Methods and apparatus for noninvasively determining a sound pressure of a source that transmits a generally periodic acoustic signal including at least one signal component to a surface of a body are provided. The signal component at the surface of the body is received and an initial phase-inverted signal component is estimated responsive to the received signal component. The phase-inverted signal component, advanced by a predetermined time increment, is transmitted toward the source via the surface. The predetermined time increment corresponds to a distance of the signal component within the body. At least one feature of the transmitted signal component is adjusted until a further received signal approximates a minimum value. The adjusted signal component corresponds to the signal component at the distance. The apparatus determines the phase-inverted signal component that corresponds to the signal component being proximate to the source.