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)

Sound wave has its own intensity which is defined by common word sound level. Sound intensity (I) is gotten by following pattern:

Sound level is gotten by following pattern:

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