an audio signal is not transmitted directly by converting it into an electromagnetic wave of same frequency.give 2 reasons.

 First reason is dat if we send directly then they will not able to propagate for long distance as they have very less energy(hv).and second reason may be that e.m.w of high frequency pass on with other getting interfered with others but if it is of low frequency then they would interfere each other..And the height of antenna is= lambda/4 which would come approx 75 km which is impossible to built..

and also power radiated =1/lambda^2  thus power will b  more required.

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1-Audio signals converted directly into an em wave would have high wavelength~20000m. Hence length of antenna required for its propagation will be around 5000m [ by L=λ/4 ] which is practically impossible. 2-Effective power radiated by the antenna would be very low.[ By Power= (1/λ)² ] Hence, Before its propagation the wave is first modulated. Hope it helps!😊
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