Learning Session Two: What is Sound?
UNDERSTANDING THE CONCEPTS
Purpose: Learn the scientific vocabulary of sound, create and observe different kinds of waves, experiment
with and identify relationships involving wavelength or frequency, wave height or amplitude, and how we
perceive sound with variations in volume and pitch.
1. Just as we saw the waves in the water produce patterns, so too, we saw that sound produced similar
patterns, both of these are due to the movement of energy in the form of waves. There are two main
types of waves, physical (mechanical) such as ocean waves and sound, and electromagnetic waves such
as visible light, x-rays and ultraviolet (UV light that causes sunburn). Physical or mechanical waves must
have something (called a medium) such as water or air for energy to move through. For example, the
energy in an ocean wave needs water to travel from one place to another. Mechanical waves cannot
travel though a space because space is a vacuum with no air particles in it at all, whereas an
electromagnetic wave can travel through a vacuum, for example visible light and ultraviolet (UV) light
from the sun.
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Show Sound in a Vacuum: A short black and white video clip demonstrating that sound travels a
medium such as air, but not in a vacuum.
Additional Activity Resources
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The Nature of a Wave: An article about waves and wavelike motion in everyday life.
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Wave Motion Gallery: Short video clips and images of waves found in everyday life.
TRANSVERSE WAVES
2. Put the following diagram up on the board.
Have students observe the following demonstrations and draw what they observe. Ask students to add
labels to their diagrams as the lesson progresses, using words from the board.
Transverse Wave Demonstration
Using a slinky from a dollar shop have a student sit on the floor and hold the other end. The teacher then
moves the slinky on the floor side to side so that there are lots of peaks/crests (the highest part of the
wave) and troughs (the lowest part). Ask the students to count how many are present as you move the
slinky. Ask them how many of them have been on the ocean and felt a little sick – the movement of the
transverse ocean wave has caused the motion sickness.
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Invite a student to draw their observation on the classroom board and other students to add the names to
the parts. Referring to the ‘Transverse Waves’ webpage, discuss and add on additional labels, showing how
to measure the wave height, wavelength and what a cycle is.
Transverse Waves
Diagram and explanation of the parts of a transverse wave, including the wave height/amplitude, wave
length and a cycle.
Longitudinal Wave Demonstration
Now using the same slinky and student holding the other end move the slinky away from you and then back
towards you in a straight line. You will see the slinky stretch and compress. This type of wave is a
longitudinal wave and is the type of wave you would feel in an Earthquake or at a rock concert as it is how
sound travels through a medium.
To compress means to squeeze, so a compression wave simply means a squeezed wave. Students may be
familiar with compressors used to squeeze air into the tyre of your car, or perhaps they have a grandparent
who uses compression bandages which help their blood circulation by squeezing an area such as a leg or
finger.
You can see the compression where the slinky is bunched up and dense because it has more pressure put
on it, and the opposite, rarefaction, where less pressure is put on the slinky allowing it to be more spaced
out (less dense).
TRANSVERSE AND LONGITUDINAL WAVES
3. Show students the video of Transverse and Longitudinal Waves: A brief video showing the difference
between the movement of energy in transverse and longitudinal waves, using a slinky.
Invite a student to draw their observation on the classroom board and ask other students to participate in
labelling the diagram. Discuss and add on additional labels, showing how to measure a/ the wave height,
also known as amplitude, which is the measurement from the undisturbed level (i.e., the middle) to the
highest part of the crest and b/ the wavelength also known as a cycle, which is measured from one part of a
wave to the same part on the next wave, for example from one crest to another crest, or the start of one
compression to the start of the next compression.
Basic Waves Theory: Labeled diagrams and information about longitudinal and transverse waves and an
interactive demonstrating amplitude.
SOUND
4. Sound travels though different mediums in different ways. Sound travels through gas or liquids in the
form of a longitudinal wave and through solids as both transverse and longitudinal waves. Not only does
the type of medium (gas, liquid, or solid) affect the type of wave, but also the speed at which sound
travels. Generally when people refer to sound waves they are talking about longitudinal waves.
To be able to compare sounds or talk about our understanding of and experiences with sounds, we need a
common language which we all understand. We have already talked about some of the words used to talk
about parts of the waves, but how can we have a conversation about volume or how high or low a sound is?
Effectively these are both ways we can measure sound. Each sound has its own unit of measurement, just
as we might measure a distance in meters, or milk in litres, we measure volume in decibels (Db) and the
pitch, how high or low a note sounds, in Hertz (Hz).
When we are measuring volume, we are measuring the density of each compression. The tighter the
squeeze, the more pressure it places on the medium it is travelling through, in our case through the fluid in
our ears. More pressure is perceived as a louder sound.
When we are measuring pitch we are measuring the frequency. The higher the frequency, the higher the
pitch. A good way to think of this is to imagine bouncing up and down on a trampoline. Each time you hit
and squeeze down on the trampoline this represents a compression. If you hit the mat once in one second
this is measured as one hertz. If you were a superhuman and could jump up and hit the mat 30 times in one
second, this would be measured as 30Hz.
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Have students work in pairs with a slinky or rope to see if they can devise a way to model louder and quieter
sounds and/or different pitches. Alternatively some students might like to explore pitch by measuring and
marking the wavelength of different notes using the table provided in ‘Physics of Music – Notes’. This activity
is best done outside on the concrete using chalk.
Additional Lesson Resources
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The NASA Files : What is Sound?: A brief outline of sound as a movement of molecules.
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The Nature of a Wave: An article about categories of waves, including information about transverse and
longitudinal and the difference between electromagnetic and physical waves, otherwise referred to as
mechanical waves.
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Physics of Music – Notes: The frequencies and wavelengths of musical notes.
MATERIALS REQUIRED
Activity 2: Rope and one slinky.
Activity 4: Ropes and slinkies, a measuring wheel or meter ruler and chalk. (The slinkies will quite possibly
be stretched beyond capacity during experimentation so it is advisable to source the cheapest possible.)
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