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Sound travels in waves that are transverse, and need to travel through a medium. The picture of the speakers represents sound. We cannot see sound waves, but we are still able to measure them. We can measure their speed by figuring out their frequency, or how much space is between every crest of the wave. The closer the crest of the wave, the higher the frequency, the higher the speed. Frequency is measured in hertz. The increment of time the sound takes to travel is called the period.
 Ultrasounds are an effective use of Doppler waves. The ultrasound machine transmits high frequency vibrations into the mother's body. The sound waves then travel and try to hit a boundary between the tissues of the uterus. Some of the sound waves will get reflected, creating an echo. Others will continue on until they hit something and bounce back. The reflected waves are then picked up by the probe and relayed into the computer. The computer calculates the distance from the probe to the tissue or other boundaries, and the period of time the wave took to get to its destination and back. The computer screen then displays the distance and intensities of the echoes on the screen, forming a picture like the one above. The mother and baby are not harmed, as there is no radiation, which is why Ultrasounds are so commonly used.
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