Acoustics
Acoustics is the science of sound.
An "elastic media" is one that vibrates if impacted by a force—it will move in one direction and return to its original position. Examples are a string that has been plucked or a tuning fork that has been struck. Liquids and gases can be elastic media, too, and vibrations can be set up in them by forces on them--pressure.
Sound is vibrations in air, a pressure wave that causes the atoms of air to vibrate back and forth, transmitting the pressure along. The same principle works in liquids and solids, too, and sound can be also be transmitted through them. All that is necessary for sound is for some object--a vocal cord, a violin string, the blow of a hammer--to start a displacement in an elastic medium. The medium then transmits the sound via waves of high and low pressure, ultimately existing in the back-and-forth motions of the medium's atoms. Modern technology has provided "acoustic transducers," which are devices to convert different forms of energy into sound. An example is a loudspeaker on a stereo system, radio, or television which converts electrical systems into sound.
Sound propagates at a certain speed, depending on the medium through which it is passing. The speed of sound in still air at 0°C (32°F) is 331.45 m/s (1,087.43 ft/s, or 741 mi/hr, also called Mach 1), but it changes at different temperatures, and, as you are probably aware, it changes if there is wind. The speed of sound in still water at 25°C (77°F) is 1,478 m/s (3,306 mi/hr), and in steel about 5,900 m/s (13,000 mi/hr). Note that the denser the medium the faster the speed of sound. Sound also depends on the speed of its source--a train moving towards us sounds different from one going away. This is an example of a Doppler shift.
Acoustics deals with sound waves of all frequencies, but only some frequencies are audible to human beings, for whom the range of hearing extends from 20 to 20,000 Hz. (One Hertz (Hz) is one complete oscillation per second.) Sound below 20 Hz is referred to as infrasonic, while that above 20,000 Hz is called ultrasonic. Dogs, for example, with a hearing range of 20 to 40,000 Hz, can hear ultrasonic waves that we cannot, which explains the "silent" dog whistle.
Sound intensity is a factor, too. Human beings cannot detect a sound below about 10-16 W/cm2, and of course very loud, powerful sounds can temporarily or permanently damage the transducers in our ears, our eardrums. Sound intensity is usually measured in decibels a logarithmic scale in which 10 decibels (dB) is about the level of a whisper, 60 dB the level of sound in a noisy office, 80 dB the sound level of heavy traffic, 120 dB the sound level of a nearby aircraft, and 130 dB about the pain threshold for the human ear.
Besides communication, acoustics has many other applications. Music soothes the savage beast, motion pictures, radio, and television entertain and inform us, and submarines and ships use sound for underwater detection, as with sonar. Ultrasonic sound waves are used for medical imaging of, for example, a fetus in a pregnant woman's womb. The relatively new fields of bioacoustics and medical acoustics explore the use of sound in medical diagnosis and therapy. Recently acoustic surgery has been performed, substituting sound for the scalpel to perform such tasks as destroying tumors and stopping internal bleeding. The acoustic scalpel works by broadcasting sound through a specially shaped set of speakers so that the sound waves converge to create a spot of very intense heat that can destroy tumor cells. In experiments in 1998, this spot was so small there was a layer of only six cells between the destroyed tissue and completely unharmed tissue--a precision finer than any scalpel. The sound waves can also heat a bleeding area to create chemical and physical changes that cauterize, or sear it.
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