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October 17, 2006
Back to the Basics: Audio in the 21st Century - Part 2Scott Janus
In part 2 of Scott Janus' book serialization, we learn more about waveforms and their characteristics. This installment: period, frequency, phase offset, phase difference, phase quadrature, in phase, out of phase, and phase opposition.
Editor's note: This article and the subsequent articles provide an introduction to Sound. It is excerpted from the book Audio in the 21st Century published by Intel Press. This excerpt is from Chapter 2 of the book. All material is copyright Intel.Waveforms Now suppose we were to pick an arbitrary point in the pipe above and record the pressure over time. At first, we would just see the ambient atmospheric pressure. The pressure would then increase as the first compression region passed over the test point. Next, the pressure would decrease to below the ambient. If we were to plot our data, the result would look like Figure 2.3. Graphs such as this one in which we plot the pressure of a sound wave versus time are known as waveforms. We will be using them frequently as we discuss audio.  Figure 2.3 Waveform From the Pipe Pure Tones A simple type of sound is the pure tone. A pure tone is a sound with sinusoidal pressure fluctuations. In other words, the pressure waveform is a sine wave, as shown in Figure 2.4.  Figure 2.4 Pure Tone Waveform
The waveform of this pure tone has several char-acteristics of interest to us. The first is the magnitude. Zero amplitude in this case refers to the ambient pressure. Points of the sine wave with posi-tive amplitude indicate increased pressure or compression. Points with negative amplitude indicate decreased pressure or rarefaction.
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