Basics of Sound

by Jared Rovny

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    00:02 Now that we've discussed the basic properties of waves as well as periodic motion.

    00:06 We're ready to discuss a particular example of periodic motion and waves which is sound.

    00:12 So let's start with talking about the physical basis of sound.

    00:15 How we measure sound, as well as some of the basic properties of sound.

    00:19 Again to contextualize what we've done is covered many of the mechanics as well as a few applications including fluids and electromagnetism.

    00:27 Since we've introduced waves, we're going to talk about sound before getting to light and then again we'll get into the smaller structure and the atoms and molecules.

    00:35 But first with sound, what we're going to do is first in this lecture talk about the basics of sound.

    00:40 Again, just how it's measured, how it's talked about and then we'll talk about some more complicated sound dynamics and how to measure a more complicated properties of sound.

    00:48 But first with the basics, what is sound? We can ask ourselves we're hearing all these things all the time and many instruments and music is obviously more and more prominent and important to our lives but where is it coming from? How does it get from one place to our ears and how do we make particular kind of sound and different frequencies and putting all those altogether? What we can do is once again imagine for ourselves maybe a guitar string.

    01:13 So we grab the string and we pluck it.

    01:15 What happens is as it moves from left to right so you can see here in this picture, guitar strings that's pinned at the bottom and pinned at the top.

    01:22 It's going to vibrate and move left to right.

    01:25 As it does that it pushes the air as it moves towards the air and it does what we say compress the air.

    01:32 We call it a compression.

    01:33 So this is a compressed amount of air and then as the string moves back away from the air.

    01:38 We have what's called a rarefaction making the air less dense and so a compression makes the air more dense as it pushes the air and then when the string moves back we have what's called a rarefaction which means less dense air.

    01:50 And as the string vibrates very, very quickly it creates many, many, many of these compressions and rarefaction is dense and less dense waves of air.

    02:01 What we can see here, is just based on our definition of the two different type of waves.

    02:06 This is a longitudinal wave because the motion of the molecules is left and right in this picture and the motion of the wave is also left to right, and so sound is a longitudinal wave.

    02:18 The speed of sound as we talked about with any wave only depends on the medium through which the sound is traveling.

    02:25 And so the sound is just traveling through air, it always has a particular velocity and so you can always know how quickly sound is traveling without having to know how fast the source was moving or whether it was made by a car or anything moving fast or slow.

    02:39 The speed will always be the same.

    02:41 The speed for sound and air is about 340 meters per second.

    02:46 It turns out that again since this speed depends on what medium you're in, what you're traveling through.

    02:52 Sound actually travels much more quickly when you go to liquids or solids.

    02:56 And there's difference in the speed sound through different materials.

    03:00 It turns out there's very important not only to the sciences, we use this as different vibrational waves or sound waves travel through the earth.

    03:07 Whether it's caused by earthquake or something else and we can measure things about the internal structure of the earth and in fact this is how we know what the structure of the earth is.

    03:16 But aside from the physical sciences we also can use this effect to study the human body.

    03:22 We can send little sound waves into the human body and then we can see what comes out or what comes out the other side and listen to it and by just simply doing the kinds of math and physics that we're introducing here.

    03:33 We can analyze whether there might have been more solid structure or less dense structure just based on how the waves of sound move through the body.

    03:42 And so there's actually a pretty dramatic clinical significance to the effects that sound has as it moves through different kinds of mediums, different kinds of materials.

    03:50 The energy from a sound wave as the sound moves out from the source of that sound is going to be spread out.

    03:59 It's going to be spread out across the surface area of the sphere.

    04:02 So you can think about the sound emanating from some source no matter what it is, that's a motion of a molecule, its vibrating molecules and those molecules can bump around in anyone of the three dimensions and so this energy is being spread and propagated in all three dimensions.

    04:17 When this happens because the energy has to be spread out across the surface area of the sphere as its traveling outwards.

    04:24 This energy is also conserved and so what we can say is that as the wave, the sound wave travels in all three dimensions and we know the surface area of the sphere depends on the square of the radius of that sphere.

    04:39 We can also say that the intensity or the energy of that sound drops as the square of the distance that you are away from the sound.

    04:47 So for example if you move twice as far away from a source of sound, you would in fact sound one fourth as loud because again the intensity of the sound will drop with the square of the distance rather than with the distance directly for the reasons we just described.

    05:02 The quartering of this sound being as loud is actually very important for the way our ears work.

    05:08 The way humans perceive sound is by distance.

    05:12 So if I move twice as far away, the way I would perceive the sound is so that my brain and I know that the source of the sound is twice as far away.

    05:20 But we just said that the intensity of that sound how loud that sound is, is one quarter as loud.

    05:27 So there's difference the fact that we want to perceive sounds based on distance so that we can know how far away something is.

    05:32 Comparing that to the fact that the energy is only a quarter as loud has some ramification for how we discuss the intensity of the sound coming to us and for that reason we need a measuring scale that expresses this as a power law.

    05:46 So that we reiterate that because it might be a little bit of a complicated point.

    05:49 My perception of sound will depend on my distance away from the sound.

    05:55 So for example if I'm next to somebody who's talking and then I moved twice as far away as I was before.

    06:00 I would want to perceive that distance correctly.

    06:02 If I close my eyes I would want to be able to know where that person is but this is added discrepancy with the actual energy of the sound as it's leaving the person as they speak because we just described how that is going to drop with the power law, a square of the distance.

    06:18 And again for that reason we have sort of a more complicated way of having to describe sound when we measure it.

    About the Lecture

    The lecture Basics of Sound by Jared Rovny is from the course Sound.

    Included Quiz Questions

    1. Sound is a pressure wave of compression and rarefaction of molecules in a medium
    2. Vibrations of a guitar string
    3. Compression and rarefaction of any musical instrument
    4. The pressure which air molecules exert on objects near the surface of the earth
    5. The force between any two parts of a musical instrument
    1. Longitudinal
    2. Transverse
    3. Parallel
    4. probability wave
    5. Perpendicular
    1. It remains the same
    2. It doubles
    3. It halves
    4. It quarters
    5. It multiplies by √2
    1. They move faster than sound waves in air
    2. They move slower than sound waves in air
    3. They are no longer based on compression and rarefaction
    4. The sound waves are now transverse instead of longitudinal
    5. Solids cannot transmit sound waves

    Author of lecture Basics of Sound

     Jared Rovny

    Jared Rovny

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