Which of the following statements is INCORRECT?
- ASound level L = Log₁₀(Q / Q₀) Bels, Where Q = sound intensity, Q₀ = reference sound intensity
- BMeasurements of the sound pressure on logarithmic scales are called levels.
- CThe sound pressure is expressed on a logarithmic scale.
- DThe sound pressure of the faintest sound that can be heard by a normal healthy individual is about 10 micropascals.Correct
Solution
Concept
The threshold of hearing for a normal healthy person is a sound pressure of about 20 micropascals, not 10.
Formula
L = log₁₀(Q/Q₀) Bels; L = 10 log₁₀(Q/Q₀) dB; Lp = 20 log₁₀(P/P₀) dB, P₀ = 20 μPa
Step-by-step solution
1. Check A: Sound level in Bels = log₁₀(Q/Q₀). This is correct. 2. Check B: Sound pressure measured on a log scale is called a 'level'. This is correct. 3. Check C: Sound pressure has a very wide range, so it is expressed on a log scale. This is correct. 4. Check D: The faintest audible sound has a pressure of about 20 μPa (2 × 10-5 Pa), not 10 μPa. This is wrong. 5. So the INCORRECT statement is D.
Explanation
The reference sound pressure (threshold of hearing) is 20 micropascals. This value is used as P₀ in the formula for sound pressure level in dB. Statement D gives 10 micropascals, so it is incorrect.
Why the other options are wrong
A: Correct definition of sound level in Bels. B: Correct, log-scale measurements are called levels. C: Correct, sound pressure is expressed on a log (dB) scale.
Shortcut
Remember: threshold of hearing = 20 μPa = 0 dB.
Common mistake
Thinking statement A is wrong because it has no factor 10. The factor 10 is needed only for decibels, not Bels.
Correct answer: D) The sound pressure of the faintest sound that can be heard by a normal healthy individual is about 10 micropascals.