Which of the following statements are correct regarding sound in environmental engineering ? 1. Sound Intensity can be defined as the time-weighted average sound power per unit area normal to the direction of propagation of the sound wave. 2. Sound Intensity can be defined as the time-weighted average sound power per unit area parallel to the direction of propagation of the sound wave. 3. Sound Pressure can be defined as the force on a unit surface area perpendicular to the direction of the sound. 4. Sound Pressure can be defined as the force on a unit surface area parallel to the direction of the sound. Select the correct answer using the code given below :
- A1 and 4
- B1 and 3Correct
- C2 and 4
- D2 and 3
Solution
Concept
Definitions of sound intensity and sound pressure
Formula
I = p² / (ρ c)
Step-by-step solution
1. Sound intensity: average sound power per unit area normal (perpendicular) to the direction of propagation (statement 1). 2. Sound pressure: force per unit area acting on a surface perpendicular to the direction of the sound (statement 3). 3. So statements 1 and 3 are correct.
Explanation
Both quantities are defined on a surface perpendicular to the direction in which the wave travels, since only the normal component of the wave motion produces a net flow of energy or a pressure force on the surface.
Why the other options are wrong
Statements 2 and 4 say "parallel", which is wrong: no energy flows across a surface parallel to the direction of propagation.
Shortcut
Intensity and pressure are measured on a surface facing the sound (normal to propagation).
Common mistake
Choosing "parallel".
Correct answer: B) 1 and 3