'A' Level Medical Option Questions - The Ear
(a) A sound source of constant output power is used to generate a sound which is measured
using a sound meter. When set to the dB scale, the sound meter displayed 60 dB as the
reading when the frequency of the sound was 1 kHz.
(i) State and explain what the reading would be for a sound of frequency 1 kHz if the meter
was changed to the dBA scale.
Reading would be 60dBA as 1kHz is the reference frequency (at the threshold of hearing).
(ii) State and explain what would happen to the reading on each scale if the frequency of
the sound was changed to 500 Hz.
dB reading would be 60dB as power is the same/not frequency dependent.
dBA reading would be less than 60dB as 500Hz has a higher threshold intensity / ear is less sensitive than at 1000Hz.
(b) A drill is operated in an otherwise silent room. The drill produces sound of power
2.0 W which is given out equally in all directions. A sound meter is placed 5.0 m from
the drill and is set to the dB scale.
Calculate the reading on the sound meter.
I0 = 1.0 × 10–12 Wm–2
Intensity at meter = 2/(4πx5x5) =6.37 x 10-3 Wm–2
Intensity reading = 10 log((2/(4xπx5x5))/1.0 x10-12)
Intensity reading = 98 dB
answer = 98 dB
(Total 6 marks)