Normal conversation has a sound level of about 60 db. How many times more intense must a 100-hz sound be compared to a 1000-hz sound to be perceived as equal to 60 phons of loudness?

5.65 times

Explanation:

60 db sound is equal to 60 phons sound when frequency is kept at 1000Hz.

But when the frequency of sound is changed to 100 Hz, according to equal loudness curves, the loudness level on phon scale will be 35 phons.

A decrease of 10 phon on phon - scale makes sound 2 times less loud

Therefore a decrease of 25 phons will make loudness less intense by a factor equal to 2²°⁵ or 5.65 less intense. Therefore intensity at 100 Hz

must be increased 5.65 times so that its intensity matches intensity of 60 dB sound at 1000 Hz frequency.