MUSICAL SOUND PERCEPTION

In order to think intelligently about sound production, we need to understand certain things about how the ears and brain make sense of the sounds that reach them. This will also help us to develop better analytical listening skills, which are invaluable in instrument making.

SOME BASICS

Sound is created when something causes small, localized fluctuations in air pressure. The fluctuations propagate outward from the source as pressure waves in the atmosphere. Should there be any ears in the vicinity, the pressure waves cause movement in the sensitive membrane that is the ear drum, and, following a series of bio-mechanical and neural transmissions, the event is interpreted as sound by the brain. A single pressure pulse doesn't amount to much of a sound; it takes a series in rapid succession to give the ear something it can respond to. The arrival of a series of pressure waves causes air molecules at a given location to move back and forth with each pulse; thus the association of sound with vibration.

An important property of vibrations is frequency, normally expressed as the number of vibratory cycles per second completed by whatever it is that is vibrating. Think of frequency in terms of complete vibratory cycles: for a vibrating object beginning at some central point, moving to one side and back to the center constitutes a half-cycle. To complete the cycle it must continue through the center point and on to the other side, and return once again to the center point. The term Hertz, abbreviated Hz, is commonly used to represent cycles per second (after the 19th century physicist Heinrich Hertz). Thus, for instance, 200 cycles per second = 200Hz.

Humans ears are responsive to frequencies falling within a range extending roughly from a lower limit of about 20Hz to perhaps 16,000 or 20,000Hz for a typical young person (this upper limit drops with age). Within this range, lower (slower) frequencies are associated with low, or bass sounds, and higher (faster) frequencies are associated with high, or treble sounds. In general, through most of the range, the human ear's acuity is quite impressive: it picks up sounds representing truly minuscule amounts of energy; and at the opposite extreme it withstands sounds carrying billions of times that energy before serious discomfort or hearing damage occurs. The ear's sensitivity is not uniform through the hearing range, however. It tapers off at both ends, and has a broad peak in the range of about 2,000Hz to 5,000Hz, corresponding to a medium-high part of the range. This means that sounds within this band sound much louder than sounds carrying comparable energy at higher or lower frequencies.

When a sound vibration occurs at a single steady frequency, you hear it as having a recognizable "note" or pitch. Pitch, in other words, is the brain's way of interpreting vibrational frequency. The ascending series of notes that you hear when someone plays a scale on a musical instrument actually represents the instrument's ability to produce sounds at a series of specific frequencies, each a little higher than the one before. (Appendix 2 at the end of this book contains a chart giving frequencies for each of the pitch names used in the standard Western musical scale.) Human ears and brains are amazingly good at recognizing steady-frequency vibrations and distinguishing one frequency from another. Frequency differences of less than one percent are e