This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Sound and the Auditory Waveform

The experience we call sound results from our detection of pressure changes in a medium such as air In a diagram of a pressure wave, peaks represent moments of relatively high pressure or compression of the air molecules. Valleys represent low pressure or partial vacuum, which is called rarefaction. A vibrating string in a piano produces alternating zones of compression and rarefaction in the air, resulting in sound.


Properties of a sound wave

What produces sound? What does Hz mean?

The distance from one peak to the next is a single cycle. A single cycle per second is called one Hertz (abbreviated Hz) after the German physicist Heinrich Hertz (1857-1894).

How do changes in amplitude or frequency affect sound? Why do piano tuners tune "by ear"?

Amplitude of a waveform-height on a graph, or amount of pressure change-correlates with loudness of a sound, although there are numerous other factors that affect perceived loudness of sounds. The wavelength or frequency of the wave in cycles per second affects the pitch of a sound: how high or low it sounds. As the frequency goes up, the sound becomes higher; as the frequency goes down, the sound becomes lower. But there are other factors besides frequency that affect perception of pitch. That is why many professional piano tuners prefer to tune a piano by ear instead of by using an electronic instrument. An electronic device would be more precise, but it might not produce a piano that sounded "in tune" to the human ear.

What is timbre?

Recall that a third characteristic of light (after brightness and hue) was saturation, corresponding to purity or complexity of the waveform. Similarly, in analyzing sounds, the word that labels the purity or complexity of the sound wave is timbre, pronounced TAM-ber. It refers to quality of a sound, as distinct from its loudness or pitch. Timbre is shown on a graph as the shape of a waveform. For example, a flute produces a very smooth sound that appears on an oscilloscope (an instrument that displays wave form) as a perfect sine wave: a smoothly rolling waveform. A piano, by contrast, produces a highly complex mix of different frequencies that looks very noisy on an oscilloscope. This happens because the timbre of a piano is very complex.

Why is a good recording of a piano useful for testing a music system?

Because the timbre of a piano is complex, it is difficult to reproduce faithfully. One way to test the fidelity of reproduction in a good music system is to play a good quality CD of a solo piano performance. Lean back, close your eyes, and see if the piano sounds realistic, as if it is in the room. If it does, you have a good music system, capable of reproducing highly complex timbres.


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