Man's physical universe



of energy or force spreading out from a central point, is inversely

proportional to the square of the distance from a source, because the

area of a sphere carrying a given amount of energy increases as the

square of the radius. Thus if the distance from the source, i.e., the

radius, is tripled, the area of the sphere will be increased nine times,

and the amount of energy in the same area of the wave would be only

one-ninth as much as at the start, because the original energy has been

distributed over nine times as much


The sensation which we call the

loudness of a sound depends upon

the intensity of the compressional

waves, and it therefore also depends

upon the amplitude of the vibrations

in the generator which caused

the compressional waves.

If all the energy of a voice of a

lecturer in an average auditorium

were absorbed and changed into


Amplitude .





heat, it would be necessary for the lecturer to talk for a hundred

years without interruption in order to raise the temperature of a

one-centimeter layer of water on the walls and ceiling one degree

centigrade, assuming that there was no gain or loss of heat during

that time.

Fig. 217. Intensity depends upon


Speaking-tubes, Ear Trumpets, and Megaphones Are Designed to

Increase the Intensity of Sound.

When sound waves spread out in every direction from a source of

sound, the energy is spread over increasingly large wave fronts, and

as a result the intensity of a sound varies inversely as the square of

the distance from the source.

Speaking-tubes and the tubes of a stethoscope prevent the sound

waves from spreading out and thus transmit the sound with little

decrease in intensity. Long corridors are similar to speaking-tubes in

their transmission of sound with little loss of intensity.

One type of airplane-detector is really a very large stethoscope,

consisting of two large conical

trumpets, mounted so that they can

be rotated about a vertical axis and having their small ends leading

to the ears through rubber tubes. The sense of direction is made possible

by the use of one trumpet for each ear.

Ear trumpets concentrate the energy of large areas of incoming

sound waves into a small area and thus increase the intensity.

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