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'A' Level Medical Option Questions - The Ear

Q8.

(a)

(i) State the main difference between the dB scale and the adapted dBA scale used to measure sound intensity levels.

The dBA scale is frequency dependent whereas the dB scale is not. 

(ii) A variable frequency sound source produces sound of equal intensity at all frequencies.

Two sound meters are placed equidistant from the source.

  • One meter is switched to the dB scale.
  • The other meter is switched to the dBA scale.

On the axes below sketch the response of the two sound meters as the frequency varies from 100 Hz to 10 000 Hz.

Label each curve dB or dBA.

dB graph: flat - same response at all frequencies 
dBA graph: correct general shape 
             most sensitive at 3 kHz 
             slightly higher than dB curve at 3 kHz 
             crosses dB line at 1 kHz 

(5 marks)

(b) A sound of intensity level 85 dB is incident on a human ear. The cross-sectional area of the ear canal is 65 × 10–6 m2. Calculate the power incident on the ear-drum. threshold intensity level, I0 = 1.0 × 10–12 W m–2

I0 = 1.0 × 10–12 W m–2

intensity level = 10 log10(I/Io)

85 = 10 x log10 (I/(1.0 x 10-12))

I = 108.5 x 1.0 x 10-12

I = 3.16 x 10-4 W m–2

P = IA = 3.16 x 10-4 x 65 × 10–6 W

= 2.1 x 10-8 W

(3 marks)

(Total 8 marks)