that is to veer the entire beam upward (If you are walking, and someone seizes your left arm slowing that part of your body, you automatically veer leftward.) At night, the situation is reversed, for the upper levels are warmer than the lower levels. The upper part of a beam of sound waves will quicken, and the whole beam will veer downward. It is for this reason that sound can usually be heard more clearly and over greater distances by night than by day. However, if we confine ourselves to the room temperature we may write Equation 12-1 as Wavelength (lambda) = 344/frequency (Equation 12-2) Until recent times, sound travelled at a velocity much greater than that of any man-made vehicle, so for practical purposes the velocity of sound did not concern the traveller. With the invention of the airplane, however, and with the steady
increase in the velocities of which it was capable the velocity of sound became of importance for reasons other than those involving the speed of communication. It is the speed of the natural rebound of molecules after compression that dictates the rate at which a compressed area restores itself to normal and compresses the next area; so it is this speed of rebound that determines the velocity of round. It is also the speed of the natural rebound of molecules after striking a speeding plane that makes it possible for air to "get out of the way" of the plane. As the plane approaches the velocity of sound then, it approaches the velocity with which the air molecules can rebound. The plane begins to “chase after" the rebounding air molecules and, with increasing speed, more and more nearly catches them. Such a plane compresses the air ahead permanently (Or at least for as long as it maintains its speed), since the air cannot get out of its way. This volume of compressed air ahead of the plane puts great strains upon the plane's structure; for a time in the 1940's, it was felt that a plane would disintegrate if it approached the speed of sound too closely. Thus, talk began to be heard of a “sound barrier," as though the velocity of sound represented a wall the plane could not break through. The ratio of the velocity of an object to the velocity of sound in the medium in which the object is travelling is called the Mach number, in honour of an Austrian physicist, Ernst Mach (1838- 1916), who toward the end of the nineteenth century first investigated the theoretical consequence of motion at such velocities. To equal the velocity of sound is to be moving at "Mach 1," to double it is to be at "Mach 2." and so on. A Mach number does not represent a definite velocity, but depends upon the nature, temperature, and density of the fluid through which the object is travelling. For normal air at room temperature, Mach 1 is 344 metres per second, or 758 miles per hour.
You've reached the end of your free preview.
Want to read all 195 pages?