This phenomenon of sound-transmission may perhaps be made more clear by the old illustration of a series of eight billiard balls in a row on a table: if the first ball is tapped lightly, striking gently against ball number 2, the latter (as well as numbers 3, 4, 5, 6, and 7) will not apparently move at all, but ball number 8 at the other end will roll away. The air-particles act upon each other in much this same fashion, the difference being that when they are set in motion by a vibrating body a complete vibration backward and forward causes a similar backward and forward movement of the particles (oscillation) instead of simply a forward jerk as in the case of the billiard balls.
Another way of describing the same process is this: the vibration of some body produces waves in the air (cf. waves in the ocean, which carry water forward but do not themselves move on continuously), these waves spread out spherically (i.e. in all directions) and finally reach the ear, where they set the ear-drum in vibration, thus sending certain sound-stimuli to the nerves of hearing in the inner ear, and thus to the brain.
An important thing to be noted in connection with sound-transmission is that sound will not travel in a vacuum: some kind of a medium is essential for its transmission. This medium may be air, water, a bar of iron or steel, the earth, etc.
4. The rate at which sound travels through the air is about 1100 feet per second, the rapidity varying somewhat with fluctuations in temperature and humidity. In water the rate is much higher than in air (about four times as great) while the velocity of sound through other mediums (as e.g., steel) is sometimes as much as sixteen times as great as through air.
5. Sound, like light, may be intensified by a suitable reflecting surface directly back of the vibrating body (cf. sounding board); it may also be reflected by some surface at a distance from its source in such a way that at a certain point (the focus) the sound may be very clearly heard, but at other places, even those nearer the source of sound, it can scarcely be heard at all. If there is such a surface in an auditorium (as often occurs) there will be a certain point where everything can be heard very easily, but in the rest of the room it may be very difficult to understand what is being said or sung.
Echoes are caused by sound-reflection, the distance of the reflecting surface from the vibrating body determining the number of syllables that will be echoed.
The acoustics of an auditorium (i.e., its hearing properties) depend upon the position and nature of the reflecting surfaces and also upon the length of time a sound persists after the vibrating body has stopped. If it persists longer than 2-1/4 or 2-1/3 seconds the room will not be suitable for musical performances because of the mixture of persisting tones with following ones, this causing a blurred effect somewhat like that obtained by playing a series of unrelated chords on the piano while the damper-pedal is held down. The duration of the reverberation depends upon the size and height of the room, material of floor and walls, furniture, size of audience, etc.
6. Sound may be classified roughly into tones and noises although the line of cleavage is not always sharply drawn. If I throw stones at the side of a barn, sounds are produced, but they are caused by irregular vibrations of an irregularly constructed surface and are referred to as noise. But if I tap the head of a kettle-drum, a regular series of vibrations is set up and the resulting sound is referred to as tone. In general the material of music consists of tones, but for special effects certain noises are also utilized (cf. castanets, etc.).
7. Musical tones have three properties, viz.:
1. Pitch.