Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 106

From Eq. (3.41) (for an omnidirectional source). Then W = 1.26 × 10⁻³ watts (the same result as Example 3.8).

Sound intensity measurements do and should give the same result as sound pressure measurements made in a free field.

Far away from omnidirectional sound sources, provided there is no background noise and reflections can be ignored:

(3.48)

(3.49)

and by taking 10 log throughout this equation

(3.50)

where L_p is the sound pressure level, L_W is the source sound power level, and r is the distance, in metres, from the source center. (Note we have assumed here that ρc = 415 ≅400 rayls.) If ρc ≅ 400 rayls (kg/m²s), then since I = p²_rms/ρc

So,

(3.51)