

Physics In Action: Sound Waves in a Flaming Pipe



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About this Lesson

  • Type: Video Tutorial
  • Length: 6:01
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 64 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Physics (147 lessons, $198.00)
Physics: Waves (19 lessons, $27.72)
Physics: Sound (5 lessons, $5.94)




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We know that sound is a wave phenomenon. Specifically, it's a pressure wave in air, for example. And as with other wave phenomena, we expect to be able to set up standing waves in appropriate situations. Remember for a string we could set up a standing wave where the string vibrates up and down, and the standing waves are standing waves because there are regions, which don't move at all. Those are nodes. There are regions, which move a lot, which are antinodes. We expect to see wavelike patterns set up in a column of air vibrating. We expect also to see regions of changing pressure; high and low pressure, and we can demonstrate that with this apparatus over here.
Now this apparatus consists of a long pipe into which we've drilled a whole bunch of holes along here, which you'll see shortly. And what we're going to do is introduce into this pipe some gas with this tube over here, and we're going to light up this pipe so you can see a whole bunch of flames standing on the pipe. Let's see how that looks. Okay, we have our pipe; gas being introduced; and the flames now are our way of telling something about the pressure inside this pipe. Now, what is of interest to us is what happens when we push the air in this pipe with some signal generator, something that's going to compress and rarefy the air, push it in and out. And that's going to be achieved by this loudspeaker, which is attached to one end of this pipe. This loudspeaker in turn is connected to a signal generator. So let's see what's going to happen.
When you turn on the signal generator, it's going to send a signal, a tone line hmmm down this pipe. The signal is going to bounce off and do whatever it wants to do, and we expect to see some standing wave pattern, which would be analogous to the standing wave pattern we saw in a vibrating string. What you expect to see is a wavelike behavior, and that's what we want to demonstrate now. You'll see that there are regions where the wave is high, regions where the wave is low, and that wavelike pattern shows us that sound, which is what is producing the wavelike pattern, is obviously a wave, which is the point of this demonstration.
Okay, so I'm going to turn on the signal generator now, and now let's see what we see. Let's take a look at the pattern. You see that there are regions where the flame is high and also somewhat yellow, and regions where the flame is low and somewhat blue. Now what's interesting is that I can change the position of the regions where the flame is high and the regions where the flame is low by changing the frequency, which is what I'm doing. I change it one way, change it another way, and let's just watch this for a few seconds and see what happens. Now visually you clearly see a visual representation of the wavelike nature of sound on this tube, the picture being transmitted to us by means of the flame, which obviously has a wavelike behavior.
We've seen that a simple tone produces simple wavelike pattern of the gas flames on this tube. Now let's see what happens if we look at a real piece of music. You see that with a real piece of music the flames jump around. As you can hear, when the music gets louder, the flames jump higher. When the music gets lower, the flames are lower. You can also see that this pattern is more complicated because a real piece of music has a more complicated mixture of different frequencies, different notes that you hear. Let's watch the music for a few seconds. Notice at various points the sound really gets loud and the flame gets higher. If we turned up the sound even more, we'd actually blow out a bunch of the flames. That reflects the fact that increasing amplitude of sound represents an increasing pressure inside the tube, and at the appropriate point the flames can get very, very high and you could see some of the flame actually going out. Notice that's happening right now.
Again, the point in this demonstration is, one, to notice the connection between the amplitude of the sound, the loudness, and the amplitude of the flames shooting up. The flame is now measuring the pressure change caused by the loud sound. And again you note that we don't get a simple pattern of evenly spaced high flames and low flames. What we get now is a more complicated pattern reflecting the more complicated nature of a real piece of music.
So now let's summarize what we've learned. We know that sound is a pressure wave, and one way of showing this is to make that pressure wave visible by means of this apparatus over here. The flames tracked the different pressures that this tube carried. There are regions where the pressure is higher; there are regions where the pressure is lower. And those regions of high and low pressure are spaced as high pressure, low pressure, high pressure, low pressure, and so on, and form a regular wave pattern. For a simple frequency, the drive in the air and the gas in the tube, we see a simple regular patter reflecting the fact that sound is, after all, a pressure wave. For a more complicated piece of music, we see a more complicated pattern but the message is still the same. Sound is a pressure wave, and those changes in pressure reflect themselves in the changes in the heights of these flames.
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