hi learners it's m from sano nerds today's video is going to be on unit 2 sound waves before we can understand what ultrasound is we need to take a look at what sound is we will take a look at some basic physical principles behind the concept of sound and be sure to define some key characteristics there are five things that you need to know about sound sound is a type of wave that carries energy sound is a mechanical wave sound is a longitudinal wave sound can only travel in a straight line and that sound cannot travel through a vacuum waves are one way in which energy can be transferred from one location to another in the case of ultrasound the machine sends out a high frequency wave that acoustic energy in the wave is transferred into the patient and then some of that energy is transferred back to the machine to create our images when we are studying ultrasound we will spend a lot of time on how the body affects the wave in energy transfer this is known as acoustic propagation properties but because we are using ultrasound on humans we also need to be aware of how the energy coming from the machine affects the biological tissue this is done through the study of biological effects or bio effects we've covered that waves carry energy and there are many types of waves but the big two are mechanical waves and electromagnetic waves some examples of mechanical waves include sound ropes springs seismic activity and water some examples of electromagnetic waves include light microwaves x-rays infrared and ultraviolet rays and radio waves the big difference between the two is that the electromagnetic waves do not need a medium to travel in they can carry energy through space or a vacuum or a medium mechanical wave must have a medium to propagate energy the mechanical wave carries energy through the particles found in a medium in ultrasound the medium is considered the body there's an old movie from about 1979 called alien and their tag line was in space no one can hear you scream this is how i remember that mechanical waves or sound waves need a medium to travel through so again mechanical waves require a medium where electromagnetic waves can travel through medium and space mechanical waves require a medium to propagate or transfer energy through the particles within the medium are going to move as the energy interacts with them the first type of mechanical wave is a transverse wave this is where particles will move perpendicular to the direction that the wave is moving ocean waves are a type of transverse wave the second type of mechanical wave is a longitudinal wave this is where the particles will move parallel to the direction the wave is moving sound waves are a type of longitudinal wave so to recap we have mechanical waves and electromagnetic waves mechanical waves require a medium electromagnetic waves can travel through medium and space and mechanical waves can be longitudinal waves or transverse waves or electromagnetic waves are just transverse so sound waves are mechanical longitudinal waves that must interact with a medium as the sound wave interacts with the particles within the medium they are going to cause those particles to go through cyclical compressions and rarefactions during compression particle motion is going to cause the particles to squeeze together and when they squeeze together they create areas of high density and high pressure refractions then are when the sound energy is weaker and starts to pull the particles apart in these areas there's going to be low density and low pressure so on the top of this image we have our particles at rest within our medium as the sound energy comes through there will be moments of compression where there is high density high pressure because of the particles being squeezed together and there will be areas of rarefaction where the particles are stretched apart and there's lower density and lower pressure than at rest to identify a sound wave there must be a cyclical change in at least one of the acoustic variables now you've already heard them but the acoustic variables include pressure which is measured in pascals and this is the force in an area density which is mass per unit of volume and density is usually expressed in kilograms per centimeters cubed and particle motion or distance and this is the distance that a particle moves as it interacts with sound energy this is usually measured in a length unit like centimeters millimeters feet or miles we often use sinusoidal waves like this one to represent a sound wave at the top we have peaks and in the bottoms we have troughs the peaks are also known as compressions and this is where the pressure is high and the density is high as the line crosses over the zero line this is the particles at rest so there's no change in pressure no change in density they are at their neutral position and then at the bottom we have rare fraction and this is where there's going to be less pressure and less density now as a side note here i want to note that it's not uncommon for transverse and longitudinal waves to be diagrammed something similar to this and i think that's kind of confusing because we often use diagrams like this to indicate sound waves and those look a lot like transverse waves but i just got done telling you that sound is a longitudinal mechanical wave so what's the deal with that well remember that when we use the sinusoidal waveform to represent a sound wave we are showing the positive and negative change of the acoustic variables when we have compression we have high density high pressure those are going to be expressed as peaks above the zero baseline and when we have low density and low pressure that's going to be expressed below the line at the trough during the rare fraction period so we've learned to identify an acoustic energy wave we must have an oscillation or change cyclically in pressure density and distance that particles move now there's also seven acoustic parameters to describe sound waves and those are going to include frequency period wavelength amplitude power intensity and propagation speed note that i've got these also expressed in sinusoidal waveforms but look at the x-axis and the y-axis depending on how we define that wave in space changes what our waveform is telling us we're going to talk a whole bunch more about these seven acoustic parameters in the next unit the last concept that i want to talk about regarding sound waves is how sound waves can interact with one another when two sound waves are out in a medium together they are going to come in contact with one another and when they do so they do interact and depending on how they interact at the times that they interact and if their compressions line up or if their rare fractions line up it's going to dictate how they interact with one another that interaction is called interference we have two types of interference the first one is constructive interference and this is going to happen when two waves that are in phase so that means that their peaks and troughs line up so that's our a wave and our b wave those are going to add together to make a bigger wave than either one of the original waves so in our example below we've got the a wave plus the b wave and those are going to add together to make a really big c wave the second type of interference is called destructive interference and this is when two waves that are out of phase which means that their peaks and troughs do not light up will subtract from one another making the new waves smaller than at least one of the original waves depending on the strength and the degree to which they are out of phase from one another the waves might look really different from the original waves or just slightly different from the original waves so in our bottom we have wave a out of phase with wave b and they deconstruct to make wave c remember that the deconstructed wave will be smaller than at least one of the components possibly both there is a special destructive interference that can occur though if you have two waves that are of the same strength and frequency and are 180 degrees out of phase then they are going to completely destroy one another and cancel each other out in our example we have an a wave and another a wave that is complete opposite 180 degrees out of phase when those two interact with one another they completely destroy each other and that learners is the end of our introductory sound wave unit make sure to go back to the workbook and work through your activities and go through your nerd check