'A' Level Medical Option Questions - The Ear
(a) State two reasons why the logarithmic dB scale is used to compare sounds of different intensities.
The ear has logarithmic response as this accommodates wide range of intensities
(b) Another scale used to compare sounds of different intensities is the dBA scale. What are the main differences between the dBA and the dB scales?
The dB scale has a flat response with frequency whereas the dBA scale is frequency compensated. this measn that for dBA, threshold intensities are different for different frequencies,
(c) A reading of 94 dB is obtained on a sound meter placed near a drill. Calculate the intensity of the sound incident on the meter.
Reference threshold intensity I0 = 1.0 × 10–12 W m–2
Intensity level =10 log (I/Io)
94 = 10 log (I/(1.0 x 10-12))
log-1(9.4) = I/(1.0 x 10-12) = 109.4
I = 1.0 × 10–12 × 109.4
I = 2.5 × 10–3 W m–2
(d) An identical drill is now placed next to the first drill and both are switched on. Calculate the new reading on the sound meter.
New intensity = twice that calculated in part (ii)
I = 2 × 2.5 × 10–3
I = 5.0 × 10–3 W m–2
New intensity level = 10 × log (I/Io)
= 10 x log ( 5.0 × 10–3/ 1.0 × 10–12)
= 10 x log (5.0 x 109)
= 97 dB
(Total 10 marks)