Measuring the Speed of Sound, Part 1

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Script

This will be an experiment to measure the speed of sound. Now we know that speed is the change in distance over the change in time. So our method is going to be how far sound travels in an amount of time, and then take the distance traveled and divide it by the amount time. Now the way this experiment is usually done is over a much larger distance then we will be using in the laboratory here. For example, you might use two people positioned on opposite ends of a football field. You have someone smash two trash can lids together, and the other end of the field you have someone with a stopwatch times how long it takes from when that person sees the trash can lids contact each other to the time when they here the sound, and there is a noticeable time delay. Now, since we are working over the distance of this table, we don't have these long distance. We will have a much shorter period of time, and wont be able to time it with a stopwatch. We will have to use another method to measure the time, but we will be able to do that successfully.

Here's some of the equipment we will be using. One will be a standard flash unit, one you might see on any camera. Let's give it a flash. The other piece is not so common, it's a sound actuated trigger, its composed of a microphone right, inside is a little audio amplifier. Whenever it receives a sound, it sets off this flash "snap" just like that. We have two of those, each with its own flash. Now here's basically the plan of the experiment, if I make a sound over here, it will set off this flash here, because it reaches this flash first. Then it will take some time to travel, and then set off the other flash. Let's try that right now. Once more. Now I'm sure you couldn't tell there was a time delay between these two flashes. That means we will need another piece of equipment to measure those two time intervals.

This is the piece of equipment we will be using to measure those short time intervals that we are going to have in this experiment. Now there is nothing special about it. It used to be a fan. I took off the fan blades and replace it with this disk, painted black with a white line on it. And you can think of this as a high-speed clock. This white line is the hand of our clock. Let's see how fast it goes. It goes so fast you normally cant see it turning. With the flashes, we will be able to see the line at certain instances in time. Let's see that right now. So ill begin by turning the flashes around to face the disk. Ill turn on this trigger. You can see that when the flash goes off, you get a sharp image of the line on the disk. And that's because the flash is only lasting about a 1/30,000th of a second. Now I'll turn the other one on. And now if I make a sound, you see two images of the hand on the clock. That's because when the sound reaches this microphone the flash goes off and you get one image. Then time passes, and the sound reaches the other microphone and you get a second image. The disk will have turned during this time, far enough so you can see the angle between them. In that angle, we will find the time that passed from the first flash to the second one.

In order to use the clock to measure time, we will need to know the angle between the two images, but we will need to know something else. We will need to know how fast the disk is turning. To find that out, we will use a stroboscope and a method like the one shown in a previous clip. I will set the stroboscope at 50 flashes a second, because I want the disk to go around 50 times per second. I have a variable speed control here, and Ill adjust that. And I get, that should be nearly 50 rotations per second. So now we know the frequency of the motor.

I will add one more piece of equipment. I'll put color filters on the flash units. A green filter for the left flash unit, and a red filter for the right flash. The reason for this is the hand that shows up on the clock, for each flash, will be a different color, so it will be easy to distinguish between the flashes and tell which one went off first. I'll make a sound here, and you can see both the red and the green image. You can also tell which one went off first, because the disk is rotating clockwise and when you look at the hands, the green one appears at about the 1:00 position and the red one appears at about the 2:00 position, the red one went off a little bit later because sound had to travel further to reach it.

Now to test the triggers, to make sure they respond in the same way, I put them together and make the sound out front, so the sound travels the same distance to each. In that case, the two images should be on top of each other, and neither look red or green. As you can see, that's the case.

Now we are ready to do the experiment. Ill begin by putting the left trigger at 10cm on the left meter stick, and put the right trigger at 70cm, meaning there is 60 cm between them. Ill produce a sound with two blocks out at the end, in line with the two microphones. That the time between the flashes corresponds to the distance between the two triggers. Here it goes. Now you can see the angle is quite a bit less than the 90 degrees we are trying to achieve. So what I need to do is move the triggers farther apart. So I am going to put the left one at the middle, at 100 cm, so the distance between them is now 90 cm. Here we go again. Still quite a bit less than 90 degrees. Let's make a bigger change. I'll move this one out to 140 cm, so the distance is 130 cm. Still a little bit less than 90, but we are getting close. Let's try almost the other end of the table, 180, so the difference between them is almost 170cm. That looks pretty close to 90 degrees. So let's use those particular results. So our data is:

Distance between the triggers: 1.70 m

The angle of the disk is 90 degrees

And the frequency of the disk is 50 rotations per second

We will know use that data to calculate the speed of sound.