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.