2.3.4 Relationship between SPL and SWL
For a given set of conditions, sound power and sound intensity can be defined in terms
of sound pressure, and vice versa.
Sound intensity = I = P2/ρV (2.8)
where P = rms sound pressure (Pa), ρ = density of air at standard conditions 1.2 kg/m3,
I = intensity, V = speed of sound in air, 344 m/sec.
Equation 2.8 can be represented in terms of pressure as follows:
Sound pressure = P = (IρV )1=2 (2.9)
Again Equation 2.8 can be described in terms of intensity.
Sound power = W = IA (2.10)
Using the above equation, the additional relationships exist between sound pressure
level and sound power level as:
Lw = Lp + 10logA (2.11)
A is defined as the surface area of an imaginary shell at distance, r, where Lp would be
the measured sound pressure level for any point on the shell