Describing a Sound Wave: Quick Revision

A brief note on Describing a Sound Wave. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Wavelength $\lambda$: compression to compression, in metres.
• Frequency $f$: waves each second, in hertz. Time period $T = \dfrac{1}{f}$.
• Amplitude: the size of the pressure change.
• The wave equation: $v = f\lambda$. The medium sets $v$; the source sets $f$.
• Loudness goes with the amplitude, pitch with the frequency, quality with the mixture of frequencies.

The facts at a glance

A worked example

A note of $480\ \text{Hz}$ goes from air ($340\ \text{m/s}$) into water ($1500\ \text{m/s}$). Find its time period and its wavelength in each.

1. Time period. $T = \dfrac{1}{480\ \text{Hz}} = 0.00208\ \text{s}$, about $2.1\ \text{ms}$, in both.
2. In air. $\lambda = \dfrac{340\ \text{m/s}}{480\ \text{Hz}} = 0.708\ \text{m}$.
3. In water. $\lambda = \dfrac{1500\ \text{m/s}}{480\ \text{Hz}} = 3.125\ \text{m}$.

Practice with new numbers

A program to try

The program finds the time period and the wavelengths of a note.

Mistakes to avoid

• 'A louder sound is higher.' Loudness is amplitude; pitch is frequency.
• 'High notes travel faster.' In the same air every sound has the same speed.
• Changing the frequency when the medium changes. Only the speed and the wavelength change.
• Using milliseconds in $f = \dfrac{1}{T}$. Change to seconds first.

Check yourself

1. A sound has a time period of $0.008\ \text{s}$. What is its frequency?

2. What happens to the wavelength of a sound in air if its frequency is tripled?

3. Which property decides how loud a sound is?

4. Why do a piano and a guitar playing the same note sound different?