Category: 3. Sound Generation and Propagation

The sound absorption coefficient α of sound‐absorbing materials (curtains, drapes, carpets, clothes, fiberglass, acoustical foams, etc.), is defined as (3.73) Note that α also depends on the angle of incidence. The absorption coefficient of materials depends on frequency as well. Thicker materials absorb more sound energy (particularly important at low frequency). See Figure 3.19. If all the sound energy is…

In a confined space there will be reflections, and far from the source the reflections will dominate. We call this reflection‐dominated region the reverberant field. The region where reflections are unimportant and where a doubling of distance results in a sound pressure drop of 6 dB is called the free or direct field (see Figure 3.18).

Near to a source, we call the sound field, the near acoustic field. Far from the source, we call the field the far acoustic field. The extent of the near field depends on: In the near field of a source, the sound pressure and particle velocity tend to be very nearly out of phase (≈90°). In the far field, the sound pressure…

In enclosed spaces the wave acoustics approach is useful, particularly if the enclosed volume is small and simple in shape and the boundary conditions are well defined. In the case of rigid walls of simple geometry, the wave equation is used, and after the applicable boundary conditions are applied, the solutions for the natural (eigen)…

There are three main modeling approaches in acoustics, which may be termed wave acoustics, ray acoustics, and energy acoustics. So far in this chapter we have mostly used the wave acoustics approach in which the acoustical quantities are completely defined as functions of space and time. This approach is practical in certain cases where the…

For a homogeneous plane sound wave at normal incidence on a fluid medium of different characteristic impedance ρc, both reflected and transmitted waves are formed (see Figure 3.13). From energy considerations (provided no losses occur at the boundary) the sum of the reflected intensity Ir and transmitted intensity It equals the incident intensity Ii: (3.64) and dividing throughout by Ii, (3.65) where R is the energy…

Sometimes noise sources are distributed more like idealized line sources. Examples include the sound radiated from a long pipe containing fluid flow or the sound radiated by a stream of vehicles on a highway. If sound sources are distributed continuously along a straight line and the sources are radiating sound independently, so that the sound power/unit…

The directivity index DI is just a logarithmic version of the directivity factor Q. It is expressed in decibels. A directivity index DIθ,ϕ may be defined, where (3.59) (3.60) Note if the source power remains the same when it is put on a hard rigid infinite surface Q(θ, ϕ) = 2 and DI(θ, ϕ) = 3 dB. EXAMPLE 3.11 SOLUTION

In general, a directivity factor Qθ,ϕ may be defined as the ratio of the radial intensity 〈Iθ, ϕ〉t (at angles θ and ϕ and distance r from the source) to the radial intensity 〈Is〉t at the same distance r radiated from an omnidirectional source of the same total sound power (Figure 3.12). Thus (3.53) For a directional source, the mean square sound pressure measured at distance r and angles θ and ϕ is p2rms (θ,ϕ). In…

The sound intensity radiated by a dipole is seen to depend on cos2 θ (see Figure 3.11). Most real sources of sound become directional at high frequency, although some are almost omnidirectional at low frequency. This phenomenon depends on the source dimension, d, which must be small in size compared with a wavelength λ, so d/λ ≪ 1 for them to behave almost…