Ultrasound Physics Chapter 2 Sound

Chapter 2: SOUND Waves • waves carry energy from place to place. • Many different types of waves exist: light, sound, heat, magnetic etc. Sound Sound is: a type of wave that carries ENERGY from place to place • Sound does not carry matter or particles it only carries energy • Sound pulses travel through media and return to our machine to create an image • Our machine creates images by sending short bursts of sound into the body Properties of Sound • Sound MUST travel through a medium • cannot travel through a vacuum • Sound travels in a straight line • Sound will travel in a straight line until it encounters something that will change the direction of the sound wave (like bumping into a wall) • Sound waves are also called “acoustic waves” Types of Waves • Transverse wave-particles move or vibrate in a perpendicular direction to the direction of the wave • If the wave is moving right to left, the particles move up and down • If the wave is moving toward toward the sky, the particles would move side to side. Waves • Longitudinal wave- particles move or vibrate in the same direction as the wave • If the wave moves right to left, the particles move right to left • If the wave moves toward the sky, the particles move toward the sky then back towards the ground. Waves • Mechanical wave - particles of a medium move (vibrate back and forth) • http://www.youtube.com/watch?v=Rbuhd o0AZDU Know this! Sound is a mechanical, longitudinal wave • Sound waves are comprised of compressions and rarefactions • Rarefactions-regions of lower particle density and lower pressure • Compressions-regions of higher particle density and higher pressure • In physics we will diagram sound waves. • While in reality, the wave actually looks like the image on the previous slide, we will diagram waves using a much simpler format. • Instead of looking at movement of energy through particles, we create a wave with peaks and troughs. • The peaks are the compressions (or areas of high pressure and particle density) • The troughs are the rarefactions (or low areas of pressure and density) • A “cycle” is one compression and one rarefaction or one peak and one trough. Acoustic Variables • When a sound wave travels, certain properties or aspects of the particles in a medium will change. • These changes are known as acoustic variables. • Sound waves are identified by changes in acoustic variables. • When one of these variables changes, we know that a sound wave is present. • The 3 acoustic variables are: • Pressure • density • distance (particle motion) Acoustic Variables • Pressure: concentration of force in an area • Density: concentration of mass in a volume • Distance: measure of particle motion • When these change, we know that a sound wave is present. Acoustic Parameters After we are aware that a wave is a sound wave, it is important to describe its features and characteristics. 7 Acoustic parameters are used to describe sound waves: • Period • Frequency • Amplitude • Power • Intensity • Wavelength • Propagation speed More on this in Chapter 3 The Acoustic Parameters • Period - The time required to complete a single cycle-can also be described as the time from the start of a cycle to the start of the next cycle • Frequency - The number of cycles that occur in one second • Wavelength – the length or distance of a single cycle • Propagation Speed - the rate that sound travels through a medium. The Acoustic Parameters • Power - the rate of energy transfer • Intensity – the concentration of energy in a sound beam • Amplitude - the difference between the average value and the maximum value of an acoustic variable Phase Relationships • Waves can be either in phase or out of phase. • In phase waves – waves that are synchronized. They peak at the same time. • Out of phase waves- waves that are not synchronized. When one wave is at its peak, the other wave is at its trough or lowest point. Phase Relationships In Phase Out of Phase Phase Relationships • Interference-when two waves overlap at the same location at the same time, they will combine to form one wave • Constructive interference - occurs when the amplitude or size of the new wave is greater than the original wave • Destructive interference - occurs when the amplitude or size of the new wave is less than that of the original waves Interference Waves of Different Frequencies • Frequency – how often cycles occur in a wave • When two waves with different frequencies interfere, at some moments the interference is constructive and at other moments the interference is destructive! So… • When the frequencies of the waves differ, destructive and constructive interference Physics: Sound Biologic Effects/Bioeffects: pg. 11 The effects of the sound wave upon biologic tissue through which it passes. Acoustic Propagation Properties: The effects of the medium upon the sound wave.