When the frequency of sound waves is higher than the audible threshold of 20000 Hz, it is classified as ultrasound. Although humans cannot hear it, dolphins and bats use it to communicate, to navigate, and to hunt for food in the darkness of caves and deep ocean. Its technology has gained applications as diagnostic and highly non-invasive surgical tools of modern medicine. It is used to look inside a person’s body to examine liquid-based tissues and organs by directing 5 to 7 million Hz sound waves into regions of the body and measure the return time of reflected waves passing through varying layer densities. The different percentages of reflected waves are then feed into computers for image processing, which in some cases can show the real time activities of a developing fetus. Ultrasounds can be used to determine the status of the nervous, circulatory, urinary, and reproductive system. Since it emits no harmful radiation and safer than certain invasive probes, it is now the preferred tool among medical technicians over the use of x-rays and surgical knives. However, it is not often used to examine bone structures for the simple reason that the skeletal system absorbs sound waves by impedance matching allowing wave energy to enter the bone marrows. Good impedance matching materials are classified as relatively soft objects such as felt, carpeting, drapes, foam, and cork. The question is how much of the absorbed energy stay inside the body? The answer is provided by knowing the average wave intensity, cross sectional area of penetration, and the time interval of incidence. Unfortunately, the physics of absolute intensity measurement does not yet exist. However, relative intensity measured in decibels is based on the faintest sounds the human ear can detect at a frequency of 1000 Hz called the threshold of hearing with experimental value of 1 trillionth of watt per square meter and the threshold of pain as the loudest sounds the ear can tolerate of 1 watt per square meter. Therefore, for all practical purposes, the absorbed energy is practically zero.