Question

A source of sound is directed along a straight line perpendicular to a large brick wall. A girl walks along the line towards the wall and notices that the intensity of sound decreases to a minimum every 50.0 cm. What is the frequency of the sound? (air temp is 20oC)

Answer 1

686 Hz

Step-by-step explanation:

Here we must remember the following relationship.

`v=\lambda f`

where v = speed of sound, λ = wavelength, and f = frequency.

Now in our case, we know that the speed of sound at 20°C is 343 m/s.

Furthermore, we also know that sound intensity decreases to a minimum every 50.0 cm; this means that the wavelength of the sound wave must be 50 cm. Why? because the wavelength is defined as the distance between two minima or two maxima.

Now, since we know that

v = 343 m/s and λ = 50.0 cm = 0.50 m, our formula gives

`\begin{gathered} v=\lambda f \\ \Rightarrow343m/s=(0.50m)f \end{gathered}`

Dividing both sides by 0.50 m gives

`f=(343\; m/s)/(0.50\; m)`
`f=686/s`
`\boxed{f=686\; Hz\text{.}}`

Hence, the frequency of sound is 686 Hz.