关于声音的几个基本概念:

What is Sound(何为声波)?

A propagating sound wave consists of alternating compressions and rarefaction's which are detected by a receiver as changes in pressure. Structures in our ears, and also most man-made receptors, are sensitive to these changes in sound pressure (Richardson et al.1995, Gordon and Moscrop 1996).

表征声波的参数主要有三个：振幅(amplitude), 波长(wavelength),频率（frequency）

　　振幅是声波能量的体现，小的振幅表明声音很弱或声波传的距离很近，小的振幅表明声音很强或声波传的距离很远。The wavelength（波长）of a wave is the distance between two successive compressions or the distance the wave travels in one cycle of vibration. The frequency of a sound wave is the rate of oscillation or vibration of the wave particles (i.e. the rate amplitude cycles from high to low to high, etc.). Frequency is measured in cycles/sec or Hertz (Hz). To the human ear, an increase in frequency is perceived as a higher pitched sound, while an increase in amplitude is perceived as a louder sound. Below are examples of sound waves that vary in frequency and amplitude.

Note that increasing the frequency of a sound in equal steps will lead to perceived increases in pitch that seem to grow smaller with each step. For example, click on the sound frequencies below, and you'll see a more noticeable difference between 200 Hz and 225 Hz than 400 Hz and 425 Hz.

Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic.

infrasonic (about 20 Hz) < human hearing < ultrasonic (about 20,000 Hz)

The Decibel Scale声波的度量单位(分贝dB)

If the amplitude of a sound is increased in a series of equal steps, the loudness of the sound will increase in steps which are perceived as successively smaller. Sound intensity is generally described using logarithmic units called decibels (dB). On the decibel scale, everything refers to power, which is (amplitude)² ; 0.0 dB corresponds to about the normal threshold of hearing and 130 dB to the point where sound becomes painful to humans.

Warning: noise levels cited in air do not equal underwater levels for reasons that will be described in the following sections.

Why use the decibel scale? Because sound "loudness" varies exponentially, we'd have to deal with a lot of zeros when doing computations involving the parameters of sound, and we'd have to multiply numbers rather than simply add and subtract them. By using the decibel scale, calculations are simplified and relative values relate more closely to perception.

Phase声波的相位

A fourth property of sound, its phase, is less directly related to perceived sound intensity. Phase is important in describing how complex sounds can be constructed from the simple sinusoidal waves. Below is an example of two sound waves with the same frequency and amplitude - only their alignment with respect to time differs. By specifying amplitude, wavelength, and phase, any sinusoid can be exactly described. By describing these parameters for all frequency components, any complex signal can be described exactly.