Question

what is the longest wavelength of audio signals to be transmitted in communication system

Answer 1

am not an expert in telecommunication but I will try to answer the question using a simple and illustrative example. The concept of modulation (at the transmitter) and demodulation (at receiving side) is the backbone of a communication system

If you want to throw a piece of paper at some distance, say 50 m then what will you do? A very simple solution is that you wrap the paper on the stone and then throw it right. So that is the philosophy of modulation. You modulate the carrier signal (stone) with your information signal (paper) and then throw it using a transmitter so that your information can travel to a longer distance. At the receiver side of the communication system the demodulater separates the information signal from the carrier and then your information signal is processed.