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Measuring the Speed of Sound with an Air Column

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This is a little experiment to measure the speed of sound. We're going to use a column of air, which is in this tube right here, but the column of air has an adjustable height. And it's adjusted with the water column right here. There's water here in this aluminum can that's connected by a tube through the bottom to the pipe right here so by adjusting this I can make the water level go up and down. We're going to produce sound with a tuning fork and this has a frequency of 384 hertz. And I'll hold the tuning fork at the top of the tube, and we're going to produce what we call resonances or I should say we're going to search for resonances. Resonances are places where the intensity of the sound is louder than normal.

Now first of all you need to listen to the normal intensity of this tuning fork. Ok, now I'm going to hold that to the mouth of the tube and you can hear that its intensified too. I managed to get the resonance in advanced by adjusting the water level. So let's go ahead and measure the level of the water and then we'll come back and talk about why we get the resonance. So let's zoom in on the meter stick. Now I want to show you how sensitive the resonance is to the position of the bottom of the water column. So let me get the tuning fork up there and I'll adjust the height of the water and you can here the intensity going down I'll come back up again and go through the resonance and the intensity comes back up again. Ok, so we should be able to find at least one more resonance. Because of the way the wave length is put in the pipe. We have a node here and an antinode here. There are different ways to do tha;t we can have just a quarter wave length in here or we could have three quarters of a wave length or some odd multiple of a quarter of a wave length. So what I'm going to do is move the water column down gradually until I find the very next resonance. And then we'll measure the length and have enough information at that point in order to calculate the speed of sound. This is quite a ways down. Just keep and eye on the water level as it descends listen for the intensification of the sound and leave the meter stick at that point. Now listen for the resonance on the way back up.

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