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Historically, loudness was measured using an ear-balancing method with an audiometer in which the amplitude of a sine wave was adjusted by the user to equal the perceived loudness of the sound being evaluated. [6] Contemporary standards for measurement of loudness are based on the summation of energy in critical bands. [7]
In air at ambient temperature, z 0 = 410 Pa·s/m, hence the reference value I 0 = 1 pW/m 2. [7] In an anechoic chamber which approximates a free field (no reflection) with a single source, measurements in the far field in SPL can be considered to be equal to measurements in SIL. This fact is exploited to measure sound power in anechoic conditions.
At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. In a similar way, compression waves in solids depend both on ...
Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m (56 ft) to 17 mm (0.67 in). Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.
In a sound wave, the complementary variable to sound pressure is the particle velocity. Together, they determine the sound intensity of the wave. Sound intensity, denoted I and measured in W·m −2 in SI units, is defined by =, where p is the sound pressure, v is the particle velocity.
This may be pictured as the superposition of two waveforms of arbitrary profile, one traveling up the x-axis and the other down the x-axis at the speed . The particular case of a sinusoidal wave traveling in one direction is obtained by choosing either f {\displaystyle f} or g {\displaystyle g} to be a sinusoid, and the other to be zero, giving
Equal-loudness contours. The phon is a logarithmic unit of loudness level for tones and complex sounds. Loudness is measured in sones, a linear unit.Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may present an identical sound pressure to a human ear, they may be psychoacoustically perceived as differing in loudness.
Quantity Unit Remarks Name Symbol Definition Name Symbol period, periodic time: T: time of one cycle: second: s: frequency: f, ν: f = 1/T: hertz: Hz: 1 Hz = 1 s −1: loudness level: L N: L N = ln(p eff /p 0) 1 kHz where p eff is the root-mean-square value of the sound pressure of a pure tone of 1 kHz, which is judged by a normal observer under standardized listening conditions as being as ...