194. A sound source moving with speed $\dfrac{1}{n}$ of the speed of sound approaches a stationary listener, and continues along the same path at the same speed. If in the first case the frequency heard by the listener is $\Delta f$ and in the second case the frequency of the source decreases, and the frequency heard by the listener is $\Delta f'$, the ratio $\dfrac{\Delta f}{\Delta f'}$ is:
- [(1)] $1$ (2) $\dfrac{n-1}{n}$ (3) $\dfrac{n+1}{n-1}$ (4) $\dfrac{1}{n+1}$
%% Page 18
Physics 120-C Page 17