Beats: Two motors in a factory are running at slightly different rates. One runs at 825 rpm and the other at 786 rpm. You hear the sound intensity increase and then decrease periodically due to wave interference. How much time elapses between successive maxima of the sound intensity?
A) 1.5 s
B) 1.4 s
C) 1.7 s
D) 1.8 s

Question

Quizplus · Accepted Answer

The beat frequency is the difference in frequencies of the two sources, which is $825 \, \text{rpm} - 786 \, \text{rpm} = 39 \, \text{rpm}$. To convert this to beats per second, divide by 60 (since there are 60 seconds in a minute), giving $39 \, \text{rpm} / 60 = 0.65 \, \text{Hz}$. The time between successive maxima is the inverse of the beat frequency, so $1 / 0.65 \, \text{Hz} = 1.54 \, \text{s}$, which rounds to 1.5 s.

Beats: Two Motors in a Factory Are Running at Slightly