cell phone signal vs speakers

I’ve noticed that a couple seconds before my cell phone rings or gets a text or voice mail notification, every time there is a series of clicking noises that come from my computer speakers that are similar to the RKO radio tower telegraph sound effect or a very short burst of morse code. It happens occasionally without my cell phone doing anything, but I assume some other cell phone in the area is being sent signals as it always happens before mine goes off in some way. What’s the origin of this and why does it affect the speakers?

It’s interference caused by your cellphone talking to the tower to negotiate the details of an incoming call. You can get higher quality speakers with RF shielding, or just move your phone further away from the speakers. Or turn your speakers or phone off.

I’m kinda simplifying and glossing over some details here, but basically, the cell phone tower and your cell phone periodically talk to each other. If nothing else, the tower is basically saying “yo, you still there?” and the phone is going “yup, I’m here”. Before a phone call or a text message, the same sort of thing happens. The tower says “still there?” and the phone says “here I am”, then the tower says “I’ve got a call for you, ready?” and the phone says “ok, send me the call information”. Then the tower sends the info, and your phone rings.

Cell phones work on radio waves, and these radio waves can easily get coupled into other devices. One of the first things you learn in electrical engineering school is how easy it is to make a radio receiver. All you need is something conductive to act like the antenna. Then you need something that conducts better in one direction than the other. And finally, you need some filtering to pick out a particular frequency. The next thing you learn in engineering school is that pretty much describes almost every electronic device in existence. We electrical engineers spend quite a bit of our time making sure that the things we make aren’t unintentional radio receivers.

Your speakers have everything they need to be a radio receiver. They have bits of wire, and metal traces on circuit boards, all of which act like antennas. They have amplifiers, which not only contain transistors (which by their nature conduct better in one direction than the other) but also amplifies the sound. This also means that they amplify any electrical noise that gets picked up from some nearby noise source, like a cell phone. Cheaper computer speakers also have very little in the way of shielding to keep out radio noise. All of this combines to make your computer speakers very good at chirping whenever the cell phone talks to the tower.

Your speakers aren’t the only things that will do this. You can often get chirps on your television if the phone is placed near the side that has the circuit board for the sound. Cell phones can cause interference with all sorts of things. That’s why they are banned on planes and in hospitals. They cause chirps in the pilot’s earphones when he’s trying to talk to the tower. Some equipment is more sensitive than others, but they can cause false readings on hospital diagnostic equipment.

Cell phones chirp because you are picking up digital communication packets, which are just sequences of bits. Sometimes you can also pick up walkie talkies and CB radios, which you will likely be able to hear the voice. Some folks have thought their computer was haunted because a voice came out of it for no reason. It usually turns out to be a nearby CB radio or a HAM radio operator.

Your speakers will chirp when your phone sets up a call, but as I mentioned above that’s not the only time that the cell phone communicates with the tower. If you listen carefully, you’ll hear it chirp periodically even when there isn’t a call.

Interesting fact: CDMA phones don’t do this.

I need a cite for this. The Ask the Pilot guy doesn’t mention it, and I’ve never heard this from any other pilot.

I don’t dispute that cell phones could theoretically cause interference, but given the amount of EM generated by, around, in, and towards aircraft, I just can’t see a cell phone doing much, especially to the radio system, which is VHF-AM and clear down at 120mhzish compared to 800+mhz cell phones.

Here are a couple of pilots complaining about it on the mythbusters message board:

From here:

From here:

I’m a pilot and when people leave their cell phone on, the interference through the headset is very annoying, particularly on approach to landing when we are very busy and trying to concentrate.

What I did: Spend a little more on those speakers and get shielded ones. You’ll never notice it again.