A physiological state of a patient is detected by at least producing and detecting pressure waves with a free wall of an implantable medical device (IMD) housing. An actuator element may contact the free wall, e.g., a portion of the IMD housing, and cause movement of the free wall that produces a pressure wave within the fluid and tissue of the patient. A detector element contacting the free wall may in turn detect reflected pressure waves received by the free wall. An acoustic module within the IMD may then determine a physiological condition of the patient, e.g., a bladder fullness state, based on the time delay between the transmitted and reflected pressure waves. In some examples in which the IMD also delivers stimulation therapy to the patient, e.g., incontinence therapy, the IMD may also automatically adjust stimulation therapy based on the determined physiological condition.