Question

If a sound wave emitted by a stationary source of frequency 680 Hz travels towards a stationary observer 150 m away, then the number of waves between the source and observer are (speed of sound in air = 340 m s\(^{-1}\))

• A. 300
• B. 150
• C. 75
• D. 450

Hint:

For wave problems: Number of waves = Distance/Wavelength. Remember wavelength $\lambda = v/f$ where $v$ is wave speed and $f$ is frequency.

Answer 1

The Correct Option is A

(A) First calculate the wavelength ($\lambda$) using the wave equation: \[ v = f\lambda \implies \lambda = \frac{v}{f} = \frac{340}{680} = 0.5\,{m} \] (B) Then determine the number of waves ($N$) in 150 m distance: \[ N = \frac{{Distance}}{{Wavelength}} = \frac{150}{0.5} = 300 \]