What would happen to the loudness of sound with increase in its frequency?

This is a difficult question to answer.

The "loudness" of a sound is measured in decibels. Every time the number of decibels is increased by ten the amount of energy the sound waves carry doubles and therefore seems twice as loud. The "loudness" of a sound is not effected by the frequency, but by the decibels. However, some frequencies (mostly at the very limits of human hearing) will seam quieter. Also many bass frequencies seem louder because (of the audible spectrum of sound) they carry the energy the most efficiently through the air around us.

A2 The loudness of a sound is a subjective phenomenon - to a human, we have a poor sensitivity to low frequemcy sounds, rising in the range of a few kilohertz, and again declining at frequencies much over 15 kilohertz. Additionally, the advertising industry for example takes advantage of the fact that we consider an advert louder if the signal is adjusted such that the sound spends much of its time at a higher level, but without the peak signals being exceded. This is done by a sophisticated form of signal clipping.
And in the audio equipment industry, 'music power' is often quoted as the equipments output, rather than the more academic 'RMS' (Root Mean Square) of the signal.

So, loudness is a subjective idea.