Physics

Sound: Screaming in space

In space no-one can hear you scream! From silent screams to voices on Mars and what music looks like – this lesson looks at everything about sound.

You will investigate the following:

• What are waves, and what type of wave is sound?

• What properties do all waves share?

• Can we see sounds?

• How well can you hear?

So turn up the volume and let’s go...

This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmosforschools.com

Introduction: Sound

Imagine you’re exploring a planet in some far-off galaxy. It’s one of those dry, dusty planets that you’ve probably seen in themovies. Like a foolish hero in one of those movies you wander away from your fellow explorers, out of sight.

That’s when you see the alien – a hideous beast with a hundred gaping mouths. How do you feel? What do you do?

If your plan is to scream then you’d better hope the planet has an atmosphere. Here on Earth we take air for granted. But it’s onlythe rhythmic waves of pressure passing through jostling air particles that allow us to talk and sing – and scream for help.

Hollywood movies from Alien to Gravity have exploited the terrifying silence of space. But new research is revealing that when youlook at it a different way, space isn’t so silent after all.

It may not have air to transmit sounds but it does have particles that vibrate in sound-like ways. These so-called plasma waves arecreated when the "solar wind" hits the magnetic fields surrounding some planets. What is the solar wind? The continual blast ofradiation from the Sun, sometimes magnified by giant storms raging on its surface.

And then there are radio waves which – like visible light – don’t need matter to get from place to place. Instead of carrying popsongs from radio stations the natural radio waves in space carry more mysterious signals from collapsed stars.

Space probes sent out by NASA have been recording radio and plasma waves and converting them into sound waves that we canhear. And some of these alien “space sounds” are much more unsettling than silence.

Read the Cosmos blog post here.

1:53

Question 1

Survey: Did you listen to the "sounds" of the planets on the blog post? Eerie aren't they? And, like screams, you might feelthat human voices as they'd sound on Mars are pretty scary. Many of the things we hear bring up strong emotions. Fill in the mindmap with different emotions, good and bad, that you have felt when you have heard certain sounds, then put in what the soundswere.

Note: you can change the prefilled nodes if you disagree with them.

Sounds

Eerie

Scary

Voice on Mars

Planet "sounds"

emotionexample

Gather: Sound

0:14

On Earth, not only can people hear your screams, you can shatter wine glasses with them!

How is that? It's because sound is a type of wave...

Sound waves

0:44

Question 1

Define: A wave is defined in the video as vibrations that:

  transfer energy, but not matter, from place to

place.

  transfer energy and matter from place to place.

  transfer matter, but not energy, from place to

place.

Question 2

Recall: Which types of wave can travel through a vacuum?

  Radio wave

  Sound wave

  Light wave

Transverse waves vs. longitudinal waves

0:26

Question 3

Label: In the sketchpad below:

1. Label the waves created by the slinky "transverse" and "longitudinal", as appropriate.

2. Draw a blue arrow in the "Direction of wave" box to show the direction the wave is moving.

3. Draw a double-ended red arrow in the "Particle movement" circle to show the direction, back and forth, the slinky coils aremoving.

Note: If you have a slinky have a go at creating these waves!

Graphing longitudinal waves

0:42

The video assumes that you are familiar with wave graphs. In a sort of coincidence, the graph of a transverse wave looks like thewave itself, but how does that type of graph represent longitudinal waves like sound?

To answer, think first of the air molecules in a silent room. They are fairly evenly distributed through the whole space, as you see inthe diagram below.

When a sound wave passes through, the air in some areas compresses, meaning the molecules there become more densely packed,and air in other areas rarefies, meaning the molecules become more spaced apart.  

As the sound travels through the space the molecules vibrate so that the areas of compression and rarefaction shift, even thoughthe molecules themselves stay roughly in the same place. You can see this in the diagrams below showing a sound wave as itmoves across a room. Two specific molecules have been marked to show how, although they shift about, they do not move alongwith the wave. 

Look at the areas marked A, B and C. 

At A, the density of the molecules is the same as in the very first diagram, where there was no sound. At B the molecules arecompressed in comparison to A, and at C they are rarefied in comparison to A. 

What if we graph the compression and rarefaction across the room?

Question 4

Graph: The top half of the sketchpad below shows part of the diagram above with the points A, B and C marked. The bottom halfshows an uncompleted graph.

The Y axis is air compression or rarefaction. The degree of density or compression of the air in the silent room is representedby the mid-point of the axis – where the X axis crosses.

The X axis represents horizontal position – it corresponds exactly to the locations in the diagram in the top half.

We have filled in part of the graph, plotting compression or rarefaction for the areas around A, B and C. Complete the graph for therest of the diagram. 

Question 5

Contrast:  The sketchpad below is similar to the previous one, but has two air molecule diagrams, Q and L. They represent thesame room with different sound waves passing through. There is one graph below both diagrams.

As above, plot the compression or rarefaction at each horizontal point in Q and L.  One point for each plot has been put in to helpyou get started.

The plots for Q and L should look similar except that the peaks and troughs for L are higher and deeper than for Q. That is, the plotfor L has a greater amplitude.

The greater amplitude represents greater compression and greater rarefaction of the air. But to push the molecules closer togetheror pull them further apart requires more energy, so the wave L has more energy than the wave Q. 

The difference in the energy of a sound wave corresponds to the sound's volume:

Low-amplitude sound waves are quiet – Q

High-amplitude sound waves are loud – L

Wavelength and frequency are explained in the video. What he doesn't mention is that for sound waves wavelength and frequencyare related to the pitch of the sound, or the note, as in music.

Sound waves with shorter wavelengths and higher frequencies we hear as higher notes.

Sound waves with longer wavelengths and lower frequencies we hear as lower notes.

Wavelength and frequency

Amplitude

Question 6

Compare: In the sketchpad below:

1. Draw a horizontal line on each of the molecule diagrams and its graph to show the wavelength (four lines in total).

2. Imagine that in each diagram the number of waves you can see is the number that would pass by in one second. Count thenumber of waves to get the frequency and type it in to the appropriate box.

3. In the remaining two text boxes type "higher" for the wave that makes the higher note and "lower" for the wave that makesthe lower note.

Process: Sound

In the last question in Gather the wave with the shorter wavelength had a higher frequency and the wave with the longerwavelength a lower frequency.

Does this inverse relationship – where one value decreases as the other increases – always hold? To help discover, we'll use thesimulator here. 

First, open the Wave on a String simulator and make the following settings:

1. Set to Oscillate in the top left field.

2. Set to No End in the top right field.

3. Click or tap to show rulers.

4. Set Tension to high.

5. Set Damping to none.

Now we will experiment with some different frequencies, but some symbols will help. We will follow standard conventions. 

The Greek letter lambda,  , represents wavelength.

The letter f represents frequency.

Investigating wavelength and frequency

λ

Question 1

Measure: Set the wave frequency to six different values. For each frequency stop the wave and measure the wavelength to thenearest 0.1 cm. Record your results in the table below.

Hint: try to get as wide a range of values as possible.

No. f (Hz) λ (cm)

1

2

3

4

5

6

Question 2

Plot: Create a graph of the data you collected.

Be sure to give the graph a title and to name the axes and enter the units used. Plot the independent variable (the values youselected) on the X axis and the dependent variable (the values you measured) on the Y axis.

Title

X-Axis

auto

Y-A

xis

auto auto auto

Series 1

x y

This graph needs some data!

You might recognize the type of plot you have created from Maths – it is a hyperbola. This gives a clue for the relationshipbetween f and  .

The speed of sound

λ

Question 3

Explore: Take three of your data points from the graph and for each multiply the x and y values – that is, the frequency and itscorresponding wavelength – together.

Your answer for each of the three data points should come out to around 6.3. (There might be some variation due to roundingerrors when you measured the wavelengths.)

That is the speed of the wave – 6.3 cm/s.  

To help understand why this is the wave speed, consider the wave where f = 3.0 Hz and   = 2.1 cm. This tells us that every secondthe wave source produces 3 waves, each one with a wavelength of 2.1 cm. So we just have to multiply the two values together tosee how fast the waves are moving away from the source.

Represented as a formula, this is:

v = f

The speed of sound depends on the medium that the sound is passing through. We can simulate this in Wave on a String.

λ

λ

Question 4

Extend: In the simulator, keep the same settings as before except:

Set Tension to the midpoint.

As before, measure the wavelength at different frequencies. This time gather two data points and multiply the f and   valuestogether for each to get the wave speed.

λ

No. f (Hz) λ (cm) v (cm/s)

1

2

With the Tension setting lowered in the simulator "string", the wave speed drops to about 3.8 cm/s. 

For sound the lowered Tension setting represents media where the molecules are less densely packed, with looser bonds betweenthem.

In materials where the molecules or atoms are closely packed, sound travels quickly.

In materials where the molecules or atoms are only in contact with each other loosely and intermittently, sound travels moreslowly.

Question 5

Match: Listed below are the speeds that sound travels in air, water and steel. Match the speeds to the appropriate media.

1,484 m/s   |   6,100 m/s   |   344 m/s

The following three questions are optional, but may help in answering the cymatics questions below.

Question 6

Play: Try some other variations in the simulator. To start, set to Manual and Fixed End and move the wrench to create one wave.What happens when the wave reaches the clamp?

Question 7

Observe: Reset the simulator, then still with Manual and Fixed End, create a wave followed a short time later with a second wave.What happens when the two waves meet?

Hint: it is a good idea to pause the simulator and then advance step by step as the two waves meet. 

Question 8

Experiment: Set to Oscillate, Fixed End, Tension high, Damping none and Frequency 1.5 Hz. Watch for at least 15 seconds. Whathappens to the amplitude of the waves?

Can you suggest reasons why the waveform behaves like it does?

Cymatics is a newly coined term meaning the visualization of sound. Watch this video and you'll understand...

Cymatics

5:19

Question 9

Hypothesize: Select one of the following cymatics effects from the video and explain what you think might be happening to createthe effect:

The Chladni plate (the square metal plate with sand on it)

The petri dish (a petri dish with vodka at subzero temperature, sitting in a speaker)

The Rubens tube (the tube with flames along the top)

Question 10

Challenge: Put your hypothesis to the test by pairing with another student who chose the same effect. Discuss your twohypotheses and decide whose is the best. Can you develop it further? Below, report:

1. How were your hypotheses different?

2. After discussion, did you agree on one of the hypotheses or develop a new one? What hypothesis did you finally settle on andwhy did it seem better than alternatives?

Apply: Sound

Experiment: Hearing range

Use headphones or earphones to listen to the audio below. You will need the classroom to be very quiet to get accurate readings. 

2:24

Question 1

Record: Note the lowest and highest frequencies you can hear. You may need to replay the clip at either end to obtain accuratereadings. 

Lowest frequency heard (Hz) Highest frequency heard (kHz)

Report: What was the bottom of your hearing range? Report to the nearest hertz.

  This poll is currently closed, so you can't vote

Poll 1

17 Hz or less

18–19 Hz

20–21 Hz

22–23 Hz

24–25 Hz

26–27 Hz 

28 Hz or more

Report: What was the top of your hearing range? Report to the nearest kilohertz.

  This poll is currently closed, so you can't vote

Poll 2

11 kHz or less

12–13 kHz

14–15 kHz

16–17 kHz

18–19 kHz

20–21 kHz

22 kHz or more

Question 2

Graph: Create a histogram for the lowest note the students in your class can hear. Your teacher will give you the results from thepoll for this.

Be sure to label the axes and put in units, where appropriate, and mark and number both axes.

Hint: if you need help creating a histogram see this site.

Question 3

Graph: Create a histogram for the highest note the students in your class can hear. 

Did you know?

There are significant differences in the hearing ranges ofdifferent species, as shown in the graph to the left. For example,elephants can hear frequencies below the human range.They produce rumbles at these levels as a part of theircommunication. 

Insect-eating bats produce short rapid "squeeks" at 20,000 to200,000 Hz, depending on the species. The sounds bounce offthe objects around them and the bats can hear the reflectedsounds. With their "squeeking" and hearing in perfect sync theycan not only avoid flying into objects in the dark, they can catchsmall insects on the wing.

Note: the graph uses a logarithmic scale. Each mark on the Xaxis is ten times the mark to its left. Such graphs are used toshow very large ranges.

In the next part of this exercise you will make comparisons between a broader range of people. If you are doing the activity athome ask your parents or other relatives or neighbours to participate. Otherwise, here are some other data you can use.

Name Subject: sex and age Lowest frequency heard (Hz) Highest frequency heard (kHz)

Lin female, 42 23 18

William male, 55 44 12

Margaret female, 63 31 15

Ajit male, 75 48 9

Question 4

Graph: Create a horizontal bar graph, similar to the one showing species' hearing ranges above, for:

1. yourself,

2. two classmates,

3. your teacher, and

4. two other people older than yourself.

Label each bar with the name, age and sex of the subject.

Question 5

Compare: How does your teacher's hearing range compare with that of your classmates and you?

Question 6

Observe: Do there appear to be any correlations in your data between the age of the subjects and hearing range? Do there appearto be any correlations in your data between the sex of the subjects and hearing range?

Which evidence, if any, seems to support such correlations, and which evidence, if any, does not. 

Question 7

Think: Would it be scientifically sound to make general conclusions for the whole human population based on any correlations – orlack of correlations – you see in your data?

Explain your answer.

Career: Sound

As a musician, Nigel Stanford doesn’t normally do science experiments at work. But one day he stumbled across somecymatics videos on YouTube and his brainwaves echoed with an idea.

Originally from Wellington, New Zealand, Nigel was alwaysfascinated by science as a kid – physics in particular. But he wasalso interested in creative studies that would allow him to makethings, so he chose to pursue art, music and architecture inschool. Later on he taught himself graphic design and beganworking with 3D graphics and computers.

These skills helped him create his cymatics video. He firstthought about the concept after he watched a documentary onsynaesthesia, a disorder that causes people to see colours whenthey hear certain sounds or hear sounds when they see brightcolours. Although Nigel doesn’t think he has synaesthesia hehas always imagined that treble frequencies would be white andbass frequencies red.

Nigel researched the science behind cymatics for five longmonths to make sure all the experiments would work perfectly.And he did them all on his own, apart from when the Tesla coilengineers helped to make sure that Nigel didn’t get zappedduring filming.

The patterns created by the vibrations and pulses wereimportant to making a good video, which meant that Nigel hadto create it backwards – starting with the visuals first. Once thescience experiments were recorded to show the sound wavesemanating from the instruments Nigel wrote and synthesizedthe musical parts.

Nigel says that work is his hobby – being able to make musicfull-time may be his job, but it certainly doesn’t feel like one!

Question 1

Expand: Nigel Stanford creates art from science. Can you think of any other areas of science that produce sounds, images, or othereffects that could be the basis of art? Explain what you know of the science and why you think there are artistic possibilities.

Cosmos Lessons team

Lesson author: Jim RountreeIntroduction author: Campbell EdgarProfile author: Megan ToomeyEditor: Campbell EdgarArt director: Wendy JohnsEducation director: Daniel Pikler

Wave simulator: PhET Interactive Simulations, University ofColorado, http://phet.colorado.eduImage credits: istock photo, Nigel StanfordVideo credits: stargazer, chasechocolate, JaHuProductions, NigelStanford, adminofthissite's channel, YouTube.