Electronic – GSM module noise

The interference comes from GSM being a time division technology and the transmitter being switched on and off quickly. Poorly designed amplifiers can unintentionally receive this signal and convert it to an audible signal. The problem could also be in the signal source.

The principle behind ferrite beads removing the interference is that they would create a high impedance for the RF signal to get to the audio amplifier. The easy way to find out if your problem is with mains (or the amplifier internals in the speaker) or audio signal cables is to disconnect the audio input. If you still get the noise with only mains connected and speakers powered, the problem cannot be fixed with ferrites on the audio cable.

I won't repeat the use information from the wikipedia page so take a look at that if you didn't.

To purposefully induce the noise for testing your filtering, you can for example call yourself - it will signal the network even though the call will not likely be connected. Try moving the phone close to the audio signal cable, the speaker itself and the signal source to find out where your set up is most vulnerable.

The buzzing is AM detected signal.

The reason of audio amplifiers being hit by GSM signal is that contemporary audio semiconductor parts are actually very functional up to high GHz range. For GSM-800-900MHz range any 80mm copper trace works like 1/4 wave antenna, or stripline resonator. The signal is AM detected on any non-linearity (transistors or diode structures in chips) on multiple points of amplifier simultaneously, also including power regulator chips and so on.

It is translated into audio range as tiny but very sharp and periodic dips or pops of averaged conductivity of non-linear parts (AM detection), which are DC powered.

Think of low speed oscilloscope trace showing straight line with beads of UHF flashes. Simple sharp spikes of consumed DC current will become audible with amplifier.