Sound: Encyclopedia II - Sound - Attributes of sound

Sound - Attributes of sound

The characteristics of sound are frequency, wavelength, amplitude and velocity.

Sound - Frequency and wavelength

The frequency is the number of air pressure oscillations per second at a fixed point occupied by a sound wave. One single oscillatory cycle per second corresponds to 1 Hz. The wavelength is the distance between two successive crests and is the distance that a wave travels in the time of one oscillatory cycle.

The wavelength of a sound wave of frequency f and travelling at speed c is given by c/f. Given a speed of 343 m/s, a 20 kHz sound wave has a wavelength of about 17 mm. For comparison, an A440 has a nominal wavelength of about 78 cm, and a 20 Hz sound wave has a wavelength of 17 m.

Suppose sound is emitted as a sine wave travelling outward spherically from a point source. The pressure (above ambient, see gauge pressure) of the sound wave can be written as

where P(r,t) is the pressure at distance r at time t, P₀ is the amplitude of the pressure variation (0 to peak), f is the frequency of oscillation, and c is the speed of sound.

Sound - Amplitude

The amplitude is the magnitude of sound pressure change within the wave, or basically, the maximum amount of pressure at any point in the sound wave. A sound wave is caused literally by increases in pressure at certain points (of a material) causing a "domino effect" outward, the high pressure points are the crests mentioned above, and behind them are low pressure points which tail them, those are the troughs mentioned above. Amplitude is the maximal displacement of particles of matter that is obtained in compressions, where the particles of matter move towards each other and pressure increases the most and in rarefactions, where the pressure lessens the most. See also particle displacement and particle velocity. While the pressure can be measured in pascals, the amplitude is more often referred to as sound pressure level and measured in decibels, or dBSPL, sometimes written as dBspl or dB(SPL). When the measurement is adjusted based on how the human ear perceives loudness based on frequency, it is called dBA or A-weighting. See decibels for a more thorough discussion.

• Chart showing decibel level of many sounds

Sound - Velocity

Sound's propagation speed depends on the type, temperature and pressure of the medium through which it propagates. Under normal conditions, however, because air is nearly a perfect gas, the speed of sound does not depend on air pressure. In dry air at 20 °C (68 °F) the speed of sound is approximately 343 m/s (approximately 1 meter every 2.9 milliseconds). The speed of sound relates frequency to wavelength. Thus, a tone of 343 Hz (F4 minus 31 cents) traveling in air has a wavelength of 1 meter.

Adapted from the Wikipedia article "Attributes of sound", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki