What are Ultrasonics? Ultrasonics are sound waves of higher frequency than 20000 Hz, which are inaudible to a human being. These are known as ultrasonics; and since velocity = wavelength × frequency, ultrasonics have short wavelengths compared with sound waves in the audio-frequency range. We shall now take a closer look at ultrasonic sound waves.
Uses of Ultrasonic Sound Waves
In recent years ultrasonics have been utilized for a variety of industrial purposes. They are used on board coasting vessels for the depth sounding, the time taken by the wave to reach the bottom of the sea from the surface and back being determined. These sounds are also used to kill bacteria in liquids and they are used extensively to locate faults and cracks in metal castings, following a method similar to that of radar. Ultrasonic sound waves are sent into the metal under investigation, and the beam reflected from the fault is picked up on a cathode-ray tube screen together with the reflection from the other end of the metal. The position of the fault can then easily be located.
Production of Ultrasonics
How are ultrasonic sounds produced? In 1881 CURIE discovered that a thin plate of quartz increased or decreased in the length if an electrical battery was connected to its opposite faces. By correctly cutting the plate, the expansion or contraction could be made to occur along the axis of the faces to which the battery was applied. When an alternating voltage of ultrasonic frequency was connected to the faces of such a crystal the faces vibrated at the same frequency, and thus ultrasonic sound waves were produced.
Another method of producing ultrasonic sound is to place an iron or nickel rod inside a solenoid carrying an alternating current of ultrasonic frequency. Since the length of a Magnetic specimen increases slightly when it is magnetized, ultrasonic sound waves are produced by the vibrations of the rod.
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Ultrasonic waves are normal sound waves but at frequencies above what humans can hear. They can be used to detect flaws in manufactured products and other materials.
Production of Ultrasonic Waves
Ultrasonic waves are normal sound waves, except that they vibrate at higher frequencies, above the audible range of human hearing (20 kilohertz). This gives them greater penetrating power. They cannot travel through a vacuum, but can be generated in gases, liquids and solids. The frequency of a sound wave is the number of vibrations that pass through a given point in a second, and is measured in Hertz (Hz). For example, middle C vibrates at 523 cycles per second in air.
To generate an ultrasonic wave, a special transducer is used to convert some other kind of energy into an ultrasonic vibration. Mechanical devices that use gas or fluid to create the vibrations include whistles and siren-type generators, but these are limited in their ability to produce high-frequency sound waves. Electromechanical devices, which are more versatile, are available that use piezoelectric and magnetostrictive materials.
The most common method for generating an ultrasonic sound wave uses piezoelectric crystals that convert electrical energy into mechanical energy (sound waves) and back again. The waves are then reflected by objects in the medium and converted back into an electrical signal by the same transducer or a separate one. This process is called excitation and deformation.
Alternatively, a ferromagnetic material such as a rod can be magnetized and then subjected to an alternating magnetic field. This causes alternate contractions and expansions of the rod, which generates ultrasonic vibrations in the surrounding medium. The resulting longitudinal waves have frequencies up to 16 GHz.
Another way to produce an ultrasonic wave is to make it from a high-frequency electronic signal. This method is often used to test and repair electronic components, such as capacitors and diodes. The high-frequency sound is particularly useful in locating faulty components that have become damaged or defective, because it can travel through a large distance quickly and detect even the smallest defects.
In medicine, ultrasonic waves are often used to examine internal body structures such as the heart and abdomen. They can also be used to help with surgery and provide a guide for needle placement. Other medical applications of ultrasonic waves include obstetric ultrasound, which provides images of the baby in the womb and the internal organs. They can also be used to clean spiral tubes and other complex electronic components.
When a transducer is placed near an object, it sends sound waves that travel through the material and reflect off of it. The reflected waves return to the transducer and are converted back into electrical signals. These signals are analyzed to detect flaws within the material. This process is similar to how echolocation works in bats and dolphins.
Unlike X-rays, ultrasound is a non-ionizing form of energy and can be used to inspect delicate objects without harming them. It is also very useful in locating soft tissue. This allows doctors to visualize internal structures of the body and avoids the need for invasive surgical techniques. Ultrasound can also provide accurate information about the internal condition of the body, such as blood flow and cell activity.
The most common method for producing ultrasonic waves is using a piezoelectric transducer. This device uses a crystal such as quartz or calcite that can undergo a form of mechanical stress and produce an electric charge. When an alternating voltage is applied to the crystal, alternately it experiences mechanical contractions and expansions. This creates a vibration that can generate ultrasonic waves.
Another way to produce ultrasonic waves is to use the magnetostriction method. This technique requires a long glass tube filled with lycopodium powder. The powder sets at the antinodes of the tube and is blown off at the nodes. When stationary ultrasonic waves pass through the tube, they form heaps of the powder at the nodes. The height of the piles can be used to determine the frequency of the ultrasonic waves.
Medical ultrasound is a diagnostic procedure that involves placing a transducer on the surface of the patient’s body and transmitting high-frequency sound waves. The wave reflections from the patient’s body tissues are detected by the transducer and converted to electrical signals that can be displayed on a computer screen for diagnosis.
Other medical applications for ultrasound include ultrasonic cleaning and lithotripsy. The former is used to clean objects that cannot be accessed through open surgery. Ultrasound can also locate and remove foreign bodies in the body. The latter is used in cases of kidney stones, gallstones, and other painful conditions.
Reflection of Ultrasonic Sound Waves
When ultrasound waves encounter an interface, some of them are reflected from the surface. In some cases the reflection is complete, in other cases only a portion of the wave is reflected (see Figure 2.5).
This reflects primarily off structures such as blood vessels and bones. As a result, ultrasound images can provide excellent information about tissue structure without having to see the tissue itself. Ultrasound also provides density information superior to that found in X-rays. This is because the intensity of the returning echoes is related to changes in tissue density.
As with all waves, the transmission of ultrasonic waves is affected by a variety of factors. For example, the frequency of the sound is important because ultrasonic waves have a higher frequency than those of normal sound. As a result, they have shorter wavelengths and more penetrating power.
The speed of the sound wave is also important. The speed of sound in air is about 340 meters per second, and this determines how fast the ultrasound wave travels through the body.
Another factor is that ultrasound is partially absorbed by tissues on its journey away from and back to the transducer. This reduces the intensity of the returned echo and can interfere with the accuracy of the imaging results.
Ultrasound is also refracted, a phenomenon that occurs when the wave meets a boundary between different tissue types. When this happens, the sound waves are redirected at an oblique angle away from the transducer. This causes the reflected signal to lose energy and becomes less clear, especially as the distance from the transducer increases.
In the case of medical ultrasonic imaging, the reflection and refraction problems can be overcome by using a special type of ultrasound transducer called a piezoelectric crystal. When a voltage is applied across the crystal it causes it to vibrate, and these vibrations are transmitted through the tissue in contact with the crystal. A computer then converts these returning signals into an image on an ultrasound monitor.
Ultrasound can be used for a wide variety of applications, including non-destructive testing of products and structures, and for producing images of the human body. For medical uses, the frequencies of ultrasonic waves must be below the range that healthy young humans can hear. The most common medical ultrasound frequencies are 2 to 15 megahertz (MHz).
Absorption of Ultrasonic Sound Waves
Ultrasound waves have a frequency higher than the upper limit of what human ears can hear. This frequency ranges from 20 kilohertz (kHz) up to several gigahertz. The physical principles of sound wave transmission remain the same, even for such high frequencies.
As sound waves travel through tissue, their intensity is reduced by refraction, scattering and absorption. Energy reduction is a function of the tissue’s attenuation coefficient. The property of attenuation in a particular material is important in determining the ability of ultrasound to penetrate the material.
When ultrasonic waves reach an interface between two different materials they cause a reflection of the reflected wave at a time proportional to the difference in their consistency, or echogenicity. Structures with greater echogenicity are reflected more strongly and can be seen on the images produced by ultrasound systems.
The reflections from flaws in a material are detected by the transducer and analyzed to determine their location and size. A computer can construct an image showing the relative position of these flaws in a specimen.
This ability to detect movement is crucial in medical ultrasound applications, such as monitoring fetal heartbeats and blood flow in veins. The frame rate of an ultrasound system is the number of frames generated per second, and it contributes to the smoothness of motion capture.
As a result of their rapid transmission speed, ultrasound waves are used for the inspection and evaluation of materials. Their ability to interact with and penetrate materials without touching them means that they can be used in the manufacture of metals and polymers, for non-destructive testing of many products, as well as in industrial production and cleaning. Ultrasound can also be used to locate and identify cracks and defects in liquids, such as water, oil or lubricants.
The speed of transmission of an ultrasonic wave in a medium is determined by its density, viscosity and the distance between the source and the target. The lower the medium’s density, the slower the wave will travel. In addition to this physical limitation, the speed of transmission will be affected by the elasticity and vibration characteristics of the medium.