AM, FM Waves and Sound To understand some of the concepts we introduced in the last module, as well as some of the strange things that happen to broadcast signals, we need to take a look at how radio works.
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 Sound waves and their uses 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 crystalwhich converts an oscillating electric field applied to the crystal into a mechanical vibration.
Piezoelectric crystals include quartz, Rochelle saltand 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 Sound waves and their uses 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 boiledbubbles 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 Rayleighwho 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.
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 sonoluminescencecan create instantaneous temperatures hotter than the surface of the Sun.
This property can be a useful tool in investigating the viscosity of materials. Ranging and navigating Sonar sound navigation and ranging has extensive marine applications.
By sending out pulses of sound or ultrasound and measuring the time required for the pulses to reflect off a distant object and return to the source, the location of that object can be ascertained and its motion tracked. This technique is used extensively to locate and track submarines at sea and to locate explosive mines below the surface of the water.
Two boats at known locations can also use triangulation to locate and track a third boat or submarine. The distance over which these techniques can be used is limited by temperature gradients in the water, which bend the beam away from the surface and create shadow regions.
One of the advantages of ultrasonic waves over sound waves in underwater applications is that, because of their higher frequencies or shorter wavelengthsthe former will travel greater distances with less diffraction. Ranging has also been used to map the bottom of the ocean, providing depth charts that are commonly used in navigationparticularly near coasts and in shallow waterways.
Even small boats are now equipped with sonic ranging devices that determine and display the depth of the water so that the navigator can keep the boat from beaching on submerged sandbars or other shallow points.
Modern fishing boats use ultrasonic ranging devices to locate schools of fish, substantially increasing their efficiency. Even in the absence of visible light, bats can guide their flight and even locate flying insects which they consume in flight through the use of sonic ranging.
Ultrasonic echolocation has also been used in traffic control applications and in counting and sorting items on an assembly line. Ultrasonic ranging provides the basis of the eye and vision systems for robots, and it has a number of important medical applications see below.
The Doppler effect If an ultrasonic wave is reflected off a moving obstacle, the frequency of the resulting wave will be changed, or Doppler-shifted. More specifically, if the obstacle is moving toward the source, the frequency of the reflected wave will be increased; and if the obstacle is moving away from the source, the frequency of the reflected wave will be decreased.
The amount of the frequency shift can be used to determine the velocity of the moving obstacle. Just as the Doppler shift for radar, an electromagnetic wavecan be used to determine the speed of a moving car, so can the speed of a moving submarine be determined by the Doppler shift of a sonar beam.
An important industrial application is the ultrasonic flow meterin which reflecting ultrasound off a flowing liquid leads to a Doppler shift that is calibrated to provide the flow rate of the liquid.
This technique also has been applied to blood flow in arteries. Materials testing Nondestructive testing involves the use of ultrasonic echolocation to gather information on the integrity of mechanical structures.
Since changes in the material present an impedance mismatch from which an ultrasonic wave is reflected, ultrasonic testing can be used to identify faults, holes, cracks, or corrosion in materials, to inspect welds, to determine the quality of poured concrete, and to monitor metal fatigue.
Owing to the mechanism by which sound waves propagate in metals, ultrasound can be used to probe more deeply than any other form of radiation. Ultrasonic procedures are used to perform in-service inspection of structures in nuclear reactors. Structural flaws in materials can also be studied by subjecting the materials to stress and looking for acoustic emissions as the materials are stressed.Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz".
In air at atmospheric pressure, ultrasonic waves have wavelengths of cm or less.. Perception Humans. The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear.
Auditory sensation can occur if high‐intensity ultrasound is fed directly. AM, FM Waves and Sound.
To understand some of the concepts we introduced in the last module, as well as some of the strange things that happen to broadcast signals, we need to take a look at how radio works.. First, let's look at the AM radio band (group of frequencies).
AM stands for amplitude modulation, which will be explained later..
AM radio ranges from to kHz (kilohertz, or. In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid..
In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20 kHz.
Video: What are Sound Waves? - Definition, Types & Uses. and create images of babies inside their mothers' wombs. Bats also use sound waves for navigation.
They send out ultrasound pulses. The reflection of sound waves is governed by two laws known as the two laws of sound reflection, The first law of reflection of sound waves is the angle of incidence = the angle of reflection..
The incident sound ray, the reflected sound ray and the perpendicular line from the point of incidence on the reflecting surface, all lie on the same plane, . Uses of Sound Waves. Echoes. Echoes are sound waves bouncing off surfaces. Sound waves obey the same first rule of reflection.
(Remember: the angle of incidence is the same as the angle of reflection.) The echo is usually quieter than the original noise as energy is lost as the wave travels along.