WAEC Physics 2013 Paper

Concept:A pipe closed at one end allows only odd harmonics because of its boundary conditions.Explanation:In a closed pipe, the closed end is a displacement node and the open end is a displacement antinode.This condition means the length of the pipe equals an odd multiple of $\frac{\lambda}{4}$.So the possible frequencies are $f_1, 3f_1, 5f_1$, etc.Hence, only odd harmonics are produced in closed pipes.Answer:C. only odd harmonics are possible in closed pipes