TYPES OF ORDINARY SOUND
In speech, music, and noise, pure tones are seldom heard. A musical note contains, in addition to a fundamental frequency, higher tones that are harmonics of the fundamental frequency. Speech contains a complex mixture of sounds, some (but not all) of which are in harmonic relation to one another. Noise consists of a mixture of many different frequencies within a certain range; it is thus comparable to white light, which consists of a mixture of light of all different colors. Different noises are distinguished by different distributions of energy in the various frequency ranges.
When a musical tone containing some harmonics of a fundamental tone, but missing other harmonics or the fundamental itself, is transmitted to the ear, the ear forms various beats in the form of sum and difference frequencies, thus producing the missing harmonics or the fundamental not present in the original sound. These notes are also harmonics of the original fundamental note. This incorrect response of the ear may be valuable. Sound-reproducing equipment without a large speaker, for example, cannot generally produce sounds of pitch lower than two octaves below middle C; nonetheless, a human ear listening to such equipment can resupply the fundamental note by resolving beat frequencies from its harmonics. Another imperfection of the ear in the presence of ordinary sounds is the inability to hear high-frequency notes when low frequency sound of considerable intensity is present. This phenomenon is called masking.
In general, reproductions of speech and musical themes are recognizable even if only a portion of the frequencies contained in the originals are copied. Frequencies between 250 and 3,000 Hz, the frequency range of ordinary telephones, are normally sufficient. A few speech sounds, such as /ð/ require frequencies as high as 6,000 Hz. For high quality reproduction, however, the range of about 100 to 10,000 Hz is necessary. Sounds produced by a few musical instruments can be accurately reproduced only by adding even lower frequencies, and a few noises can be reproduced only at somewhat higher frequencies.
