Category: 3. Sound Generation and Propagation

Electrical analogies have often been found useful in the modeling of acoustical systems. There are two alternatives. The sound pressure can be represented by voltage and the volume velocity by current, or alternatively the sound pressure is replaced by current and the volume velocity by voltage. Use of electrical analogies is discussed in chapter 11 of…

In cases where the geometry of the acoustical space is complicated and where the lumped‐element approach cannot be used, then it is necessary to use numerical approaches. In the late 1960s, with the advent of powerful computers, the acoustical finite element method (FEM) became feasible. In this approach, the fluid volume is divided into a number of…

3.19.1 Acoustical Lumped Elements When the wavelength of sound is large compared to physical dimensions of the acoustical system under consideration, then the lumped‐element approach is useful. In this approach it is assumed that the fluid mass, stiffness, and dissipation distributions can be “lumped” together to act at a point, significantly simplifying the analysis of…

Waveguides can occur naturally where sound waves are channeled by reflections at boundaries and by refraction. Even the ocean can sometimes be considered to be an acoustic waveguide that is bounded above by the air–sea interface and below by the ocean bottom (see chapter 31 in the Handbook of Acoustics [1]). Similar channeling effects are also sometimes…

Standing‐wave phenomena are observed in many situations in acoustics and the vibration of strings and elastic structures. Thus they are of interest with almost all musical instruments (both wind and stringed) (see Part XIV in the Encyclopedia of Acoustics [19]); in architectural spaces such as auditoria and reverberation rooms; in volumes such as automobile and aircraft cabins;…

The sound radiation from plates and cylinders in bending (flexural) vibration is discussed in Refs. [9, 27] and chapter 10 in the Handbook of Acoustics [1]. There are interesting phenomena observed with free‐bending waves. Unlike sound waves, these are dispersive and travel faster at higher frequency. The bending‐wave speed is cb = (ωκcl)1/2, where κ is the radius of gyration h/(12)1/2 for a rectangular…

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…

The critical distance rc (or sometimes called the reverberation radius) is defined as the distance from the sound source where the direct field and reverberant field contributions to p2rms are equal: (3.79) thus, (3.80) Figure 3.23 gives a plot of Eq. (3.78) (the so‐called room equation).

If we have a diffuse sound field (the same sound energy at any point in the room) and the field is also reverberant (the sound waves may come from any direction, with equal probability), then the sound intensity striking the wall of the room is found by integrating the plane wave intensity over all angles θ, 0 < θ < 90°. This involves a…

In a reverberant space, the reverberation time TR is normally defined to be the time for the sound pressure level to drop by 60 dB when the sound source is cut off (see Figure 3.20). Different reverberation times are desired for different types of spaces (see Figure 3.21). The Sabine formula is often used, TR = T60 (for 60 dB): where V is room volume (m3), c is the…