With reference to interference, explain why the intensity of sound alternates along line AB.

The sound has a maximum intensity at P. Calculate the distance along line AB to the next intensity maximum when S1 and S2 emit a musical note of frequency 2.5 kHz.

S1 and S2 are moved so that they are now 3.0 m apart. They remain at the same distance from line AB. Discuss the changes, if any, in the rate at which the intensity of sound alternates when a person is walking along line AB at half the speed.

the waves are coherent so interference occurs;

high intensity sound corresponds to a position where sound constructively interferes/superposes / low/zero intensity sound corresponds to a position where sound destructively interferes/cancels;

high intensity is where the path difference is an integral number of wavelengths;

low/zero intensity of sound is where the path difference is $n + \frac{1}{2}$ wavelengths;

$\lambda  = \frac{{330}}{{2500}} = 0.132{\text{ m}}$;

$x = \left( {\frac{{n\lambda D}}{d} = \frac{{1.0 \times 0.132 \times 8.0}}{{1.5}} = } \right){\text{ }}0.70(4){\text{ m}}$;

distance doubles so fringe width is halved;

so fringes are encountered at the same rate, no change;

Most described the interference in (a), without giving a cause, such as path difference.

Many answered (b) and (c) well.

Many answered (b) and (c) well.