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Speed of Sound
Activities
These materials were originally written by Kelly Cooke for
middle-school teachers and their students. With modification, they're
also appropriate for high-school students. The speed of sound
measurement using the high-speed clock makes a good demonstration at
the middle-school level and above.
Introduction
This program is designed to use hands-on methods to give students
of 6th-8th grade level an understanding of the magnitude of the speed
of sound by using both visual and auditory senses for measurement. The
program consists of an introductory discussion and outdoor activity on
the speed of sound, a session on measuring the speed of sound with
high-speed techniques, and follow-up exercises that could be led by
the students' science or mathematics teachers. The exercises require
multiplication and division of fractions and proportional reasoning.
- Parts I and II are
written as teacher's guides.
- Parts III and IV (rtf
format) guide the students through the mathematics needed to
calculate the speed of sound. Follow-up problems are given.
These parts are provided in rich-text format to allow for
convenient editing by teachers.
Part I. Introductory activity and discussion
Materials
meter stick
large clear area (at least 100 meters in length)
large cymbals or bass drum
* note: if these items are unavailable, others can be used; the
only requirements are that it needs to be large enough for the
children to see distinct movement, and it needs to make a very loud,
sharp sound (two metal trash can lids, two large blocks of wood,
etc.)
Procedure for Outdoor Activity
(1) Begin with discussion of sound travel, pointing out the
measurable difference between the speeds of sound and light. The
following example might be helpful.
"Let's think about a thunderstorm. When lightning strikes,
do you see the lightning or hear the thunder first? That's right,
you see the lightning usually a few seconds before you hear the
thunder, if the storm is a few miles away. This is because sound is
a lot slower than light. One way to tell how far away you are from
the storm is to count the number of seconds between when the light
from lightning reaches your eyes and when the sound of thunder
reaches your ears. Every 5 seconds you count means the storm is
about 1 mile away."
(2) Take the students out to a field which has been pre-measured
and marked off at a distance of 100 meters. (It need not be extremely
accurate: take 1m ~ 1 adult giant step). Have at least one student
stand at one end of the field with a pair of cymbals or other
noisemaker. The other children should stand at the other end. Let them
watch and listen as the cymbals (or pieces of wood, etc.) are clapped,
making sure they notice the difference between the moment they see the
two objects hit and the moment they first hear the sound. There should
be at least a noticeable difference in the two times.
(3) Next the children will need to measure the field in units of
their armspans. Simply string the students out (fingertip to fingertip
or holding hands) across the field. It is unlikely that there will be
enough of them to cover the entire field. If this is the case, an
explanation beforehand using the meter stick should make the activity
a bit easier for them to follow. Show them that to measure a distance
of 1.5 meters with only one meter stick, one must first measure the
meter, then swing the stick around its endpoint to measure the other
0.5 m. In this way the students might measure the field without having
a large enough number to stretch all the way across. Stretch part of
the way, then hold the child on the end still while the others swing
around him to measure the rest.
(4) After taking the measurement, you will want to show the
students how to break up large units of distance into smaller ones.
This might be done with a short talk about, "If it takes sound a
certain length of time to go the whole 100 meters (60 children), how
long does it take sound to go half that distance, or 50 meters (30
children)? It takes about half the time. What about to go 1/4 of the
distance, or 25 meters (15 children)? It takes about 1/4 of the
time." Now the students should be ready for the next activity on
the speed of sound.
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Part II. High-speed measurement of the speed
of sound
The method for this measurement is described in the html
document: A Demonstration Measurement of
the Speed of Sound.
The script below is written in a free-flowing style intended to
provide notes for the demonstrator.
- this disk is a sort of clock, but it's not exactly like the
ones you normally see: what are the differences? it has no
numbers and it moves very quickly, so it can be used to measure very
fast things
- to be able to use the clock to time things, we must first time
the clock to see how fast it spins; we can do this by using this
stroboscope light
- the stroboscope is a light that you might have seen before,
because it's often used at parties to make motion look choppy;
it does this by a very fast flashing light; because it can show
things in short jumps instead of continuous motion and show us
things that happen really fast; it is also used to study machinery
- for instance, if we shine the light on the disk, we can now see
a few lines on it; this is because although the disk has only
one line painted on it, the stroboscope light is flashing when the
line is in different positions, so that we see it as many different
lines
- to measure how fast the disk spins, we need to make the light
flash once every time the line moves in a complete circle;
what we will see is one stationary line, because each time the light
flashes, the line, having rotated one complete turn, will be in the
same place as the time before
- when we see that stationary line, then the frequency at which
the light is flashing will also be the frequency at which the disk
is spinning; we can see that now, when the stroboscope is
flashing 50 times per second
- since the disk is rotating at the same time the stroboscope is
flashing, we know that the motor moves at about 50 rotations per
second, or does one rotation in about 1/50 s.
- using this, we can measure how long it takes sound to travel a
certain distance
- for instance, I wonder how long it takes sound to travel my
armspan, from fingertip to fingertip? you can try to measure it with
a normal classroom clock, but I bet it won't work. Let's use this
high-speed clock instead.
- if we set up these two sound triggers as far apart as the
distance from one of my hands to the other (about 1.7 meters), with
each one connected to a flash unit, then make a loud sound, we
should be able to measure the time it takes for the sound to move
the distance of my armspan
- what will happen is that when we make the sound in line with
the sound triggers, it will pass the first trigger first, causing a
flash to go off and show us the position of the line on the disk
- then it takes a certain amount of time for the sound to travel
from the first to the second trigger, and in this time the disk
keeps spinning, so that when the second trigger finally
"hears" the sound and causes the second flash to go off,
the disk has moved a certain istance
- so the time it takes sound to go from one trigger to the other
(the distance of my armspan) can be seen by the difference in the
position of the white line on the clock; let's try that now
- since the two images of the line are one-quarter turn (90
degrees) apart, we know that the time it took sound to get from the
first trigger to the second (to go across my armspan) is the same
time it took for the clock to rotate one-quarter turn
- we can look at this now like a regular clock
- it takes the second hand of a clock 60 seconds to move around
one time
- if it goes one-quarter turn, it takes 1/4 times 60 seconds, or
15 seconds
- remember that each complete rotation of the disk takes about
1/50 s; so for one-quarter turn, it takes about (1/4) x (1/50 s) =
1/200 s
- we can see now that it takes sound about 1/200 s to travel
about 1.7 meters, or about one adult armspan
- so when I hold my arms apart and snap the fingers of one hand,
it takes the sound 1/200 s to reach the other hand
- if the sound traveled 1.7 m in 1/200 s, how far would it travel
in 1 s? 200 times as far or 1.7 m x 200 = 340 m, so the the speed of
sound is 340 meters per second
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