Sound, physics acoustic waves

The phenomenon sound comes about by periodic pressure waves, which are called acoustic or sonic waves. The term acoustic is sometimes reserved for vibrations that are in the audible range of frequencies, nominally from 20 to 20,000 Flz. Fligher frequencies are referred to as ultra-sonic and lower frequencies as infra-sonic. In the physics of sound and acoustics they play a similar role as the electromagnetic waves in the field of light and optics. Acoustics were unified with mechanics during the development of theoretical mechanics, in the same way as optics were unified with electromagnetism by the famous theory of Maxwell in the nineteenth century. 

Figure 10.4. (A) Acoustic lens focused on the front (left) and back (right) the acoustic waves. (B) Measured sound echoes from both zones. Above front and back echo with the lens focused on the sample s back. Below front echo with the lens focused on the sample front. (Reproduced with permission of the American Institute of Physics, Ref [34].)...

Coherent reflections at the top and bottom boundaries of the plate give way for a set of standing acoustic waves between the two main surfaces of the plate. Due to the piezoelectric nature of quartz two sets of resonance frequencies exist for each mode, depending on the electrical boundary conditions. The first set corresponds to a plate with open-circuit boundary conditions. From the physical point of view charges will be collected on the electrodes building up a potential difference and hence an electrical field from the electrical point of view the electrodes are unconnected. This resonance is termed anti-resonance in the piezoelectric literature and parallel resonance in electronics literature. The second set of resonance frequencies corresponds to a plate with short-circuit boundary conditions. The electrodes are connected and a potential difference cannot be built up. The respective names are resonance in piezoelectric and series resonance in electronics Hterature. The differences arise from piezoelectric stiffening accompanied by differences in the sound velocity. The anti-resonance (parallel) frequencies of each of the three acoustic modes are completely decoupled giving ... 

Stoll R.D., 1974. Acoustic waves in saturated sediments. In Hampton L. (ed) Physics of sound in marine sediments. Plenum Press, New York, pp 19-39... 

Borisov Yu Ya, Gynkina NM. On acoustic drying in a standing sound wave. Soviet Physics-Acoustics 8(1) 129-131, 1962. 

Kim, Y.-H. 2010a. Acoustic Wave Equation and Its Basic Physical Measures. Sound Propagation, 69-128. NJ John Wiley Sons, Ltd. 

The purpose of this paper is to emphasize some analogy between the mechanical response of the two most studied phases of liquid crystals nematics and smectics A. In particular, we show that in nematics a new acoustical wave could be observed. This new wave has some formal analogy with the second sound in the smectics, although the physical origin is completely different. 

Miscellaneous Properties. The acoustical properties of polymers are altered considerably by their fabrication into a ceUular stmcture. Sound transmission is altered only slightly because it depends predominandy on the density of the barrier (in this case, the polymer phase). CeUular polymers by themselves are, therefore, very poor materials for reducing sound transmission. They are, however, quite effective in absorbing sound waves of certain frequencies (150) materials with open ceUs on the surface are particulady effective. The combination of other advantageous physical properties with fair acoustical properties has led to the use of several different types of plastic foams in sound-absorbing constmctions (215,216). The sound absorption of a number of ceUular polymers has been reported (21,150,215,217). 

When a sound wave comes in contact with a soHd stmcture, such as a wall between two spaces, some of the sound energy is transmitted from the vibrating air particles into the stmcture causing it to vibrate. The vibrating stmcture, in turn, transmits some of its vibrational energy into the air particles immediately adjacent on the opposite side, thereby radiating sound to the adjacent space. For an incomplete barrier, such as a fence or open-plan office screen, sound also diffracts over the top and around the ends of the barrier. The subject of this section is confined to complete barriers that provide complete physical separation of two adjacent spaces. Procedures for estimating the acoustical performance of partial barriers can be found in References 5 and 7. 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

When the room to be simulated doesn t exist, we can attempt to predict its impulse response based on purely physical considerations. This requires detailed knowledge of the geometry of the room, properties of all surfaces in the room, and the positions and directivities of the sources and receivers. Given this prior information, it is possible to apply the laws of acoustics regarding wave propagation and interaction with surfaces to predict how the sound will propagate in the space. This technique has been termed auralization in the literature and is an active area of research [Kleiner et al., 1993]. Typically, an auralization system first computes the impulse response of the specified room, for each source-receiver pair. These finite impulse response (FIR) filters are then used to render the room reverberation. 

Many problems in ultrasonic visualization, nondestructive evaluation, materials design, geophysics, medical physics and underwater acoustics involve wave propagation in inhomogeneous media containing bubbles and particulate matter. A knowledge of the effect of voids or inclusions on the attenuation and velocity of sound waves is necessary in order to properly model the often complex, multilayered systems. 

Acoustic (sound) waves are also transmitted by atomic vibrations. Atomic vibrations are often called phonons, i.e., "particles" of sound. Theories of heat transfer [13] in insulators usually attempt to relate X to other physical properties which are mainly determined by atomic vibrations, such as the velocity of sound and the heat capacity. 

Acoustic methods Measures the attenuation of sound waves as a means of determining size through the fitting of physically relevant equations... 

The propagation of small pressure pulses or sound waves is a physical effect which can be a source of information for single and multiphase systems. Ultrasonic methods (between 20 kHz and 100 MHz) are easy to use, safe, non-destructive and non-invasive. MorbideUi et al. [19] used this acoustic method, with success, for the determination of the evolution of conversion in various copoly-meric systems in the dispersed phase (emulsion polymerization). The same methodology has been applied to bulk and solution systems by Gavin et al. [20, 21] and Zeihnann et al. [22] for monitoring high-soUds content polymerization of styrene and MM A. 

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