All musical tones, including the phonetic sounds of words, have four characteristics: pitch, loudness or intensity, quality or tone-color, and duration. The last, of course, needs no definition.
Pitch is dependent on the number of vibrations per second. The greater the number of vibrations, the higher the pitch and the more 'acute' the tone. The lowest pitch recognizable as a tone (as distinguished from noise) is 8 vibrations a second; the highest pitch the ear can hear is between 20,000 and 30,000 a second. In normal English speech among adults the voice ranges from about 100 to 300 vibrations, but in animated speaking this range is greatly increased.
Loudness is a comparative term for the strength of the sensation of sound in the ear. It is determined by the energy or intensity of the vibrations and varies (technically speaking) as the product of the square of the frequency and the square of the amplitude (I=n2A2). But for ordinary purposes it is sufficient to regard loudness and intensity as the same. The distinction, however, is clear in common practice; for whether one says "father" loudly or quietly, there is a relatively greater intensity of sound in the first syllable than in the second. In speech this intensity is called accent or stress.
The third characteristic, variously called quality, timbre, tone-quality, tone-color, is that which distinguishes sounds of the same loudness and pitch produced by different instruments or voices. It is the result of the combination of the partial tones of a sound, that is, of the fundamental and its overtones. In music, tone-quality is of the utmost importance, but as an element of speech rhythm it is practically non-existent, and may be wholly neglected, though it plays, of course, a prominent part in the oral reading of different persons.[2]
What is the relation of these physical attributes of sound to sound rhythm? The answer lies in a closer examination of the nature of rhythm, especially as it concerns the rhythm of speech.
Rhythm means measured flow or succession. Now first, in order that any succession may be measured, there must be something recognizable which distinguishes one unit from the next. In spatial rhythms the point of division is almost always easily perceived; hence the greater difficulty of analyzing the simplest time-rhythms as compared with the most complex space-rhythms. Moreover, the basis of measurement, that by which the 'distance' between any point of division and that which follows it is determined, must, by definition, be duration of time. Suppose, however, that the time-distance between successive points of emphasis or division is equal, is the rhythm therefore necessarily regular? No, because the points of emphasis themselves may vary in force or energy. Thus if in the following scheme (´ = point of emphasis; - = equal time-distance):
´-´-´-´-´-´- etc.
every ´ is not of the same value, the result might be (´´ = twice as much emphasis as ´; ´´´ = three times as much):
´´-´-´´´-´´-´´´- etc.