'A' Level Medical Option Questions - The Ear
(a) The threshold of hearing is quoted as 1.0 × 10–l2W m–2. Explain what is meant by the threshold of hearing and state the frequency at which the threshold has this value.
The threshold of haring is the lowest level of sound (the minimum intensity of sound) which a normal ear can detect when listening to a frequency of 1 kHz.
(b) Sound intensity levels are usually measured in decibels. Give two reasons why this logarithmic scale is used.
Any two points from:
- The ear has a logarithmic response OR the log scale is chosen to match the (perceived) response of the ear,
- It is needed to accommodate very wide range of sound intensities to which ear can respond
- The perceived change in loudness is proportional to fractional change in intensity
- Ten-fold increases in intensity are perceived as steps of equal increase in loudness
- The log scale means that numerical values on the scale represent ratios of two sounds, expressed as the log of that ratio
(c) Why was it necessary to introduce an adapted scale referred to as the dBA scale, which is used on some sound level matters?
The dBA scale takes account of the frequency dependence of the sensitivity of the ear
OR to match the ear's frequency response or meters calibrated on a dBA scale to give frequency-weighted readings
(d) Modern hi-fi equipment and televisions often have volume controls which allow the sound volume to be increased in steps. If each of these steps produces an increase in the sound intensity level of 2.0 dB, calculate
(i) the ratio by which the sound intensity is increased for each step up in volume,
2.0 = 10 x log10 (I2/I1)
(I2/I1) = 1.58
(ii) the ratio by which the sound intensity is increased for a total of 10 identical steps up in the volume.
10 steps = 10 × 2dB = 20dB
(I2/I1) = 100
1.5810 = 100
(Total 9 marks)