Designing Effective Speech Interfaces

Other Properties of Sound

In addition to frequency and amplitude, sine waves have several other properties that contribute to the human perception of sound. These properties include phase, timbre, and envelope.

Phase

When two sine waves occur simultaneously, they interact with one another and create a composite sound wave. If the two waves have exactly the same amplitude and frequency, the composite wave will be the sum of the amplitudes—in other words, twice the volume. For example, having two audio speakers instead of one provides double the volume. However, altering the frequency or amplitude of one sine wave would put them “out of phase.”

Decibel Level	Sound
130 dB	Emergency siren; threshold of pain
120 dB	Jet airplane
110 dB	Jackhammer
100 dB	Heavy thunderstorm
90 dB	Motorcycle
80 dB	Lawn mower
70 dB	Heavy traffic
60 dB	Loud conversation
50 dB	Average office
40 dB	Average household
30 dB	Quiet conversation
20 dB	Whisper
10 dB	A pin dropping
0 dB	Absolute quiet; threshold of hearing

Table 3.2 Decibel Level of Certain Sounds

Timbre

The timbre of a sound is its tonal character, sometimes called its tone color. Most sounds are not pure, or single, sine waves. Most sounds are composite waves comprised of a fundamental wave (the sound's lowest frequency) and additional frequencies called harmonics. Additional frequencies change the human perception of tonal character and thus the perception of the overall sound. For example, consider a man singing ...