Below is plotted the rms signal voltage received, normalized to the rms value of the drive signal to the transducers. Both were digitized about 3 times per second during the 10-day period shown. The 4-hour averages represent the average of about 20,000 values. One can say that the signal attenuation varied by no more than 30% throughout the 10-day period. One can also say that there was considerable variation of the order of 30%.