Experimental demonstration of the frequency dependence of the effective scatterer number density

In previous papers, a data reduction method for determining a frequency- dependent 'effective scatterer number density' in ultrasound was reported [Chen et al., J. Acoust. Soc. Am. 95, 77-95 (1994); 96, 2556-2563 (1994)]. The effective scatterer number density is the actual number density of scatterers multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of the scatterers. Evaluation of this technique based on measurements applied to a phantom with a distribution of scatterer diameters is reported here. This condition yields an effective scatterer number density that depends on the dominant scatterer concentration at the analysis frequency. Measured scatterer number densities at 3.5, 5, and 8 MHz were in agreement with theoretical computations of this parameter for the phantom.