# Man's physical universe

SOUND PRODUCED BY VIBRATIONS IN MATTER 449

The singing of teakettles shortly before the boiling-point is reached

is due to the fact that the condensation and collapse of steam globules

as they rise into the colder water near the surface set up vibrations

within the liquid. This process is called cavitation.

Sound waves differ from electromagnetic waves in that the former

are transmitted by matter, while the latter are transmitted through

empty space. The fact that sound waves require matter for their

transmission can be readily demonstrated by ringing a bell in a jar

which can be evacuated. As the air is removed, the intensity of the

sound decreases until it can no longer be heard. It is important to

remember that waves are disturbances that travel in a medium but

that the material of the medium itself does not travel.

Sound Waves Travel Much More Slowly than Light Waves.

The thunderclap is heard some time after a distant flash of lightning

The jet of steam from a distant steamer is seen before the sound

is seen.

of the whistle is heard. Other common observations lead to the conclusion

that an appreciable time is required for sound to travel from

one point to another.

The velocity of sound in dry air is 1085 feet per second at 0° C. and

increases at the rate of 2 feet per second per degree C. rise in temperature.

Corrections for both the temperature and moisture content of

air must be made in calculating the speed of sound in air, because the

velocity imparted to molecules depends upon their masses rather than

their number. Slight changes in barometric pressure do not afTect the

speed of sound because they merely determine the number of molecules

present. Temperature affects the speed of sound in air because it

changes the natural speed of the molecules. It thus becomes evident

that the presence of water vapor changes the density of the atmosphere

by introducing molecules of different mass, while changes in the density

of the atmosphere due to barometric changes are the result of changing

the number of molecules present in a given volume. The velocity of

sound in water is more than four times as great as that in air, and in

steel it is nearly fifteen times as great as that in air.

Sound waves, like light waves, may be reflected. Echoes are reflected

sound waves. Thus the distance between two objects may be

determined by noting the time required for a given sound to return to

its source as an echo. The combined effect of wind, humidity, and

temperature on the passage of sound through air often causes soundlocating

instruments to make serious errors; for example, three trained

anti-aircraft units in England on May 23,

1917, reported that airships

were nearly overhead, though in reality they were 25 miles distant.

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