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There is evidence that bats and dolphins may also sense the velocity of an object such as prey reflecting their ultrasound signals by observing its Doppler shift. Figure 7. Ultrasound is partly reflected by blood cells and plasma back toward the speaker-microphone. Because the cells are moving, two Doppler shifts are produced—one for blood as a moving observer, and the other for the reflected sound coming from a moving source.
The magnitude of the shift is directly proportional to blood velocity. Ultrasound that has a frequency of 2. Assume that the frequency of 2. The last question asks for beat frequency, which is the difference between the original and returning frequencies. Identify knowns. The beat frequency is simply the absolute value of the difference between f s and f obs , as stated in:.
The Doppler shifts are quite small compared with the original frequency of 2. It is far easier to measure the beat frequency than it is to measure the echo frequency with an accuracy great enough to see shifts of a few hundred hertz out of a couple of megahertz.
Furthermore, variations in the source frequency do not greatly affect the beat frequency, because both f s and f obs would increase or decrease.
Industrial, retail, and research applications of ultrasound are common. A few are discussed here. Ultrasonic cleaners have many uses. Jewelry, machined parts, and other objects that have odd shapes and crevices are immersed in a cleaning fluid that is agitated with ultrasound typically about 40 kHz in frequency.
The intensity is great enough to cause cavitation, which is responsible for most of the cleansing action. Because cavitation-produced shock pressures are large and well transmitted in a fluid, they reach into small crevices where even a low-surface-tension cleaning fluid might not penetrate. Sonar is a familiar application of ultrasound. Sonar typically employs ultrasonic frequencies in the range from Bats, dolphins, submarines, and even some birds use ultrasonic sonar.
Echoes are analyzed to give distance and size information both for guidance and finding prey. In most sonar applications, the sound reflects quite well because the objects of interest have significantly different density than the medium in which they travel.
When the Doppler shift is observed, velocity information can also be obtained. Submarine sonar can be used to obtain such information, and there is evidence that some bats also sense velocity from their echoes. Similarly, there are a range of relatively inexpensive devices that measure distance by timing ultrasonic echoes. Many cameras, for example, use such information to focus automatically. Some doors open when their ultrasonic ranging devices detect a nearby object, and certain home security lights turn on when their ultrasonic rangers observe motion.
Sinks in public restrooms are sometimes automated with ultrasound devices to turn faucets on and off when people wash their hands. These devices reduce the spread of germs and can conserve water. Ultrasound is used for nondestructive testing in industry and by the military. Because ultrasound reflects well from any large change in density, it can reveal cracks and voids in solids, such as aircraft wings, that are too small to be seen with x-rays.
For similar reasons, ultrasound is also good for measuring the thickness of coatings, particularly where there are several layers involved. Basic research in solid state physics employs ultrasound. Its attenuation is related to a number of physical characteristics, making it a useful probe. Among these characteristics are structural changes such as those found in liquid crystals, the transition of a material to a superconducting phase, as well as density and other properties.
These examples of the uses of ultrasound are meant to whet the appetites of the curious, as well as to illustrate the underlying physics of ultrasound. There are many more applications, as you can easily discover for yourself. Why is it possible to use ultrasound both to observe a fetus in the womb and also to destroy cancerous tumors in the body? Ultrasound can be used medically at different intensities.
Lower intensities do not cause damage and are used for medical imaging. Higher intensities can pulverize and destroy targeted substances in the body, such as tumors.
Doppler-shifted ultrasound: a medical technique to detect motion and determine velocity through the Doppler shift of an echo. Because the wavelength is much shorter than the distance in question, the wavelength is not the limiting factor; b 4. Skip to main content. Physics of Hearing. Search for:. Ultrasound Learning Objectives By the end of this section, you will be able to: Define acoustic impedance and intensity reflection coefficient.
Describe medical and other uses of ultrasound technology. Calculate acoustic impedance using density values and the speed of ultrasound. Calculate the velocity of a moving object using Doppler-shifted ultrasound. Characteristics of Ultrasound The characteristics of ultrasound, such as frequency and intensity, are wave properties common to all types of waves.
Example 1. Calculate Acoustic Impedance and Intensity Reflection Coefficient: Ultrasound and Fat Tissue Using the values for density and the speed of ultrasound given in Table 1, show that the acoustic impedance of fat tissue is indeed 1.
These days, sound wave which is not intended to be heard is also called ultrasonic wave. In a solid, there simultaneously exist two types of elastic waves. One is an elastic wave that has a displacement in the same direction of the propagation direction of the wave called longitudinal wave or density wave and another is an elastic wave that has a displacement to the vertical direction of the propagation direction of the wave called traverse wave or shear wave.
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