We know that υ∝√T
where υ= frequency, T = tension
The decrease in the tension of a string decreases its frequency. So let us assume that original frequency uA of A is more than the frequency υB of B.
Thus υA−υB=±6 Hz (given)
andυA=324 Hz
∴ 324−υB=±6
or υB=324±6=318 Hz or 330 Hz.
On reducing tension of A, Δυ=3 Hz
If υB=330 Hz and on decreasing tension in A, υA will be reduced i.e. no. of beats will increase, but this is not so becauseno. of beats becomes 3.
∴ υB must be 318 Hz because on reducing the tension in string A, its frequency may be reduced to 321 Hz, thus giving 3 beats with υB=318 Hz.