Fig. 1 shows a sound level meter beside a festival loudspeaker. The meter is positioned 8.0 m from the speaker and reads 94 dB. An organiser moves the meter...
Question 1 Report
Fig. 1 shows a sound level meter beside a festival loudspeaker. The meter is positioned 8.0 m from the speaker and reads 94 dB. An organiser moves the meter to 16.0 m away while the speaker output remains constant. The organiser expects the sound intensity to decrease because the same sound energy spreads over a larger area. A worker wears ear defenders while adjusting a cable behind the stage. The figure shows circular wavefronts spreading from the speaker, although real reflections from buildings are ignored.
(a) What does dB measure in this situation? [1]
(b) Describe what happens to sound intensity as distance from the speaker increases. [1]
(c) Explain why the intensity decreases. [2]
(d) Give one reason for the worker to wear ear defenders. [1]
The diagram shows a hiking group using an echo to estimate the distance to a rock face. One hiker makes a sharp clap at point A. Another hiker measures 1.50 s between the clap and the returning sound. The air temperature is such that the velocity of sound is 330 m/s. The group is well away from the rock face and hears only one clear echo. Their guide explains that the sound wave is reflected from the hard rock surface. The diagram labels the outgoing and returning paths.
(a) What is the total distance travelled by the sound? [2]
(b) Complete the calculation to find the distance from A to the rock face. [1]
(c) Explain why the guide uses a sharp clap rather than a long shout. [2]
Answer Details
Festival loudspeaker
(a) dB measures sound level, also called sound intensity level, in this situation. [1]
(b) Sound intensity decreases as distance from the speaker increases. [1]
(c) The sound energy spreads over a larger area as the wavefronts move farther from the speaker. [1] Therefore less energy passes through each square metre each second, so the intensity is lower. [1]
(d) Ear defenders reduce the sound reaching the worker's ears and so reduce the risk of hearing damage. [1]
Echo from the rock face
(a) The 1.50 s is the time for the complete outward-and-return journey:
\[\text{total distance}=330\ \text{m s}^{-1}\times1.50\ \text{s}=495\ \text{m}\]
The total distance travelled is 495 m. [2]
(b) The rock face is one half of the round-trip distance away:
\[\text{distance from A to rock face}=\frac{495\ \text{m}}{2}=247.5\ \text{m}\approx248\ \text{m}\]
[1]
(c) A sharp clap is a short pulse with a clear start and end. [1] The returning echo can therefore be timed separately from the original sound. [1] A long shout could overlap with its echo.
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