 Now, you experience longitudinal waves every day, including at this moment as you are listening to the video. When sound travels, air molecules form a series of compressions and rarefactions, as shown in this sound wave, frozen in time. At compression, the air pressure is higher than normal, and at rarefraction, the pressure is lower than normal. So here's a plot of the air pressure versus distance. We can easily recognize this as a wave. It looks like the displacement versus distance curve of a transverse wave. And because of that, some may forget that the sound wave is a longitudinal wave, not a transverse wave. Don't forget the difference. Sound waves are longitudinal waves. Sound waves are not transverse waves. The sound wave generates pressure variations. Here's a plot showing how that pressure varies with distance. These pressure variations are what allows us to hear sound. And to create sound, we generate pressure variations. How big is this pressure difference? Very small. Here's a chart of the pressure variation for various sounds, including a jet engine and a trumpet. For a loud human voice, the pressure difference is about 110 Pascal's. Now, a Pascal is a unit of pressure, and atmospheric pressure is 101,325 Pascal's, so the pressure variation from a loud human is a little more than a thousandth of normal atmospheric pressure. We cannot feel this pressure difference, but we can hear it. As I speak, sound waves are traveling towards the microphone. In the microphone is a diaphragm, which deforms or vibrates with the same frequency as the pressure wave. This diaphragm motion can be turned into an electric signal that records the pressure wave. We can look at the electrical signal displayed on an oscilloscope. This is an oscilloscope that you can interactively use on the web. It shows the electrical signal generated by the microphone. This is proportional to the pressure at the microphone position as a function of time. So at an earlier time, the pressure at the microphone was low, and then it oscillated up and down in time. The amplitude is proportional to the pressure variation in the medium. The period is the time that it takes for the pressure wave to advance one wavelength, or the time between consecutive crests. Let's see how the oscilloscope responds in real time. I'll slow it down so that you can see that the pressure wave moves from right to left as time advances. Now I'll sing into the microphone. I've just frozen the wave in time, so you can see that this is the amplitude of my voice and this is the period. And you can measure the amplitude. Remember you are measuring an electrical signal, volts. So each division is associated with a specified voltage shown here. Now we don't know the relationship between voltage and pressure variation, but if the voltage is high, that means the pressure variation is high. The amplitude is proportional to the loudness or volume of the sound. Large amplitude sound is loud, small amplitude sound is barely heard. You can test this at home.