How We Process Sound

Many of the distinctive qualities of speech relate to the physics of sound. Sound is a forceful, physical medium. Any disturbance that vibrates air-an explosion, a plucked guitar string, or the movement of a cricket's legs-creates a traveling wave of energy. The brain's recognition networks contain specialized modules for processing sound, located within the auditory cortex. When a wave of energy reaches our ears and the air pressure pounds on our eardrums, the energy is transformed into patterns of nerve impulses that the processing modules within our auditory cortex recognize as sound.

Here is a key point. The patterns of vibration that reach our ears are transient. For example, as you pronounce the word permanence, the first syllable, "per," is gone before you pronounce the second syllable, "ma." You can record sound to review later, but when you play it back, it's transient again. Although a digital recording, such as the one illustrated in Figure 3.2, might enable you to "see" a segment of a sound wave, interpreting that segment requires specialized expertise. It's just not possible to generate a true "still shot" of sound. In fact, time is inseparable from sound because the brain responds to the number of waves per second. A word spoken, a car horn, and a sonata are all patterns in time, which cannot be frozen and examined. Sound perception is thus intimately connected to transient sequences in time-and thus to memory.

d - Figure 3.2 - A Sound Wave

Think of speech as a particularly rich subcategory of sound. Human vocal chords generate extremely complex patterns, and the ability to recognize the source of these sounds (the particular vibration patterns of the sender) and the meaning of vocalizations is one of our most remarkable traits.