Acoustic isolation

The interferometer assembly has been modified to provide acoustic isolation from both building vibrations and airborne noise (7). These improvements have greatly enhanced the signal to noise. The spectra presented here show a signal to noise ratio in excess of 500 for silica samples and in excess of 100 for the alumina samples the differences are due to different sample porosities. No smoothing of the spectra has been performed, and all spectra reported are direct reproductions of the plotter output from the spectrometer. 

Potato of the first kind involves acoustic coupling to the resonator of vibrations in the field modulation coils that arise from forces determined by the cross product of current and magnetic field. It is controlled by mechanical damping and acoustic isolation. 

Figure 5-32 is a block diagram of the ANL acoustic cross-correlation test section (Sheen and Raptis, 1986) for monitoring solid/gas flows. Three pairs of wide-band transducers are clamped directly on the pipe. In each pair, one transducer acts as a transmitter and the other as a receiver. To avoid acoustic crosstalk, each pair of transducers is acoustically isolated from the others. The isolation is achieved by using viton gaskets between flanges. During operation, the transmitters delivered a continuous wave with a frequency of 1 MHz. Receivers were conditioned with amplifiers and band-passed filters set at 100-1... 

We are indebted to H. P. Layer for providing us with a detailed design of the NBS He-Ne iodine stabilized laser, to J. L. Hall for informing us of his design of an acoustically isolated interferometer chamber, to K. L. Foster for assistance in all phases of their fabrication, and to F. Walls for help with ultra-stable crystal oscillators. This work is supported in part by the U.S. Office of Naval Research. 

Fig. 5.47. Experimental realization of an acoustically isolated table for a wavelength-stabilized laser system...

It is useful at this point to consider the definition of an additive as given by the EC (European Commission) an additive is a substance, which is incorporated into plastics to achieve a technical effect in the finished product, and is intended to be an essential part of the finished product. Since the very early stages of the development of the polymer industry, it was realized that useful materials could only be obtained if certain additives were incorporated into the polymer matrix, in a process normally known as compounding. Additives confer on plastics significant extensions of properties in one or more directions, such as general durability, stiffness and strength, impact resistance, thermal resistance, resistance to flexure and wear, acoustic isolation, etc [86]. 

The refractory wire inside the pipe would be either centered in the flow stream or attached to the inner liner surface. The wire waveguide would remain acoustically isolated from all other components both inside and outside of the pipe. Materials would have been carefully chosen for the internal waveguide and horn that would allow attachment to the internal surface and would not diffuse into the gas stream at elevated temperatures. The horn and waveguide materials on both sides of the inlet should have similar acoustic impedances to the inlet material in order to minimize reflections at the interfaces. 

---