Playing with smartphone feedback
In this laboratory session, we will play around with some of the weird sounds you get when you use one phone to call another phone which is very nearby.
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This is a simple experiment, but you do need two mobile phones.
What do I need to prepare?
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Two smartphones
All you need for this experiment is two smartphones. Any smartphone should work for this, as long as it has a speaker.
Be careful!
Make sure that you have free calls, or enough minutes on your phone plan to call another phone, otherwise you might be charged!
Instructions
Position your phones next to each other
Position the two phones beside each other, so that they are very close other.
Call one phone with the other phone and create some feedback
Call one phone with the other phone and put both phones into speaker mode. You can now make noises, and listen to the different types of feedback that is created by the phones.
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You should hear a kind of echoing screeching noise, which is a distorted version of whichever sound you are making.
Products
Why does calling two phones near to each other create feedback?
When a sound is received by phone A, it is sent to the nearest cell-signal transmission mast at the speed of light. Once it arrives at this mast there is a brief delay while the signal is converted from microwaves to infrared light and sent through fibre optic cables to the receiving phone's nearest mast tower.
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Because there is a delay for the signals to be processed, they travel much slower than the speed of light, which means there is a noticeable delay between when we make the sound, and when the sound comes out of the 2nd phone.
Why do the sounds get distorted?
This effect is called the Larsen effect. The phones will trade delayed versions of the sound signal that we create which causes a feedback loop.
The original signal is received by the first phone and played out of the second phone. This delayed signal is then passed out of the loudspeaker of the second phone, where it is picked up by the first phone again. The delayed sound from the first phone is then received by the microphone again, amplified further, and passed out through the 2nd phone again and so on and so on.
What happens if we call via the internet instead of via cell signal?
If you do the same experiment but using Skype or some other internet calling program?
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You'll find that since the delay is longer, the echoing and feedback effect does not build up as strongly. Internet calling sites usually also have significant sound processing steps in order to compress and remove sound from call data, so depending on the program, you might not get any static at all!
