The value of "e" is found in many mathematical formulas such as those describing a nonlinear increase or decrease such as growth or decay (including compound interest), the statistical "bell curve," the shape of a hanging cable or a standing arch. "e" also shows up in some problems of probability, some counting problems, and even the study of the distribution of prime numbers. In the field of nondestructive evaluation it is found in formulas such as those used to describe ultrasound attenuation in a material. The sound energy decays as it moves away from the sound source by a factor that is relative to "e." Because it occurs naturally with some frequency in the world, "e" is used as the base of natural logarithms.