Sound attenuation composite

Lim, R. and Hackman, R. H., "Fundamental Analysis of a Novel Composite Sound Attenuator Using the T-Matrix Approach," NCSC Technical Report U2120-88-40. 1988. 

A corresponding composite probe with the same frequency and crystal size, however, detects the test flaw much better the echo has a 12 dB higher amplitude (see Fig. 4) and in addition, the noise level is much lower, resulting in an improved signal to noise ratio. This effect is especially observed at high sound attenuation. However, in materials with low attenuation or in case of shorter sound paths the standard probe yields a comparable good signal to noise ratio. 

Ultrasound is used to obtain information about the properties of a material by measuring the interaction between a high frequency sound wave and the material through which it propagates. This interaction depends on the frequency and nature of the ultrasonic wave, as well as the composition and microstructure of the material. The parameters most commonly measured in an ultrasonic experiment are the velocity at which the wave travels and the extent by which it is attenuated. To understand how these parameters are related to the properties of foods it is useful to consider the propagation of ultrasonic waves in materials in general. 

From Eq.21 it follows that the imaginary party of the effective bulk modulus K is much larger than the imaginary part of K. Therefore, sound waves are strongly attenuated in this type of material. The viscoelastic polymer-air microbubbles composites are particularly useful in the design of broadband, transition type anechoic coatings for underwater... 

It is obvious from the above review that a wide variety of composite materials and composite structures have been developed for attenuation of sound. Nevertheless, there is a continuing need and opportunity for new concepts for attenuation of air-borne and water-borne sound waves, and to reduce structural vibrations. In particular there is a continuing need for coatings which operate over a wide range of frequencies, over a range of temperatures, and in some cases over a range of pressures. 

Since sound speed is a frequency dependent complex quantity, it therefore follows that the characteristic impedance of the media will also be frequency dependent and complex. If the frequency dependence of sound speed is not known, it can be estimated from the attenuation coefficient as follows. For the rubber composites of interest here, usually a A is essentially independent of frequency. Using Kramers-Kronig relationships (5) it can then be shown that ... 

While the design of such coatings is straightforward, selection of appropriate materials is not. Usually materials with the properties required for a particular application are not readily available, and some custom laboratory fabrication is necessary. This usually involves selecting a polymer composite which somewhat approximates the required physical properties. Then minor alterations to the chemical constituents or fillers are used on a trial basis and the acoustic properties (some combination of Young s Modulus and damping factor, sound speed, attenuation, density, and front-face reflectivity) of these sample formulations are measured. This continues until a suitable formulation is achieved. 

Steady attenuation of sound indicates constant composition in the product... 

In a similar manner to light, other types of radiation, e.g., electromagnetic waves in the X-ray domain, high-frequency electric fields, or acoustic waves, offer ways to monitor changes in composition and structure of suspensions. For instance, dielectric spectroscopy was used to investigate the moisture uptake and stability of cosmetic creams (Sutananta et al. 1996 Tamburic et al. 1996), and acoustic parameters (resonance frequency, attenuation, sound speed) were shown to correlate with sol-gel transition in suspensions of coUoidal silica (Senouci et al. 2001), as well as with the phase transition of renneted milk (Bakkali et al. 2001). 

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