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Physics Module 1


sound the transmission of mechanical vibrations through a medium
wave a mechanism which transfers energy, not matter
compression region of highest acoustic variables (pressure, density, temp, & particle motion)
rarefaction region of lowest acoustic variables (pressure, density, temp, & particle motion)
mechanical waves need a medium to propagate (sound is an example)
electromagnetic waves able to propagate in either a medium or a vacuum (light, heat, gamma rays, TV signals are all examples)
transverse waves particle motion that is PERPENDICULAR to the wave propagation direction
longitudinal waves particle motion that is PARALLEL to the wave propagation direction; moves along/in the same direction
acoustic variables mechanical changes in the medium that are measured in 4 physical quantities
pressure (P) force/unit area
acoustic pressure the force of compression applied to a unit area of the medium by the sound wave
density (p) mass per unit volume
temperature the degree or intensity of heat present in a substance or object
particle motion particles vibrate back & forth in their original position & do not travel with the wave
frequency (f) a measure of how many times an event occurs in time
infrasound below human's ability to hear (0 Hz - 20 Hz)
audible sound 20 Hz - 20 KHz
ultrasound above human's ability to hear (>20 KHz)
diagnostic ultrasound 2 MHz - 12 MHz
period (T) time needed to complete one cycle
wavelength the physical distance one cycle occupies
propagation velocity (c) the distance sound can travel per unit of time
amplitude (A) the maximum variation of an acoustic variable from its mean value; a measure of how big or how strong
power (W) the rate at which energy is transferred
intensity (I) the concentration of power per unit area
elasticity the ability of an object to return to its original shape after being mechanically deformed by an external force
compressibility a measure of how much the volume of an object changes when pressure is applied to it
density mass/unit volume
stiffness the inverse of compressibility
bulk modulus similar to stiffness
decibels (dB) units used to measure the intensity of a sound by comparing it with a given level on a logarithmic scale
reflection a change in direction of the propagating acoustic energy as it strikes the boundary between two mediums & returns to the source
specular reflection occurs when an ultrasound wave is incident on to a large, smooth surface
diffuse reflection occurs when the ultrasound wave is incident on to a rough, irregular surface
Rayleigh scattering occurs when the reflecting structures are much smaller than the wavelength of the incident ultrasound wave
acoustic impedance the resistance to ultrasound travelling through a medium
angle of incidence formed between the direction of the ultrasound wave (beam direction) & the line perpendicular to the interface
normal incidence occurs when the ultrasound beam is orthogonal (perpendicular) to the interface
oblique incidence occurs when the ultrasound beam is not orthogonal to the interface
refraction occurs when an incident ultrasound beam changes direction or bends on the transmission side of the interface
angle of transmission the angle made by the direction of the transmitted ultrasound wave (beam direction) and the perpendicular to the interface (normal line)
Snell's law the ratio of propagation speeds for two mediums in contrast is proportional to the ratio of the Sines of two angles: the angle of incidence & the angle of transission
degree of refraction the absolute difference between the incident and transmitted angles
critical angle incident angle at which total internal reflection occurs
attenuation the reduction in intensity & amplitude of an ultrasound wave as it travels through a medium
attenuation coefficient the amount of attenuation per cm (unit dB/cm) per MHz of frequency
half intensity depth depth at which the intensity is reduced to 1/2 of its original value
simple harmonic motion the vibration of a source to create motion
meter the distance light travels in a vacuum
acoustic intensity acoustic power per unit area of the sound beam
transmission portion of the energy transmitted through an interface
Created by: ginaliane
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