Designing Effective Speech Interfaces

Measurement of Sound

Sound travels at a speed of 1130 feet per second (called the speed of sound). It travels through the air in a manner similar to waves traveling across the ocean surface. As sound travels, it contracts and expands, disturbing the air pressure around it. The contractions are called compressions and the expansions are called rarefactions. This is because the contractions compress air particles while the expansions cause the air particles to be more rare in that physical space. These compressions and rarefactions form a sound wave, or sine wave named after the trigonometric function. The sine wave is typically represented as a graph showing increases and decreases in atmospheric pressure over time, as shown in Figure 3.1. The center line of the graph represents normal atmospheric pressure. Compressions, the increases in pressure, are called peaks. Rarefactions, the decreases in pressure, are called troughs. The distance between a peak and a trough is a single wavelength, also known as a cycle.

Frequency

The number of cycles a wave completes in one second is called its frequency. An example of a 4-cycle wave is shown in Figure 3.2. The shorter a wavelength is, the higher its frequency. The longer the wavelength is, the lower its frequency. Frequency is measured in hertz, which means cycles per second. This is abbreviated as Hz. One thousand hertz is one kilohertz, abbreviated kHz. The range of sound, or vibrations the human ear is capable of hearing, is between ...