“Seeing Sound”
Author: Dean Papadakis
Discrepant Event - Teacher's
Guide
SED 695B; Fall 2007
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Here is a picture of the embroidery loop with saran wrap tightly stretched over it
and a tiny mirror glued to the saran wrap, along with a green laser pointing at the
mirror. The embroidery loop is placed in front of a radio speaker.
The discrepant event here is in the title, which suggests that we can
“see” sound waves. However, we really can’t “see” sound waves.
The
We can hear certain frequencies of sound. So in this demonstration,
discrepant
we are “transforming” the sound waves from a radio speaker, into
event
light waves.
The Set-Up
 First you obtain a wooden embroidery loop and you either glue a sheet
of saran wrap to the surface of the circular edge or you tighten the saran
wrap in between the inner loop with the outer loop, using the set screw
and clamp, as is done in the picture above.
 Next you must obtain a tiny mirror, which can be cut from a larger piece
of mirror using a glass cutter (or you can just break off a small piece
using a pair of pliers). The mirror should be small (~ 3 mm) so that its
mass is small and therefore its inertia is small.
 Next, you hang the embroidery loop with the small mirror in front of the
speaker of a radio. Now set up a clamp and ring stand so that a laser
pointer can be clamped into a position where it can be aimed at the
embroidery loop.
 The mirror will reflect the laser light onto a wall or screen. When the
radio is turned on, the sound waves will hit the saran wrap and cause it
to vibrate, just like the head of a drum will vibrate when hit. As the
saran wrap vibrates, so does the mirror and so does the reflected laser
light and very amazing figures of light are produced. Of course, do this
in a darkened room.
 Students will be amazed and entertained at the visibility of light waves
being produced from sound waves.
Many Principles Are Ilustrated in This Demonstration

Light can be reflected off a mirror and obeys the “Law of Reflection”.

Sound waves have energy that can cause something else to also have energy of motion (e.g. film
of saran wrap).

Wave transformations from transverse to longitudinal and back can be traced back to the original
source. Radio wave entering radio is a transverse wave traveling at the speed of light. The
sound coming out of the radio is a longitudinal wave traveling much slower at the speed of
sound. The sound wave is causing a light wave from the laser to oscillate onto a screen.
The Principles
Standards
4. Waves have characteristic properties that do not depend on the type of wave. As a basis for
understanding this concept, students know:
a. waves carry energy from one place to another.
b. how to identify transverse and longitudinal waves in mechanical media such as
springs, ropes, and the Earth (seismic waves).
c. how to solve problems involving wavelength, frequency, and wave speed.
d. sound is a longitudinal wave whose speed depends on the properties of the medium in
which it propagates.
e. radio waves, light and X-rays are different wavelength bands in the spectrum of
electromagnetic waves whose speed in vacuum is approximately 3x108 m/s (186,000
miles/second).
Questioning Script
Prior knowledge & experience:
 Students should already have an understanding of how longitudinal waves differ from transverse waves.
 They should also know examples from the two categories of waves.
 Students should also understand the concept of reflection and specifically the “Law of Reflection”.
 They should also understand the idea that waves, whether they are longitudinal or transverse, can carry energy from
one place to another.
Root question:
How can we see Sound?
Target response:
The way that we can see sound waves is by relying on the principle that sound waves have energy and therefore they can
transport their energy to something else. They do this by traveling through some medium, such as air, as they leave the
radio speaker, and they cause the saran wrap, which is in front of the speaker, to vibrate or oscillate due to the energy of the
sound waves.
An additional variation to this demonstration can be done by, adding an additional laser of red wavelength. Now you have
two sources of light, (green and red), which are slightly offset in where they reflect to the white board and you get a
“Dueling Lasers Light Show”.
Common Misconceptions:
 The sound waves exiting a radio are transverse waves traveling at the same speed as light.
 We are listening to the “radio waves” when they come out of the radio.
 We can directly see “sound waves” with our eyes.
 Sound waves have no energy.
The embroidery
loop is directly in
front of the radio
speaker, which is
playing music.
This is an image
of the white board,
in a darkened
room, which has
the reflected laser
light from the
mirror in front of
the radio speaker,
showing
wonderful
patterns of light.
The varying light
patterns are the
result of different
amplitudes of
sound waves and
different
frequencies of
sound waves
causing the saran
wrap to vibrate in
different waves.
Sound waves of a
person speaking,
look different from
the sound waves
of music playing.
Different types of
music will also
look different.
References & Links:
I first saw this idea in a children’s book of demonstrations where a coffee can has a rubber balloon stretched over one open
end of the can and then a small mirror is glued to the rubber balloon. The other end of the can is also open and placed in
front of a radio speaker.