Sound waves

Introduction

Sounds are waves generated through compression and expansion of any medium that is solid, liquid or gas into certain frequencies. To human ear sound is defined as 20 to 20000Hz. Waves above 20000Hz are called ultrasound and below 20Hz are called infra-sound.

Sound waves are called longitudinal mechanical waves since the medium contents oscillate in the direction of wave propagation.

Figure 1. Longitudinal wave (http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html)

Math

Sound wave has its own intensity which is defined by common word sound level. Sound pressure in any media is not constant. It changes continuously. For that we have acoustic pressure (P) which is the difference between the instantaneous and the average pressure. Particle displacement (ξ) is particles distance of a movement from an equilibrium position (spot where wave hits first). With these two characteristics we can calculate intensity of a sound (I) which is the transferred power per unit area. 

Acoustic pressure:

I=Pξ 

Sound intensity level (SIL) is logarithm of ratio of sound intensity to sound intensity standard reference value (I₀).

Sound intensity standard reference value:

Sound intensity level:

Figure 0. Sound pressure pattern (http://wideawakeparanormal.com/main/images/stories/spl.jpg)

Figure 2. Sound pressure level table (https://www.brechbuehler.ch/typo3temp/pics/e1b7211d3c.jpg)

Sound travelling speed depends on temperature and material. 

Figure 3. (http://www.ctgclean.com/tech-blog/wp-content/uploads/Speed-of-Sound-Table.jpgh  

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Sources

Handbook of Modern Sensors by Jacob Fraden,2010, Springer

http://en.wikipedia.org/wiki/Sound_intensity

http://en.wikipedia.org/wiki/Sound_pressure

 http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

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