|
2 THE EFFECTS OF ELASTIC WAVES ON BIOLOGICAL OBJECTS
2.1 Elastic waves in nature, science, engineering, technology, medicine
The elastic waves that exist in nature differ primarily in frequency and intensity. Let us consider the physical features and practical use of elastic waves in different frequency ranges and intensities. Particular attention will be paid to waves of the ultrasonic range, since it is these that have received the widest practical application and are of the greatest interest [108].
Frequency ranges of elastic waves. Any wave is the propagation of a disturbance in a medium in which energy rather than matter is transferred.
Mechanical waves are subdivided according to what forces restore the unperturbed state of the medium. Mechanical waves on the surface of the liquid are of two kinds: gravitational, i.e. those in which the equilibrium after perturbation of the liquid surface is restored by gravity; and capillary, in which the equilibrium is restored by surface tensioning forces. In a mechanical elastic wave, equilibrium after perturbation of the medium is restored due to elastic forces that arise in the medium during propagation of compressions and rarefactions.
Elastic waves are otherwise known as sound waves: this name emphasizes the fact that although human-audible sound occupies a relatively narrow range of frequencies, the physical nature of elastic waves is unified. Elastic waves are divided by frequency: up to 20 Hz - infrasound, 20 to 20-103 Hz - sound in the narrow sense or audible sound, from 20-103 to 109 Hz - ultrasound and above (up to 1012 - 1013 Hz) - hypersonic waves [108].
Infrasound waves. Infrasound oscillations are most often produced when air currents are broken up by various obstacles, by wind interaction with sea waves, electrical discharges, explosions, crustal displacements.
Ultrasound. Although ultrasonic waves are inaudible (and maybe because of that!), they are of great practical importance. Therefore, let us consider them here in a little more detail, focusing on ultrasound in natural conditions.
Apart from butterflies, many insects use ultrasound: crickets emit sounds at 8, 11 and 16 kHz in the audible range, 24 and 32 kHz in the ultrasonic range, locusts respond to 90 kHz ultrasound, cicadas hear 40 kHz ultrasound; honeybees emit sound waves at 20-22 kHz when swarming and detecting food.
Here are some data on the upper hearing threshold by frequency for animals: cat - 50 kHz, dog - 135 kHz, chimpanzee - 33 kHz, mouse - 100 kHz, rat - 80 kHz [110].
Ultrasound and pets. The above is the reason why humans use ultrasound when interacting with pets. This is the earliest practical application of ultrasound.
Nowadays ultrasound with a frequency of 20-25 kHz is used to train or frighten dogs. The deterrent effect is based on the fact that these frequencies are uncomfortable for animals. It causes dogs to learn certain commands (e.g. "foo"). In England, ultrasonic whistles are used to train police and hunting dogs [108].
Table 2.1 gives the upper limits of frequencies perceived by some
animals and insects.
Table 2.1 - Upper limit of frequencies (in kHz) perceived by the hearing organ
Name of animals and insects
|
Unit, kHz
|
Name of animals and insects
|
Unit, kHz
|
Seagulls
|
8
|
Grasshopper
|
100
|
Frogs
|
30
|
Bat
|
150
|
Dogs
|
60
|
Butterflies
|
160
|
Cats
|
100
|
Dolphins
|
200
|
Ultrasound in science, engineering and technology. Measurement of the velocity of ultrasound, studies of interference, diffraction, dispersion, absorption, scattering and other ultra-acoustic phenomena make it possible to study the physical properties and structure of matter. In technical and technological processes ultrasound is most commonly used at frequencies of tens of kilohertz: ultrasonic cleaning, chemical and electrochemical processes, dispersion, emulsification, atomization, crystallization, metallization and brazing, machining, welding, plastic deformation - this is not a complete list of technical applications of ultrasound [108].
Table 2.2 - gives approximate values of the ultrasonic speed in different media at 36°C.
Table 2.2
Ultrasonic velocities in different media at 36°C
Do'stlaringiz bilan baham: |
