I. Introduction

Ultrasound is defined by a sound wave with a fequency over 20 KHz and below 10^13 Hz, at which point the wave becomes hypersound (aka praetersound). At these very high frequencies, it becomes very difficult to propogate the sound waves through solids, and even liquids. In fact, at 1.25x10^13 Hz, longitudinal waves cannot propogate at all. Although sound above 20 KHz is inaudible to humans, many animals have auidtory ranges far exceeding ours, even as high as 150 KHz. The following is a table of some common animals, and their auditory frequency ranges:
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
(figure adapted from Encyclopedia Britannica)

II. Generating Ultrasonic Waves

Ultrasound is generated similarily to regular sound, using a transducer to convert electrical or magnetic energy into waves. There are three basic types of transducers, all using different elements to produce the waves.

A. Piezoelectric Transducer
Piezolelectric transducers are the most popular and versatile transducers. When a crystal having piezoelectric properties (such as Quartz or Rochelle salt)is subjected to changes in pressure, it becomes electrically charged. The varying current produced by the crystal can be read, and translated into the waveforms of the waves hitting it. The converse is also true, which allows the crystal to transduce ultrasonic waves. When applying an oscillating AC current to a piezoelectric crystal, the crystal becomes active and vibrates with the wave form and frequency of the current. An important factor in the use of piezoelectric crystals is also their natural mechanical (resonant) frequency. The major con of using piezoelectric crystals is that they must be cut to be resonant at the frequency they produce/detect. Cutting a crystal to proper dimentions is a complicated and precise operation. A large ultrasonic emmiting array, unfortunately, would contain a huge number of single crystals, making the technique infeasible. Instead, ceramic materials containing varying piezoelectric fields work very well, but require a very strong electric field to operate. The ultrasonic waves can also be concentrated to a specific point by using a concave shaped crystal or ceramic.

B. Gas/Liquid Driven (pnuematic/hydrodynamic) Transducers
Pnuematic and hydrodynamic transducers are very simple, and examples of them include whistles (such as an inaudible (ultrasonic) dog whistle), vibrating blades, and other simple oscillators. The applications for these transducers are limited, however, because they cannot generate high ultrasonic frequencies, but they are found in industrial settings such as cleaning, drying, and fuel injection.

C. Magnetostrictive/Electrostrictive Transducers
The magnetostrictive effect dictates that an oscillating magnetic field is applied to an electmagnet's coil, the magnetic rod at the center of the coil will expand and contract, generating compressional waves. The electrostrictive effect is similar to the magnetostrictive effect, except an alternating current (AC) is used. Unlike piezoelectric crystals, cutting to a specific resonant frequency is not necessary.

III. Applications Of Ultrasound

Ultrasonics have a very broad range of applications, ranging from heavy industry to mother nature. Ultrasonic waves, by nature, will travel through a uniform substance until they hit an impurity, which allows for medical and material testing capabilities. This capacity of ultrasonic waves also is applied to echolocation, sonar, and ranging.
A. Sonar
Sonar is a very popular method of finding ocean craft. By emitting ultrasonic waves into the water, then recording the echos made by waves reflecting off hard substances, like the metal used in the hull of ships. Ultrasonic waves are used in sonar similarily to how radar waves work in a Radar device. Bats also use a form of sonar by emitting an ultrasonic noise, which bounces off obstacles and the bugs they prey upon.
B. Ultrasonic Ranging
Ultrasonic waves bounce off non-liquids much like light (photons) bounce off a mirror. Thus, waves can be used to find the distance of an object, and can be transmitted through water. Many ships use a ranging device to determine how far away the bottom is, to prevent from running aground. Fishing ships also use ranging to find schools of fish, increasing effienciency considerably. A considerable amount of the ocean floor has also been meticulously mapped using ranging to create detailed depth charts used in navigation.
C. Ultrasonic Doppler
When an ultrasonic wave hits a moving object, the reflection has a slightly different frequency, and using equations, the frequency shift can be translated into the speed of the moving object. This is also known as the Doppler Effect. Sonar utilizes doppler to determine the speed of the craft it tracks utilizing ultrasonic waves.
D. Ultrasonic Nondestructive Materials Testing
Ultrasound can be used at certain frequencies to actually move through a peice of metal. When these ultrasonic waves pass through the metal, a fault, hole, impurity, or corrosion, will create an impedence, or anomaly, indicating weaknesses in the metal. This method is used in many applications, such as testing of nuclear reactors, and testing of railroad rails.
E. Ultrasonic Cleaning
High intensity ultrasonic waves have found applications in cleaning. By taking a small item, such as jewelery (commonly used in ultrasonic cleaning), and immersing it in water, it can then be bombarded by ultrasonic waves through the medium (water). Cavitation of the water, combined with the high intensity vibrations, can loosen dirt and other unwanted substances from the object being cleaned. Degreasing agents are also used in ultrasonic cleaning, as the vibrations work to activate the degreasing qualities of the agents used.
F. Ultrasonic Machining
The vibrations created by high intesity ultrasonic waves are employed in Ultrasonic Machining to move and adjust machine tools. The action of an Ultrasonic Machining tool could be compared to the use of the commercial product "Dremmel" to chisel soft stone. By vibrating a metal or even diamond tool, the substance can be removed by means of friction.
G. Ultrasonic Soldering
Ultrasonic waves are used to clean areas about to receive solder, obsoleting the need for solder flux. The high intesity waves can remove the normal thin layer of oxide even present on aluminum, making it possible to weld aluminum, and to clean other substances. Ultrasonic Welding is usually used for unusual and difficult welds, as it makes the bonds stronger, requiring less material.
H. Ultrasound and Medicine
Ultrasound has diverse medical applications, ranging from an imaging device similar to X-ray, MRI, and CAT systems, to elimination of kidney and bladder stones through vibration. Ultrasound is used to treat joints through vibrational massage of damaged tissues, and the heat created by the vibrations. It is also used in special cases to remove tumors where highly localized and focused heat is necesarry. Ultrasonic imaging is used commonly to scan a fetus in early developmental stages, to determine physical health, deformities, developmental advancement, and even the gender of the unborn child. It is also used to image some internal organs to find anomalies. Ultrasound is prefered over other scanning techniques because it does not give off harmful radiation.

Note: I have used the several resources to compile this report, none of which I copied anything from, with the exception of the hearing range chart from Encyclopedia Britannica. I encourage msgs critiquing this writeup, and I would like to continue to work on it, and possibly split up the sub topics into seperate nodes.

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