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The ultrasonic nondestructive characterization of heterogeneous media is difficult due to the inherent inhomogeneity of the material. Previous studies have shown that the amplitude information is not fully correlated to the medium characteristics. Our main interest is to evaluate the mean spacing between heterogeneities randomly distributed in the medium. The mean spacing between heterogeneities is related to the autocorrelation function for the echoes. However, this function in the case of highly heterogeneous media does not reflect the medium characteristics: the impulse response of the whole measurement set (especially the transducer) biases the obtained signature. Our studies are based upon the introduction of an original method giving a nonbiased signature of highly heterogeneous media derived from the spectrum of the rf backscattered signal. This signature immediately gives the mean heterogeneities spacing parameter.