry both of them around in close proximity. I noticed that when the phone contacts the cell phone network (when you would normally hear those beeping/electronic noises on headphones or speakers), the volume on the iPod next to it jumps all around.

My computer scientist boyfriend doesn't believe me, but I can now sort-of consistently reproduce the effect by switching the phone into airplane mode and then back, so it has to re-contact the cell phone network.  

They seem to need to be pretty close together, and nothing else on the iPod seems to be affected (it only changes volume, the track doesn't seem to jump around or turn on/off).

What would cause this interaction between the cell phone and the iPod?

Thank you and great work!

What do you think?


Offline surreyscientist

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Why does my cell phone change the volume on my iPod?
« Reply #1 on: 19/01/2009 00:19:24 »
This is something called electromagnetic interference.

Basically (i think) the waves sent to/from your phone to the network cause interference i your ipod (the wires, for want of a better word, inside it act as little antenna). This then induces currents in your ipod which alters the volume. (Also why your hear the beepy noise just before you get a text and your near a radio).


Offline erickejah

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Why does my cell phone change the volume on my iPod?
« Reply #2 on: 20/01/2009 22:27:05 »
Depends in what mode your cellphone is, if you have it in vibration the electromagnetic field generated by the vibrator coil may be the cause.

The headphones have a coil

The vibrator also have a coil

Both parts generate inductance. If they are close there is a mutual inductance, just like transformers.
That is my theory. [:)]


Offline techmind

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Why does my cell phone change the volume on my iPod?
« Reply #3 on: 23/01/2009 00:27:13 »
A mobile-telephone is a radio transmitter, and at close range (less than 30-40cm) the radiofrequency field emitted is quite strong, "concentrated" if you like.
This induces electric currents in other nearby circuits (which is why you often hear beeping noises if you operate the phone near a radio or hifi, or even a wired landline phone).
If I operate my phone very close to my radio-controlled clock, it makes the clock reset to 00:00 01 Jan 1997.
If I operate my phone within 30cm of my electronic kitchen scales then the weight reading jumps about all over the place.
When you know about electronics and physics, such behaviour isn't that surprising.

Basically, at close range the radiofrequency field (or transmissions) from the phone handset are far stronger than the other piece of electronics is designed and tested to be immune to. Of course more recent electronics from the better manufacturers designed since mobile phones became ubiquitous is usually made more robust - but it can still be an issue. There's no legal requirement for them to be immune to RF radiation of that strength.

These interference issues (EMC - electromagnetic compatibility) are examples of why it might not be advisable to have a phone switched on in close proximity to medical or life-critical equipment. Such equipment ought to be designed to high immunity standards, but this might not always be guaranteed, especially on older designs.
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"