Why do speakers make pulsing noise

Any electrical circuit emits two types of electromagnetic waves viz. radiative emission and conductive emission.

Radiative waves can travel in unguided medium and are generated due to the antenna effect of the circuit.

Conductive waves need a guided channel to propagate.

In the similar way, any electrical circuit absorbs these two types of waves.

The audio speakers are designed to respond to any signal in a typical range of 20Hz – 20KHz, which is same as the audible range of human ears. 

The carrier frequency of the mobile signals is hundreds of Mhz (eg. 800-900MHz for GSM phones).

However, when a cell-phone receives or sends a phone call or an SMS, it transmits and receives tokens in a fixed time interval with the base stations.

This time interval corresponds to about 4 KHz.

The connecting wires between the amplifier and the speakers act as a reception antenna.

The 4Khz radiative signals from the mobile phones are absorbed by these connecting wires, and thus we hear a pulsing noise.

Cure: The absorption of radiative emissions can be reduced by using electromagnetic shields for the entire length of the cable wires connecting the amplifier and the speakers.

Aluminium foil wrapped around the connecting wires can act as a shield. Alternatively, one can reduce the length of the connecting wires to the speakers, thus reducing the antenna length. One can also use wireless speakers, which eliminate the connecting wires and hence the impact of mobile emission is very minimal.