Sound refers to mechanical vibrations of particles of an elastic medium (air, water, metal, etc.), which are subjectively perceived by the organ of hearing. Sound sensations are caused by fluctuations in the environment, occurring in the frequency range from 16 to 20,000 Hz. Sounds with frequencies below this range are called infrasound, and above, ultrasound.

 
Sound pressure - variable pressure in the medium due to the propagation of sound waves in it. Sound pressure p estimated by the strength of the sound wave per unit area and expressed in newtons per square meter (1 n / meter square = 10 bar).
 
Sound Pressure Level - Ratio of Sound Pressure p to zero, for which sound pressure is taken p_0 = 2 * 10 ^ {- 5} n / square meter:
 
N = 20log (\ frac {p} {p_0})
 
The speed of sound depends on the physical properties of the medium in which mechanical vibrations propagate. So, the speed of sound in air is 344 m / s at T = 20 ° C, in water 1,481 m / s (at T = 21.5 ° C), in a tree 3,320 m / s and in steel 5,000 m / sec
 
Sound power (or intensity) - the amount of sound energy passing per unit time through a unit area; measured in watts per square meter (w / m2).
 
It should be noted that sound pressure and sound power are related by a quadratic dependence, i.e., when the sound pressure is 2 times higher, the sound power increases 4 times.
 
Sound power level - the ratio of the strength of a given sound I to zero (standard) level, for which sound power is taken I_0 = 10 ^ {- 12} W / m2, expressed in decibels:
 
N = 10log (\ frac {I} {I_0})
 
Sound pressure levels and sound strengths, expressed in decibels, are the same.
 
The threshold of audibility is the quietest sound that a person can still hear at a frequency of 1000 Hz, which corresponds to sound pressure 2 * 10 ^ {- 5} n / m2.
 
Sound volume - the intensity of the sound sensation caused by a given sound in a person with normal hearing. Volume depends on the strength of the sound and its frequency, changes in proportion to the logarithm of the strength of sound and is expressed by the number of decibels by which the sound exceeds the sound accepted for the audibility threshold. The unit of volume is the background.
 
Threshold of pain - sound pressure or sound strength, perceived as pain. The threshold of pain does not depend much on frequency and occurs at a sound pressure of about 50 n / m2.
 
Dynamic range - the volume range of sound, or the difference in sound pressure levels of the loudest and quietest sounds, expressed in decibels.
 
Diffraction - a deviation from the linear propagation of sound waves.
 
Refraction - a change in the direction of propagation of sound waves caused by differences in speed at different sections of the path.
 
Interference is the summation of waves of the same length arriving at a given point in space along several different paths, as a result of which the amplitude of the resulting wave at different points is different, with the maxima and minima of this amplitude alternating among themselves.
 
Beats are the interference of two sound vibrations that differ little in frequency. The amplitude of the oscillations arising from this periodically increases or decreases in time with a frequency equal to the difference of the interfering oscillations.
 
Reverb - residual “after-sounding” in enclosed spaces. It is formed due to repeated reflection from surfaces and the simultaneous absorption of sound waves. Reverb is characterized by a period of time (in seconds) during which the strength of sound decreases by 60 dB.
 
Tone is a sinusoidal sound wave. Pitch is determined by the frequency of sound vibrations and increases with increasing frequency.
 
The fundamental tone is the lowest tone created by the sound source.
 
Overtones - all tones, except the main one, created by the sound source. If the frequencies of the overtones are an integer number of times greater than the frequency of the fundamental tone, then they are called harmonic overtones (harmonics).
 
The timbre is the “coloring” of sound, which is determined by the amount, frequency and intensity of overtones.
 
Combination tones - additional tones arising due to the nonlinearity of the amplitude characteristics of amplifiers and sound sources. Combination tones appear when two or more oscillations with different frequencies are exposed to a system. The frequency of the combination tones is equal to the sum and difference of the frequencies of the fundamental tones and their harmonics.
 
Interval - the ratio of the frequencies of two compared sounds. The smallest distinguishable interval between two adjacent musical sounds in frequency (each musical sound has a strictly defined frequency) is called a semitone, and the frequency interval with a 2: 1 ratio is called an octave (a musical octave consists of 12 semitones); an interval with a ratio of 10: 1 is called a decade.
 
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