Question

A sound point source, situated 1.8 m above a brick floor, is emitting sound at 5 ✕ 10⁻⁵ watts.

(a)

Find the sound intensity level (in dB) due to that source at a listening point 3.2 m above the floor and 14 m away. (Assume that 14 m is the horizontal distance between the sound point source and the listening point. Do not take reflection from the floor into account.)

dB

(b)

Find the sound intensity level (in dB) at that point when the reflection from the floor is taken into account. (Assume 100% reflection.)

dB

Answer 1

The distance between the source and the point into consideration is:

since the source is a point, the power will be distributed over the surface area of a sphere.

So, intensity will be:

convert this to dB

b] If a reflection occurs, effectively there will be another point source at the ground.

The distance of this point source from the origin can be found using the fact that angle of incidence = angle of reflection

=> x = 5.04 m

so, distance from this source to point P is:

D = [(14 - 5.04)² + (3.2)²]^1/2 = 9.514 m

the total intensity at P will then be the sum of this and the initial intensity due to the real point source. Use this to determine the intensity in dB.