ultrasonics
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What are ultrasonics?
Ultrasonics involves the study and application of ultrasonic vibrations or compressional waves, which are sound waves beyond the human ear's audible range.
High-intensity ultrasonics are used in cleaning, machining, and soldering. Ultrasonic cleaning uses vibrations in liquid tanks to clean items like jewelry and surgical instruments. Ultrasonic machining employs high-intensity vibrations to move a machine tool, while ultrasonic soldering is valuable for soldering difficult materials and applications requiring extreme cleanliness.
In medicine, ultrasound uses high-frequency sound waves to produce images of internal body structures and can examine organs such as the heart, liver, and kidneys.
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How are ultrasonic waves different from sound we can hear?
Ultrasonic waves are sound waves with frequencies beyond the human ear's audibility limit, which is about 20,000 hertz. Humans can typically hear sounds ranging from 20 to 20,000 hertz.
Many animals, such as cats, dogs, bats, and rodents, can hear ultrasonic frequencies.
Ultrasonics have many applications, including medical imaging, cleaning, and machining. In medicine, ultrasound waves are used to produce images of structures within the human body. Ultrasonic cleaning uses ultrasonic vibrations to clean objects. Ultrasonic machining uses high-intensity vibrations to move a machine tool.
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What are some common sources of ultrasonic waves?
Ultrasonic waves, which are vibrations above 20 kilohertz, are produced using devices called ultrasonic transducers that convert other forms of energy into ultrasonic vibrations.
Common sources of ultrasonic waves:
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Piezoelectric crystals These convert an oscillating electric field applied to the crystal into a mechanical vibration. They are versatile and used across the entire frequency range.
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Magnetostrictive devices These make use of magnetic material; an applied oscillating magnetic field squeezes the atoms of the material together, creating a periodic change in length and producing a high-frequency mechanical vibration.
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Mechanical devices Gas-driven (pneumatic) transducers like whistles, and liquid-driven transducers such as hydrodynamic oscillators and vibrating blades, are used. These are limited to low ultrasonic frequencies and have applications such as ultrasonic cleaning and drying.
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How do scientists generate ultrasonic waves in a lab or industry?
Ultrasonic waves are generated in labs and industries using ultrasonic transducers, which convert other forms of energy into ultrasonic vibrations. These transducers can be classified by their energy source and the medium they generate waves in.
Mechanical devices like whistles and hydrodynamic oscillators are used for low-frequency applications such as drying and ultrasonic cleaning. Electromechanical transducers, including piezoelectric and magnetostrictive devices, are more versatile. Magnetostrictive transducers use an oscillating magnetic field to squeeze the atoms of a magnetic material, creating high-frequency mechanical vibrations. Piezoelectric crystals, such as quartz, convert an oscillating electric field into mechanical vibration and are used across the entire frequency range. High-intensity ultrasound is utilized in ultrasonic cleaning and machining.
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What are some uses of ultrasonics in medicine or technology?
Ultrasonics, employing high-frequency sound waves, has diverse applications in medicine and technology. In medicine, ultrasound is used to produce images of internal body structures, diagnose conditions, guide procedures like biopsies, and monitor fetal development. Unlike X-rays, ultrasound is non-ionizing, making it safer for many applications.
Beyond medicine, ultrasonics is used in nondestructive testing to detect flaws in materials, clean jewelry and surgical instruments, machine hard materials, and solder unusual materials. Ultrasonic techniques also enable the study of cavitation and viscosity in research, offering insights into intermolecular forces and the properties of living cells.
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How does ultrasound imaging work to create pictures inside the body?
In medicine, ultrasound imaging uses high-frequency sound waves to create images of structures inside the human body. Ultrasonic waves, which are above the audible range for humans, are produced by the electrical stimulation of a piezoelectric crystal and directed at a specific body area. As these waves travel through tissues, they bounce back when they encounter changes in tissue density, such as the borders between different organs. An electronic device receives these reflected echoes, determining their intensity and the position of the tissues from which they originate. These data points create static images or, with rapid scanning, moving pictures of the body's interior.
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ultrasonics, vibrations of frequencies greater than the upper limit of the audible range for humans—that is, greater than about 20 kilohertz. The term sonic is applied to ultrasound waves of very high amplitudes. Hypersound, sometimes called praetersound or microsound, is sound waves of frequencies greater than 1013 hertz. At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 1.25 × 1013 hertz it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly enough.
Many animals have the ability to hear sounds in the human ultrasonic frequency range. A presumed sensitivity of roaches and rodents to frequencies in the 40 kilohertz region has led to the manufacture of “pest controllers” that emit loud sounds in that frequency range to drive the pests away, but they do not appear to work as advertised.
Some ranges of hearing for mammals and insects are compared with those of humans in the table.
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| animal | frequency (hertz) | |
|---|---|---|
| low | high | |
| humans | 20 | 20,000 |
| cats | 100 | 32,000 |
| dogs | 40 | 46,000 |
| horses | 31 | 40,000 |
| elephants | 16 | 12,000 |
| cattle | 16 | 40,000 |
| bats | 1,000 | 150,000 |
| grasshoppers and locusts | 100 | 50,000 |
| rodents | 1,000 | 100,000 |
| whales and dolphins | 70 | 150,000 |
| seals and sea lions | 200 | 55,000 |
Transducers
An ultrasonic transducer is a device used to convert some other type of energy into an ultrasonic vibration. There are several basic types, classified by the energy source and by the medium into which the waves are being generated. Mechanical devices include gas-driven, or pneumatic, transducers such as whistles as well as liquid-driven transducers such as hydrodynamic oscillators and vibrating blades. These devices, limited to low ultrasonic frequencies, have a number of industrial applications, including drying, ultrasonic cleaning, and injection of fuel oil into burners. Electromechanical transducers are far more versatile and include piezoelectric and magnetostrictive devices. A magnetostrictive transducer makes use of a type of magnetic material in which an applied oscillating magnetic field squeezes the atoms of the material together, creating a periodic change in the length of the material and thus producing a high-frequency mechanical vibration. Magnetostrictive transducers are used primarily in the lower frequency ranges and are common in ultrasonic cleaners and ultrasonic machining applications.
By far the most popular and versatile type of ultrasonic transducer is the piezoelectric crystal, which converts an oscillating electric field applied to the crystal into a mechanical vibration. Piezoelectric crystals include quartz, Rochelle salt, and certain types of ceramic. Piezoelectric transducers are readily employed over the entire frequency range and at all output levels. Particular shapes can be chosen for particular applications. For example, a disc shape provides a plane ultrasonic wave, while curving the radiating surface in a slightly concave or bowl shape creates an ultrasonic wave that will focus at a specific point.
Piezoelectric and magnetostrictive transducers also are employed as ultrasonic receivers, picking up an ultrasonic vibration and converting it into an electrical oscillation.
Applications in research
One of the important areas of scientific study in which ultrasonics has had an enormous impact is cavitation. When water is boiled, bubbles form at the bottom of the container, rise in the water, and then collapse, leading to the sound of the boiling water. The boiling process and the resulting sounds have intrigued people since they were first observed, and they were the object of considerable research and calculation by the British physicists Osborne Reynolds and Lord Rayleigh, who applied the term cavitation to the process of formation of bubbles. Because an ultrasonic wave can be used carefully to control cavitation, ultrasound has been a useful tool in the investigation of the process. The study of cavitation has also provided important information on intermolecular forces.
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Research is being carried out on aspects of the cavitation process and its applications. A contemporary subject of research involves emission of light as the cavity produced by a high-intensity ultrasonic wave collapses. This effect, called sonoluminescence, can create instantaneous temperatures hotter than the surface of the Sun.
The speed of propagation of an ultrasonic wave is strongly dependent on the viscosity of the medium. This property can be a useful tool in investigating the viscosity of materials. Because the various parts of a living cell are distinguished by differing viscosities, acoustical microscopy can make use of this property of cells to “see” into living cells, as will be discussed below in Medical applications.