If you made it through the first installment of this series, give yourself a high-five.
Better yet, bring those two hands together to create an impulsive sound with a propagating omnidirectional wavefront!
This time, we’ll have a look sound particles, acoustic pressure, propagation medium density, particle velocity, acoustic energy, volume velocity, and acoustic impedance.
Abstract concepts? Nope - far from it. All are related to the physics of sound propagation in an elastic medium that result in changes to the displacement, pressure, density, temperature, and velocity within the medium.
In other words, this is at the basis of everything we hear, the scientific essence of sound.
Sound Particles. Sound requires a medium for the transmission of vibrations, the most common being air. Understanding sound propagation can be difficult because it’s not visible unless special opto-acoustical instruments are used.
One way to “visualize” sound propagation is to imagine the vibrations acting on invisible particles as the vibratory energy passes through a given spatial region. The acoustic “particle” is a small volume unit of air whose physical dimensions are smaller than the propagating sound wavelength. The particles move about a fixed equilibrium position as a function of time as the acoustical energy propagates through the medium.
The collision of neighboring particles transmits energy through the medium. From the standpoint of elementary mechanics, the particles undergo displacement, velocity, and acceleration, just as any moving body does.
Acoustic Pressure. Sound consists of a series of pressure maxima (compressions) and minima (rarefactions). The unit of acoustic pressure (p) is the pascal, abbreviated Pa. Acoustic pressure can be considered as the difference between the instantaneous pressure at a fixed point in a spatial region with the sound source present and with the source absent. The pressure maxima and minima oscillate above and below normal atmospheric pressure (po) in direct response to the acoustic particle motion. (Figure 1)
Figure 1: Fluid particle positiones through one complete oscillatory cycle. Y axis is acoustic pressure with 0 equal to atmospheric pressure, and X axis is distant (or time) corresponding to wave propagation. (Coutesy Pierce)
A certain amount of acoustic pressure is necessary to evoke the sensation of hearing. For individuals with “normal” hearing, and unfortunately this may exclude some of our dear readers, the minimum acoustic pressure necessary for the hearing sensation is 20 X 10-6 Pa (20 μPa).