Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 88

If the sound pressures p₁ and p₂ at a point produced by two independent sources are combined, the mean square pressure is

(3.25)

where 〈〉_t and the overbar indicate the time average .

Except for some special cases, such as two pure tones of the same frequency or the sounds from two correlated sound sources, the cross term 2〈p₁ p₂〉_t disappears if T → ∞. Then in such cases the mean square sound pressures and are additive, and the total mean square sound pressure at some point in space, if they are completely independent noise sources, may be determined using Eq. (3.26).

(3.26)

Let the two mean square pressure contributions to the total noise be p²_rms1 and p²_rms2 corresponding to sound pressure levels L_p1 and L_p2, where L_p2 = L_p1 − Δ. The total sound pressure level is given by the sum of the individual contributions in the case of uncorrelated sources, and the total sound pressure level is given by forming the total sound pressure level by taking logarithms of Eq. (3.26)

(3.27)

where L_pt is the combined sound pressure level due to both sources, L_p1 is the greater of the two sound pressure level contributions, and Δ is the difference between the two contributions, all in dB. Equation (3.27) is presented in Figure 3.6.