Full transcript
0:00we don't have a lot to cover today
0:03because we covered reflection as you
0:05remember specular we have three forms of
0:08reflection specular reflection which
0:10needs a large smooth interface and an
0:12impedance mismatch diffuse reflection
0:15which is also called scattering which
0:16needs a largely irregular surface but
0:20also an impedance mismatch and of course
0:25Rayleigh scattering which is it is a has
0:29an impedance mismatch but the objects
0:31are so small that oftentimes no
0:33reflection actually occurs but some
0:35reflection could occur and then we
0:38talked about refraction which is the
0:41bending of sound which is based on the
0:43velocity mismatch between one medium and
0:46the next we mentioned that there are
0:49three types of refraction one if the
0:54medium velocity and the velocity medium
0:56two is greater that it will create a
1:00angle of transmission greater than the
1:03incident angle and that will Bend away
1:06from line norm which is that fictitious
1:08line drawn perpendicular to the
1:10interface if the velocity medium too is
1:13slower it'll Bend back towards line norm
1:15and create a less angle of transmission
1:17and then case three Snell's law says if
1:21the angle of incidence exceeds the
1:23critical angle which in most part is 22
1:26degrees or greater and the velocity
1:29medium - that's greater that it will
1:30skip across the surface causing total
1:33internal reflection so that was kind of
1:35a summary of what we discovered last
1:37week everybody's up to speed on that
1:40right crazy
1:41another interaction with matter is
1:43diffraction diffraction is what happens
1:47when a wave travels through an aperture
1:50and aperture is simply a fancy word for
1:53narrowing sorry so what happens is if we
2:03were sending a sound wave towards a
2:08narrowing of some sort
2:10now obviously some of this wave energy
2:14I think of it like a water wave energy
2:16if these were waves hitting two objects
2:18some of that energy would be bounce back
2:20from these two objects but some of that
2:23energy would make through this narrowing
2:26or aperture after it makes it through
2:29the two options are that it just
2:32continues on this very extremely focused
2:34beam but diffraction and like and the
2:38concept of divergence is that sound as a
2:40natural property spreads out the further
2:43it travels from its source creating this
2:47pie shaped image so divergence is a
2:50natural property diffraction is what
2:52happens when the wave energy pressure
2:56enter the energy travels through and a
2:58narrowing or aperture so divergence or
3:02diffraction is simply the spreading of
3:03sound the further travels from the
3:06narrowing or the further travels from
3:09its source all right I just want to make
3:12sure this case is okay yeah we got that
3:17so far all right another interaction
3:19with matter is interference so as we see
3:26here we have where two ways that there
3:50compression cycles and rarefaction
3:53cycles match we will produce a higher
3:55amplitude wave think of it this way to
3:59water waves come together what happens
4:01they splash together creating a huge
4:04wave right if they're in what they call
4:07it a summation of waves if they are in
4:10phase with each other so the compression
4:12waves here and here
4:14match and there rarefaction portions
4:17match exactly the same time we produce a
4:20much higher amplitude signal this is
4:23true in home stereo equipment if you
4:26turn on your
4:27your home stereo and only have one
4:31speaker playing then go in and plug in
4:33the other space speaker you will notice
4:35that your sound is twice as loud not
4:38because you turned off your volume or
4:40the amplitude applied to that one
4:42speaker it's because you have two of
4:44them pushing particles at the same time
4:47it becomes much louder if you hooked up
4:50another four speakers it becomes even
4:52louder if you look up another eight
4:54speakers not because you're pushing out
4:56any more energy it's just because you
4:57have more things doing work so so this
5:01is called constructive interference the
5:04other thing that happens is destructive
5:08interference it must be endless the next
5:09page yes if they're out of phase with
5:14each other
5:14and that we have a compression zone here
5:18matching a rarefaction zone and vice
5:22versa rarefaction meeting freshen zone
5:24it completely annihilate the energy
5:27together you can also try this with your
5:32home speakers if you reverse the
5:34polarity in your speakers this has
5:38happened to me once back in the day when
5:42I was in high school yes it happened I
5:45can remember actually going and
5:47purchasing a brand new stereo for my car
5:51where my stereo was worth much more than
5:53my entire cart together and I purchased
5:58new speakers and a new stereo for it and
6:00back then you have you dropped your car
6:03off for days to install your stereo
6:06today you just drop it off and it's done
6:07within a couple hours but you would
6:09leave your car for days for them to
6:11install a stereo in your car but when I
6:13picked it up I spent all his money on
6:15the stereo and it was not loud or
6:22earth-shattering like I thought well
6:23even my previous stereo would shake the
6:27entire car but this one nothing happened
6:30so I immediately turned around said put
6:32my own stuff back I can't handle this
6:34they spend all this money and it sounds
6:35like crap so the guy turn down the
6:38stereo pop my trunk and went back
6:40and switch the polarity of just one
6:42speaker and I swear I thought my windows
6:46were literally gonna shatter because
6:48while one speaker was pushing pressure
6:52the other one was sucking it out
6:54they were completely annihilating each
6:56each other's energy now you heard some
6:59sound but it was not very clear because
7:01they were interacting with each other
7:03but there is absolutely no puncher base
7:06to it which is primarily what I
7:07purchased it for but it has everything
7:10to do that the speakers were D
7:12constructively interfering with each
7:14other's energy therefore literally
7:16making it a lower you turned it up the
7:19less you could actually hear which is
7:21really weird so D constructive
7:24interference is another way that sound
7:27can interact with another sound wave so
7:30as we send sound waves out to the
7:32patient as other sound waves are coming
7:34back they can hit each other and
7:36completely annihilate each other out in
7:38fact when you get into the second
7:40physics course we talked about this is
7:42actually something we do on purpose we
7:45will actually send waves out of
7:47different faces in order to interact
7:49with certain frequencies we don't want
7:51to hear and purposely interfere with
7:55those frequencies coming back so they
7:57never make it back to the transducer so
7:59they're not process it's pretty
8:00complicated stuff but we'll talk about
8:03that in a second physics course the
8:06other two interactions with matter we've
8:09discussed a little bit before we talked
8:13about attenuation here this is the
8:17gradual reduction of energy as the sound
8:21propagates through a medium I explained
8:24if I had a stereo here and it was
8:26playing it on a volume v and then I
8:28walked away from you the farther and
8:30walk away from you the less and less we
8:32would hear correctly remember that that
8:34that process associated with with
8:38distance specifically is attenuation so
8:43it says we lose 0.5 to 1.0 decibels in
8:47energy every centimeter we travel for
8:51every megahertz we use the megahertz
8:54again is of course related to
8:56frequencies and how higher frequencies
8:57attenuate much quicker and lower
9:00frequencies do not attenuate and that's
9:02why you hear lower frequencies for
9:04longer periods of time eventually I said
9:08there'd be a point that I walked so far
9:10away from you that you would not hear
9:13the sound coming from that radio that is
9:17known as absorption when the energy no
9:21longer exists because it's completely
9:23attenuated within the medium that's
9:26known as absorption an ultrasound
9:30absorption leads to heat and then the
9:35side note this is why some philosophers
9:42including Oh drawing blank on this named
9:48crazy hair guy mustache I'm fine thank
9:51you
9:51why am i drawing a blank on them
9:54Einstein suggested that this is one of
9:56the main reasons there must be life
9:59after death because energy doesn't cease
10:02to exist it simply changes from one form
10:04to another and an ultrasound this
10:08pressure wave changes to heat it stops
10:12existing as a pressure wave and now
10:15exists as heat so just a little bit of
10:20interesting tidbit hopefully that will
10:21help you remember what ultrasound turns
10:25into and it completely absorbs the
10:27process of attenuation is the gradual
10:29reduction until it's completely
10:30annihilated within the medium and that's
10:33called absorption and that I believe in
10:39what 15 minutes finishes chapter 3 and
10:44all the interactions reflection they are
10:47specular diffuse reflection and Rayleigh
10:50scattering
10:51there's Snell's law which describes
10:54refraction which is the bending of sound
10:56of which there are three different types
10:58you should know what the three types are
11:02causes them and what their results are
11:07discuss divergence and diffraction which
11:09are similar just the spreading of sound
11:11as a moves from its source or from an
11:13aperture or narrowing constructive and
11:16destructive interference as well as the
11:18ten uation and absorption as long as you
11:21understand all of those interactions
11:23this is the shortest chapter in this
11:26semester questions are endless about any
11:31of that information
11:35no is there more that's it
11:43wait well some other things we have to
11:50do today one I know next week is spring
11:54break but when you come back from Spring
11:57Break is your midterm examination really
12:00you're excited about that these midterm
12:03you're like halfway through yeah I'm
12:05halfway through yeah like you know
12:12that's part of what we have to do today
12:14is that you will have your studying out
12:15I decided this time to give you all of
12:21the questions on the midterm so you'll
12:22come all those to study over spring
12:25break and when we come back the week
12:27after that okay the next thing is I was
12:30notified that you were randomly selected
12:32to participate in a research assignment
12:36that may take up to 45 minutes to
12:38complete yay so I have to hand out and
12:42administer that to you today wait what
12:45yeah wait what exactly but before we get
12:48there I need to know yeah Morgan has any
12:52additional questions or whether Kim has
12:55questions from her online group that I
12:57need to answer and why they're thinking
12:59of that I need you to ask me any
13:00questions related to chapter three so we
13:03can shut down this this webinar and then
13:06move on to the other business so do you
13:08have any questions about chapter three
13:11no one here not here your students are
13:15good Kim yep okay or no all right more
13:20do you have any questions okay all right
13:25and I'm going to close this down and
13:27then we'll go to that research
13:30assignment and then I'll hand out your
13:31practice test okay so you see guys