Chapter 2 - Sound

DMS 216 - Ultrasound Physics

media

biologic tissue

sound

a mechanical wave in which particles in the medium move. the molecules in the medium vibrate back and forth from a fixed position. sound travels in a straight line. sound waves are longitudinal waves. also known as acoustic waves.

compressed

squeezed together

rarefied

stretched apart

acoustic propagation properties

the effects of a medium upon the sound wave

biologic effects

the effects of the sound wave upon the biologic tissue through which it passes

acoustic variables

pressure, density, distance

pressure

concentration of force in an area. Units: pascals (Pa)

density

concentration of mass in a volume. Units: kg/cm cubed

distance

measure of a particle motion. Units: cm, feet, mile

7 acoustic parameters

period, frequency, amplitude, power, intensity, wavelength, propagation speed

transverse wave

particles move in a direction that is perpendicular to the direction that the wave propagates

perpendicular

at right angles, 90 degree angle

longitudinal wave

particles move in the same direction that the wave propagates. sound is a longitudinal wave.

in-phase

when the peaks of a pair of waves occur at the same time and at the same location. similarly, the troughs of the waves occur at the same time and place. in-phase waves are “in step” just like the members of a synchronized marching band.

peaks

maximum values

troughs

minimum values

out-of-phase

when two waves peaks occur at different times, and so do their troughs

interference

when multiple beams arrive at an identical location at exactly the same time. the waves lose their individual characteristics at that moment and combine to form a single wave.

constructive interference

the interference of a pair of in-phase waves results in the formation of a single wave of greater amplitude than either of its components.

destructive interference

the interference of a pair of out-of-phase waves results in the formation of a single wave of lesser amplitude that at least one of its components.

complete destructive interference

when two out-of-phase waves are of equal amplitude, complete destructive interference may occur

constructive

when waves of different frequencies interfere and are in phase

destructive

when waves of different frequencies interfere and out of phase