Second Monday Music: Sound

The physical world and 
human perception

By André Duvall

An attempt to answer the well-known question “if a tree falls in the forest and no one is present, does it make a sound?” highlights one difference between physical and perceptual descriptions of sound. A falling tree will produce vibrations both within itself and against the ground it falls on. These vibrations will then send off repeated compressions and decompressions (rarefactions) of the molecules in the surrounding airspace. So, physically, yes, there are vibrations and resultant changes in pressure of air molecules, the occurrence of which is essential for sound production. However, there is no human ear, brain, and body present to perceive, process, and react to these changes in pressure, and therefore there is no experience of this sensation, at least by a human being.
    Another example of the difference between the physical and perceptual concepts of sound is a comparison of frequency to pitch. Frequency is the rate of regular compressions and rarefactions (i.e., a single sound wave) per given unit of time. Thus, it is an objective and quantifiable physical phenomenon. Sound waves at extremely low frequencies, like those emitted from black-hole regions of deep space, are not discernible to human ears. The fact that they cannot be processed and transcribed into a sound (and thus “heard”) by the ear does not change the fact these vibrations still occur.
    Even within the range of human perception, frequency is not quite the same as pitch. While most people will perceive higher frequency sound waves as higher-sounding pitches than their perception of lower frequency sound waves, pitch is the unique experience of each individual; a tone sounding at a given frequency may create a slightly different sonic experience from one person to the next. This is exemplified by the fact that certain people are naturally more adept at perceiving slight changes in pitch, whether or not they are musicians. The threshold of perceived change in frequency varies among individuals. On the other hand, trained musicians may be more conditioned to recognize slight differences in pitch and timbre, partly because they have been cultivating this awareness for years in their musical studies.

Amplitude is another physical, quantifiable measurement. While changes in amplitude (basically a measure of energy) certainly affect our perception of loudness, the relationship is not always a direct proportion. Fletcher-Munson curves are used in studies of acoustics to illustrate the fact that the same amount of amplitude at particularly low or high frequencies within our perceptive range will produce different degrees of loudness. The implications of this phenomenon lead to adjustments in the amplification of sound along the frequency spectrum for music played over the radio.

Timbre, or tone quality, is a perceptual descriptor of sound (e.g., the difference in the way a violin sounds compared with a guitar sounding identical pitches). Its physical equivalent is waveform, which is the composite pressure disturbance resulting from the often multiple vibrations (and correspondingly different frequencies) produced by a vibrating object. There is a fairly direct correlation between waveform and timbre, since each waveform can be divided into a unique combination of simpler sound waves of various strengths. These simpler sound waves often consist of a fundamental sound wave, which is what we hear as the actual pitch, and potentially multiple higher frequencies known as overtones. Think of the sound of a large cast-iron bell, the type often found hanging in cemeteries or town squares—there are many strong overtones present in the sound, giving the bell its rich quality. However, sounding multiple bells of these types together creates a degree of cacophony rather than harmony due to the strong presence of overtones, even if the fundamental pitches are in harmony. This is similarly why chords played in the lowest register of a piano are often unrecognizable.
    Physically, the waveform can be selected and objectively created, as in the instance of creating bells that sound overtones of various strengths. Perceptually, different individuals will have different emotional reactions to the timbre; some will simply “like” this timbre more than other timbres, based on any number of personal experiences or traits. Perhaps the sound of multiple cast-iron bells, even though they do not produce clear chords, is a pleasing sonic experience to certain individuals, but too chaotic for others.

The fourth physical descriptor of sound is time, and its perceptual counterpart is duration. As an object vibrates in time, it may continue to vibrate at the same rate, as in an organ pipe, but it also may quickly or gradually lose energy, as in a plucked guitar string or a struck piano string (the sound energy in a piano string is actually a bit more complex than this, but that’s the basic idea). These correspond to varying combinations of differences in waveform and amplitude, depending on the physical traits of the vibrating object. While the connection between time and duration is basically a direct relationship, duration does vary according to individual perception to a degree. For instance, a person who is hard of hearing will perceive a sound as dying away faster than a person with average hearing capabilities, yet the timing of the physical vibrations of the object are an absolute measurement.

Does a tree falling in the forest make a sound? And if it does, one might ask, how intense was that sound, and what did it sound like, and for how long did the sound continue? The answer may not be quite as set as one might think!
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Copyright © 2014 by André Duvall

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