Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 131

If we are situated in the reverberant field, we may show from Eq. (3.78) that the noise level reduction, ΔL, achieved by increasing the sound absorption is

(3.81)

(3.82)

Then A = S is sometimes known as the absorption area, m² (sabins). This may be assumed to be the area of perfect absorbing material, m² (like the area of a perfect open window that absorbs 100% of the sound energy falling on it). If we consider the sound field in a room with a uniform energy density ε created by a sound source that is suddenly stopped, then the sound pressure level in the room will decrease.

By considering the sound energy radiated into a room by a directional broadband noise source of sound power W, we may sum together the mean squares of the sound pressure contributions caused by the direct and reverberant fields and after taking logarithms obtain the sound pressure level in the room:

(3.83)

where Q_θ,ϕ is the directivity factor of the source (see Section 3.9) and R is the so‐called room constant:

(3.84)

A plot of the sound pressure level against distance from the source is given for various room constants in Figure 3.23. It is seen that there are several different regions. The near and far fields depend on the type of source [21] and the free field and reverberant field. The free field is the region where the direct term Q_θ,ϕ /4πr² dominates, and the reverberant field is the region where the reverberant term 4/R in Eq. (3.83) dominates. The so‐called critical distance r_c = (Q_θ,ϕ R/16π)^1/2 occurs where the two terms are equal.