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Current time:0:00Total duration:5:35

Video transcript

the human ear can hear frequencies from around 20 Hertz to about that would be 20 Hertz is a very low base to up to around 20,000 Hertz this is way up there and if it's a frequency above this this is the range we can hear if the frequency lies above this range we give it a special name we call it ultrasound or ultrasonic and this does more than just annoy animals this has a practical purpose if you wanted to image do some medical imaging or figure out what's going on in the human body so say here's a portion of the human body and there's may be a vital organ or some tissue here or some tissue over here you're worried something's going wrong you got to figure out what's going on inside you can operate but that obviously sucks you want to try to avoid an operation if possible you can do x-ray but too much exposure to exposure to radiation is bad too so a very good option is usually ultrasounds we can take a what's called a transducer you put that transducer up to the skin this transducer takes electrical energy you plug it into the wall turns it into sound energy you send out sound waves you send out a pulse so this transducer sends out a pulse this pulse travels toward anything in here and it turns out it will reflect it will reflect anytime there's a difference in the medium so anytime there's an interface between the two media which in this case will make it simple let's just say there's tissue from blood or other things or sorry tissue from organs and then the red will represent the blood so this is going to keep traveling here it keeps traveling once there's an interface here between blood and tissue it'll reflect comes back and this transducer is always timing its nose when it's sent out the pulse and it knows knows when that pulse got reflected back it also knows the speed of sounds so it can calculate all right if it took that long to get back it must have reflected this far away something's at this point and that's not done yet though this way it's going to travel on in fact most of this wave travels through keeps going here's another interface between tissue and blood so it's going to reflect again this reflects back we'll get another pulse this is at some later time and the transducer knows all right took that long now there must have been something else there my one pulse got reflected two times so there's something here and here's the end of it but that's not done either this keeps on going it will reflect against this interface between blood and tissue I'm drawing these sound waves crooked just so you can see them they'd really be right on top of each other along this line but that takes another amount of time so it keeps doing this and it knows that you'll have points right here difference between interfaces right here and interface between two different tissues and you can get a image of this whole cross-section if you just have a transducer that sends out pulses along this whole face of the transducer you can image this whole region so you can start imaging all these points you can figure out what is inside of here what's the shape of it what are any particular lesion Zoar lumps going on inside of here so that's ultrasound that's one way it's useful it actually uses ultrasound frequencies you might wonder why why would we have to use ultrasound well I mean one reason why is if you took this transducer and you were using audible frequencies you take this noisy thing you hold it up to a patient that patient is going to be like you sure that's okay to hold up to me doc that might be upsetting but another more practical reason for using ultrasound is high frequencies and that is to say low wavelengths and these two are the same because remember speed of a wave is wavelength times frequency so if the frequency is high the wavelength is low because the speeds are not determined by either of those the speed is determined by the medium itself it turns out for high frequency low wavelength you get less diffraction and diffraction is an enemy of making clear pictures because what diffraction is is this is a spreading out of waves so if I had my wave coming in here wave coming in and there was some sort of barrier let's say this barriers right here and I've got a small hole in it waves spread out but if it's a high frequency wave it won't spread out much it's going to enter through this hole and it'll spread out a little bit it's going to get a little bit wider but if it were a low frequency may be audible region high wavelength the spreading would be bigger and this would be a problem because if it spreads out think about it if this wave was coming in here and this wave is coming in here and then it started it curves around corner so another thing diffraction does is it causes waves to curve around corners the spreading happens now you've got all this bending of sound waves sound waves reflecting off of things confusing the transducer you get a blurry image that's why we want to use high frequencies there's less diffraction you get a clearer image so this is one application of ultrasound for medical imaging