(i) Energy carried by low frequency audio waves (20 Hz to 20000 Hz) is very small.

(ii) For efficient radiation and reception of signal, the transmitting and receiving antennas should be very high approximately 5000 m.

(iii) The frequency range of audio signal is to small that overlapping of signals create a confusion.

`text(Types of Modulation :)`

(a) Amplitude Modulation
In this type of modulation, the amplitude of high frequency carrier wave is varied in accordance to instantaneous amplitude of modulating signal.

Band width required for amplitude modulation = twice the frequency of the modulating signal.

(b) Frequency Modulation
In this type of modulation, the frequency of high frequency carrier wave is varied in accordance to instantaneous frequency of modulating signal.

(c) Pulse Modulation
In this type of modulation, the continuous waveforms are sampled at regular intervals. Information is transmitted only at the sampling times.