Sound Waves — Print · Grade 8 Science

Chapter: Waves, Sound & Light

Sound Waves

Learning goals

Readings for this unit

Before this unit

Know that gases contain particles and waves can be longitudinal.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions. Study the four visual parts across the scheduled sessions, then use the illustrated worked case and independent transfer questions. Extend or revisit a session when the learner cannot explain the figure; completion time is a proposed allocation, not a mastery guarantee.

Start with something to explain

What would explain the observations in sound waves?

Illustrative evidence case. An illustrative echo returns 0.30 s after a short sound. The modeled sound speed is 340 m/s, and the path goes from source to wall and back.

A sound disturbance through matterA sound disturbance through matterCompressed regionMore spread regionDisturbance travels →Particles mainly move back and forth locally.Illustrative model • use the labels and stated assumptions
The dots model local particle crowding rather than literal air colors. Compressions and rarefactions form a traveling disturbance while particles mainly oscillate locally. The two arrow types distinguish disturbance propagation from local motion; the schematic omits particle-scale distances and random thermal motion.

Notice and wonder

Do these dots need to travel from speaker to ear? Write two observations from the supplied record without guessing their cause. Include a quantity and its unit when given. An observation is what was recorded; an inference is what you think explains it.

Write one question about why the pattern occurs and one question about a comparison that could distinguish competing explanations. A testable question identifies what could be observed or measured.

Your first model

Before reading the model answer, sketch the named objects or particles and label what you think changes. Find one-way distance and explain the medium’s role. Distinguish particle motion from the travelling disturbance. Keep this first version so you can compare it with your later explanation.

Build the language from the idea

longitudinal wave
A wave where particles vibrate back and forth in the SAME direction the wave travels.
compression
A region of a longitudinal wave where particles are squeezed close together.
rarefaction
A region of a longitudinal wave where particles are spread farther apart.
pitch
How high or low a sound seems, determined by its frequency.
medium
The matter (solid, liquid, or gas) that a wave travels through.
model
A representation that explains selected features and makes predictions; it leaves other details out.
evidence
Relevant observations or records used to support or challenge a claim.
system boundary
The chosen limit separating the objects being studied from their surroundings.
assumption
A condition taken as true for a model or calculation and needing to be checked against the case.

Sound is a longitudinal wave, and that word describes how it moves. Unlike the up-and-down ripples on water, the particles carrying a sound wave vibrate back and forth in the very same direction the wave is traveling. As they do, they create a repeating pattern of two regions: compressions, where the particles are momentarily squeezed close together, and rarefactions, where they're stretched farther apart. A sound wave is really this pattern of squeeze-and-stretch rippling outward from its source.

Sound as compressions and rarefactionsSound is a pressure variation that needs a mediumcompressionrarefactiontravels through the mediumIn a vacuum there are no particles to compress, so sound does not travel
A sound wave is a pattern of crowding and spreading passing through matter. Drawing it as particles rather than as a curve makes the requirement obvious: remove the particles and there is nothing left to carry the pattern.

Ordinary sound needs a material medium such as air, water, or a solid. Local particle interactions pass pressure or elastic disturbances along, so sound does not propagate through a vacuum. Speed depends on the medium’s elastic properties and density, as well as conditions such as temperature. Many solids transmit sound faster than air, but particles being closer together is not a sufficient explanation by itself. Use the speed supplied for the specific medium when calculating distance.

Frequency largely determines a tone’s perceived pitch. Amplitude and intensity relate to loudness under comparable conditions, while frequency, distance, and individual hearing also affect perception. A higher-frequency tone is not necessarily louder, and a louder tone does not necessarily have higher frequency. Compare one feature at a time with supplied records. Quiet observations are sufficient; learners do not need loud sound exposure to study these relationships.

An echo includes outward and return travelAn echo includes outward and return travelSource andreceiverReflecting wallOutward pathReturn pathTotal time: 0.40 sSpeed model: 340 m/sTotal path 136 m → one-way distance 68 mIllustrative model • no loud-sound experiment is needed
The source and receiver are at the same location in this straight-path model. Total distance is 340 m/s × 0.40 s = 136 m, shared between equal outward and return legs, so the wall is 68 m away. The drawing is not to distance scale. Dividing by two would not be valid for a delay that already describes one-way travel.

Sound travels at a steady but fairly leisurely pace — about 343 m/s in room-temperature air. That's quick by everyday standards, but far, far slower than light. This speed gap is exactly why you see a distant lightning flash almost instantly but hear its thunder seconds later: the light reaches you nearly at once, while the sound has to plod across all those kilometers. You can even estimate the distance using distance = speed × time — roughly every three seconds of delay means the storm is about a kilometer away.

Grasping how sound waves work explains a surprising amount of daily life — why a thick wall muffles noise (it's a poor medium for passing the vibration through cleanly), why your voice sounds different underwater, and why a tuning fork or a plucked guitar string produces one clear pitch. It's also the foundation of technologies like ultrasound scans, the sonar that maps the ocean floor, and the microphones and speakers that turn vibrating air into signals and back again.

Sound in air consists of pressure variations with compressions and rarefactions. Air particles move back and forth locally while the disturbance travels. Sound needs a material medium, so ordinary sound does not cross a vacuum.

Frequency largely determines pitch, and amplitude relates to loudness under comparable conditions. Sound speed depends on the medium and temperature. A higher-pitched sound in the same air does not simply travel faster because it has a higher frequency.

An echo involves a round trip from source to reflector and back. Multiply travel time by speed to find total path length, then divide by two for the distance to the reflector. Protect hearing: experiments do not require loud sounds or listening close to speakers.

Build your understanding

Where this fits

This unit helps answer the chapter question: How do waves and circuits transfer energy and information? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Model sound as pressure variations

Sound in air consists of pressure variations that pass through a material medium. Air particles move back and forth locally, creating compressions and rarefactions while the disturbance travels. An ordinary sound wave does not cross a vacuum because no medium particles are present to support that process.

Sound as compressions and rarefactionsSound is a pressure variation that needs a mediumcompressionrarefactiontravels through the mediumIn a vacuum there are no particles to compress, so sound does not travel
A sound wave is a pattern of crowding and spreading passing through matter. Drawing it as particles rather than as a curve makes the requirement obvious: remove the particles and there is nothing left to carry the pattern.

Read the visual model

What travels from a speaker to the listener in this model?

Visual reasoning — after your attempt

The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.

Draw a line of particles with alternating close and less-close spacing. Show travel and local motion along the same axis in the longitudinal model. Do not show one particle traveling all the way from a speaker to a listener.

Pause and explain

What travels from a speaker to the listener in this model?

Check your reasoning after answering

The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.

Part 2 · Distinguish pitch from loudness

Frequency largely determines perceived pitch. Amplitude and intensity relate to perceived loudness under comparable conditions, but distance, hearing, and frequency also affect perception. A louder tone need not have higher frequency, and a high-pitched tone is not necessarily louder. Protect hearing when discussing measurements.

Pitch and amplitude are different changesPitch and amplitude are different changesAPressure amplitude: 1 relative unit(s); 2 cycles in 0.01 sBPressure amplitude: 1 relative unit(s); 4 cycles in 0.01 sCPressure amplitude: 2 relative unit(s); 2 cycles in 0.01 sShared time interval: 0–0.01 sIllustrative model • no loud-sound experiment is needed
All three pressure records use the same time interval and vertical scale. A and C are 200 Hz but C has twice the pressure amplitude. B is 400 Hz at A’s amplitude. This supports separate pitch/amplitude comparisons under comparable receiver conditions; it does not assign an exact perceived loudness or require playing these tones.

Read the visual model

What changes if a tone’s amplitude increases while frequency stays fixed?

Visual reasoning — after your attempt

Under comparable conditions its intensity and perceived loudness generally increase, while its basic pitch remains linked to the unchanged frequency.

Compare two supplied records with equal cycle rate but different pressure amplitudes. They can have the same basic pitch and different loudness at the same listener position. The example does not require playing loud tones.

OpenSciEd graph A of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd Graph A tracks source position rather than air pressure or the path to an ear. Compare repeated peaks over the same 0.01 s window: B has the shortest period and C the longest, while their vertical excursions are similar. Read amplitude from the mid-position to a peak; partial cycles at the window ends should not be counted as complete cycles. Sound-source vibration graph A · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
OpenSciEd graph B of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd Graph B tracks source position rather than air pressure or the path to an ear. Compare repeated peaks over the same 0.01 s window: B has the shortest period and C the longest, while their vertical excursions are similar. Read amplitude from the mid-position to a peak; partial cycles at the window ends should not be counted as complete cycles. Sound-source vibration graph B · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
OpenSciEd graph C of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd Graph C tracks source position rather than air pressure or the path to an ear. Compare repeated peaks over the same 0.01 s window: B has the shortest period and C the longest, while their vertical excursions are similar. Read amplitude from the mid-position to a peak; partial cycles at the window ends should not be counted as complete cycles. Sound-source vibration graph C · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.

Read A/B/C in the figure as a separate supplied example: divide 2 or 4 cycles by 0.01 s for 200 or 400 Hz. A/B changes frequency while amplitude stays at one relative unit; A/C changes amplitude while frequency stays at 200 Hz. The graph is pressure variation at a location versus time, not the path of a moving air particle. Loudness remains a perception influenced by more than one condition.

Pause and explain

What changes if a tone’s amplitude increases while frequency stays fixed?

Check your reasoning after answering

Under comparable conditions its intensity and perceived loudness generally increase, while its basic pitch remains linked to the unchanged frequency.

Part 3 · State the conditions for sound speed

Sound speed depends on properties such as elasticity, density, and temperature of the medium. A dense material is not automatically a faster medium; stiffness also matters. Many solids carry sound faster than air, but particle closeness alone is an incomplete explanation. Use the supplied speed for a calculation.

Sound speed needs a named mediumSound speed needs a named mediumQuantity (m/s)Air model340Water model1500Solid model5000Illustrative model • no loud-sound experiment is needed
These supplied approximate model speeds are examples under specified medium conditions, not universal constants for every air, water or solid sample. The bars compare speed rather than pitch or density. Elastic properties, density and temperature matter; particle closeness alone does not explain all differences. Use the problem’s stated speed for a calculation.

Read the visual model

Why should a sound-speed value include a medium or condition?

Visual reasoning — after your attempt

Speed depends on the medium’s properties and temperature. One number is not a universal value for all gases, liquids, solids, or temperatures.

For introductory air calculations near room temperature, a supplied value around 340 m/s may be used. Higher pitch in the same ordinary air does not simply make sound travel faster. Temperature changes can alter the appropriate speed.

For the figure’s supplied models, a one-second disturbance travels 340, 1,500 or 5,000 m respectively. Those labels identify three different medium examples, not three pitches in the same air. Real composition and conditions change the appropriate value, so a medium label and assumptions belong beside the speed.

Pause and explain

Why should a sound-speed value include a medium or condition?

Check your reasoning after answering

Speed depends on the medium’s properties and temperature. One number is not a universal value for all gases, liquids, solids, or temperatures.

Part 4 · Use the full path for echoes

An echo time covers travel to a reflecting surface and back. Multiply speed by total time to find round-trip path length, then divide by two for one-way distance when source and receiver are at the same location. A one-way delay needs a different interpretation.

An echo includes outward and return travelAn echo includes outward and return travelSource andreceiverReflecting wallOutward pathReturn pathTotal time: 0.40 sSpeed model: 340 m/sTotal path 136 m → one-way distance 68 mIllustrative model • no loud-sound experiment is needed
The source and receiver are at the same location in this straight-path model. Total distance is 340 m/s × 0.40 s = 136 m, shared between equal outward and return legs, so the wall is 68 m away. The drawing is not to distance scale. Dividing by two would not be valid for a delay that already describes one-way travel.

Read the visual model

When should you divide speed×time by two?

Visual reasoning — after your attempt

When the measured time includes a round trip from the colocated source/receiver to a reflector and back, under the stated path and speed assumptions.

At 340 m/s, an echo after 0.5 s traveled 170 m total, so the reflector is 85 m away in the simple straight-path model. State the round-trip assumption before dividing by two.

Use the figure’s 0.40-second record as another worked case. The clock runs during both arrows, giving 136 m total and 68 m one way. Mark the measured time interval on your drawing before deciding whether to halve the calculated path. A thunder delay describes sound arriving after a light flash and is an approximately one-way case, so it is not treated like an echo.

Pause and explain

When should you divide speed×time by two?

Check your reasoning after answering

When the measured time includes a round trip from the colocated source/receiver to a reflector and back, under the stated path and speed assumptions.

Apply the whole unit · Interpret an echo and two tones

Illustrative record: an echo returns after 0.40 s; use sound speed 340 m/s. Tone A is 250 Hz and tone B 500 Hz, with otherwise comparable measurement conditions.

Your task

Calculate reflector distance, sketch the round-trip path, and compare basic pitch. Explain why frequency alone cannot establish which tone is louder or which arrives faster in the same ordinary air.

What your response should show

  • Use the supplied observations and measurements accurately; include units where needed.
  • Explain the science behind your answer, rather than giving only a number or label.
  • Show a labeled model, calculation, or evidence comparison as requested.
  • State an assumption or limitation and suggest a useful next measurement.
Response guidance — after your attempt

Round-trip path=340×0.40=136 m, so reflector distance is 68 m. B has higher basic pitch because its frequency is higher. Loudness needs amplitude/intensity and listening conditions; frequency alone is insufficient. Under the stated ordinary-air approximation both have essentially the supplied speed. The distance calculation assumes a direct out-and-back path.

Scientific references and adapted teaching resources

Parent / teacher teaching notes

Use a path drawing before division. Keep tests quiet and optional; supplied data are sufficient. Replace explanations based only on particles being closer with medium properties and stated conditions. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation.

The key idea

Sound is a longitudinal wave of compressions and rarefactions that needs a medium; frequency controls pitch, and amplitude controls loudness.

Distance from Speed and Time

d = v × t

v is the speed of sound (about 343 m/s in air); t is time in seconds; d is distance in meters.

Worked examples

Example 1

Lightning is seen instantly, but its thunder is heard 6 seconds later. About how far away did the lightning strike, using the speed of sound as 343 m/s?
  1. Use distance = speed × time.
  2. d = 343 × 6.
  3. = 2,058.
Answer: About 2,058 m (roughly 2 km) away

Example 2 · Worked application

A sound echo returns after 0.4 s. Use sound speed 340 m/s to find the distance to the wall.
  1. Sound travels to the wall and back.
  2. Round-trip distance=340×0.4=136 m.
  3. One-way distance=136/2=68 m.
Answer: The wall is 68 m away in this model.
Check the reasoning

Remember the return path; the echo delay is not one-way time.

Example 3

A sound wave in air has a frequency of 686 Hz. Using the speed of sound as 343 m/s, find its wavelength.
  1. Rearrange v = f × λ to solve for wavelength: λ = v ÷ f.
  2. λ = 343 ÷ 686.
  3. = 0.5.
Answer: 0.5 m

Example 4

An echo returns after 0.4 s. Using sound speed 340 m/s, find the distance to the reflecting wall.
  1. Total path=340×0.4=136 m.
  2. The path goes to the wall and back, so wall distance=136/2=68 m.
Answer: 68 m to the wall in this model.

Example 5 · Find and fix the mistake

Context: A sound echo returns after 0.4 s. Use sound speed 340 m/s to find the distance to the wall. A student claims: “A louder sound always has a higher pitch.” Evaluate the claim and write a corrected explanation.
  1. Test the claim against the stated conditions: Remember the return path; the echo delay is not one-way time.
  2. Loudness relates to amplitude and perception; pitch relates primarily to frequency. They can vary independently.
Answer: Loudness relates to amplitude and perception; pitch relates primarily to frequency. They can vary independently.
Check the reasoning

Remember the return path; the echo delay is not one-way time.

Example 6 · Interpret the visual model

Sound as compressions and rarefactionsSound is a pressure variation that needs a mediumcompressionrarefactiontravels through the mediumIn a vacuum there are no particles to compress, so sound does not travel
A sound wave is a pattern of crowding and spreading passing through matter. Drawing it as particles rather than as a curve makes the requirement obvious: remove the particles and there is nothing left to carry the pattern.
What travels from a speaker to the listener in this model?
  1. Identify the represented objects, axes, labeled quantities and stated conditions before choosing a rule.
  2. Draw a line of particles with alternating close and less-close spacing. Show travel and local motion along the same axis in the longitudinal model. Do not show one particle traveling all the way from a speaker to a listener.
  3. The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.
Answer: The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.
Check the reasoning

Keep the model’s stated limits explicit: A sound wave is a pattern of crowding and spreading passing through matter. Drawing it as particles rather than as a curve makes the requirement obvious: remove the particles and there is nothing left to carry the pattern.

Avoid these mistakes

Check your understanding

An echo returns after 0.4 s. Using sound speed 340 m/s, find the distance to the reflecting wall.

Discuss the answer

68 m to the wall in this model.

Why does sound need a medium while light can cross space?

Discuss the answer

Sound in air consists of pressure variations with compressions and rarefactions. Air particles move back and forth locally while the disturbance travels. Sound needs a material medium, so ordinary sound does not cross a vacuum. Frequency largely determines pitch, and amplitude relates to loudness under comparable conditions. Sound speed depends on the medium and temperature. A higher-pitched sound in the same air does not simply travel faster because it has a higher frequency.

Return to the phenomenon

An illustrative echo returns 0.30 s after a short sound. The modeled sound speed is 340 m/s, and the path goes from source to wall and back. Find one-way distance and explain the medium’s role. Distinguish particle motion from the travelling disturbance. Revise your first explanation using a named idea from the lesson. Mark the changed part and the evidence that supports it.

Compare explanations after your attempt

The round-trip path is 340×0.30=102 m, so one-way distance is 51 m. Ordinary sound needs matter, with successive compressions and expansions transmitting a disturbance. Individual air particles mainly oscillate near their positions rather than travel the complete path. The arithmetic assumes the stated speed and clear timing.

What should we investigate next?

Which condition could be changed to test a remaining uncertainty in this explanation? State the predicted observation and what result would challenge your model.

Investigate, explain and revise

Use these tasks after the three teaching sessions as needed. Discuss an attempted explanation before opening response guidance; a matching sentence is less important than a supported mechanism.

Activity 1 · Explain the new evidence case

Assigned reading and preparation

Use the visible scenario above, Parts 1–4 and your first notebook model. No outside reading or equipment is required.

Your task

An illustrative echo returns 0.30 s after a short sound. The modeled sound speed is 340 m/s, and the path goes from source to wall and back. Find one-way distance and explain the medium’s role. Distinguish particle motion from the travelling disturbance. Write 100–150 words or give a labeled drawing with an equivalent oral explanation.

Success criteria

  • Use two relevant stated records or conditions.
  • Explain the mechanism rather than only repeat a definition.
  • Label observation and inference.
  • Name a changed model feature or a remaining uncertainty.
Hint

Read the stated boundary and conditions before selecting a relationship. The unit’s worked examples show intermediate steps.

Response guidance — discuss after your attempt

The round-trip path is 340×0.30=102 m, so one-way distance is 51 m. Ordinary sound needs matter, with successive compressions and expansions transmitting a disturbance. Individual air particles mainly oscillate near their positions rather than travel the complete path. The arithmetic assumes the stated speed and clear timing.

Activity 2 · Read and extend the figure

Assigned reading and preparation

Study the new labeled figure and its caption, then compare it with the associated teaching part.

Your task

Do these dots need to travel from speaker to ear? Explain what each important label represents. Add one missing feature that would be needed for a different question, and explain why the original figure leaves it out.

Success criteria

  • Answer the figure question with its stated labels.
  • Connect the visual representation to the scientific idea.
  • Identify a specific limitation from the caption.
  • Propose a useful change without misrepresenting supplied evidence.
Hint

The dots model local particle crowding rather than literal air colors. Compressions and rarefactions form a traveling disturbance while particles mainly oscillate locally. The two arrow types distinguish disturbance propagation from local motion; the schematic omits particle-scale distances and random thermal motion.

Response guidance — discuss after your attempt

No. Successive interactions can transfer the disturbance while the particles mainly move near their local positions. The dots model local particle crowding rather than literal air colors. Compressions and rarefactions form a traveling disturbance while particles mainly oscillate locally. The two arrow types distinguish disturbance propagation from local motion; the schematic omits particle-scale distances and random thermal motion.

Words to know

longitudinal wave
A wave where particles vibrate back and forth in the SAME direction the wave travels.
compression
A region of a longitudinal wave where particles are squeezed close together.
rarefaction
A region of a longitudinal wave where particles are spread farther apart.
pitch
How high or low a sound seems, determined by its frequency.
medium
The matter (solid, liquid, or gas) that a wave travels through.
model
A representation that explains selected features and makes predictions; it leaves other details out.
evidence
Relevant observations or records used to support or challenge a claim.
system boundary
The chosen limit separating the objects being studied from their surroundings.
assumption
A condition taken as true for a model or calculation and needing to be checked against the case.

Sound travels faster through solids and liquids than through air

Sound needs particles bumping into their neighbors to travel, so the more tightly packed those particles are, the faster a sound wave can move through them. Sound travels through air at about 343 m/s, but through water at about 1,480 m/s — over four times faster — and through solid steel at roughly 5,960 m/s, more than seventeen times faster than air. That's why old Western movies show characters pressing an ear to a train track: solid metal rails carry the sound of an approaching train far faster (and farther) than the air ever could.

The Doppler effect: why a siren's pitch drops as it passes you

In 1842, Austrian physicist Christian Doppler proposed that a moving sound source should seem to shift pitch for a listener — sound waves bunch closer together in front of a source moving toward you (raising the frequency and pitch you hear) and stretch farther apart behind it (lowering the pitch). Three years later, Dutch scientist Buys Ballot tested the idea in a memorable experiment: trumpet players performed a single sustained note aboard a moving train, while musicians with sharp pitch memory listened from the platform and confirmed the pitch really did shift exactly as Doppler predicted. You hear this exact effect every time an ambulance siren passes you on the street.

Decibels: measuring loudness on a logarithmic scale

Like the pH scale, the decibel (dB) scale used for loudness is logarithmic, not a simple straight-line ruler: every increase of 10 dB means the sound's intensity is 10 times greater, not just '10 units louder.' A whisper measures around 30 dB, normal conversation around 60 dB, and a loud rock concert around 110 dB — meaning that concert isn't just 'somewhat louder' than a conversation, its sound intensity is roughly 100,000 times greater (a 50 dB gap = 10⁵). That's exactly why sustained exposure to loud concerts and headphones can damage hearing so much faster than people expect.

Echolocation: seeing with sound

Bats and dolphins navigate and hunt using echolocation: they emit rapid ultrasonic pulses — sound waves with frequencies above 20,000 Hz, higher than human ears can detect — and listen for the returning echoes to build a detailed picture of their surroundings, including the size, shape, distance, and even texture of nearby objects and prey. Humans borrowed the same trick technologically: sonar lets ships and submarines 'see' underwater using sound pulses, and medical ultrasound imaging uses the same principle of interpreting echoes to safely picture a developing fetus or internal organs without any radiation.

In the real world

Counting the seconds between seeing lightning and hearing thunder lets you estimate how far away a storm is.

Exit check

Do these dots need to travel from speaker to ear? Explain the relevant mechanism and one model limitation.

Check your explanation after answering

No. Successive interactions can transfer the disturbance while the particles mainly move near their local positions. The dots model local particle crowding rather than literal air colors. Compressions and rarefactions form a traveling disturbance while particles mainly oscillate locally. The two arrow types distinguish disturbance propagation from local motion; the schematic omits particle-scale distances and random thermal motion.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Using the speed of sound as 340 m/s, how far away is a thunderstorm if thunder is heard 4 seconds after the lightning flash?
Answer and explanation

1360 m

distance = speed × time = 340 × 4 = 1,360 m.

Problem 2

Sound is what type of wave?
  1. transverse
  2. longitudinal
  3. electromagnetic
  4. standing only
Answer and explanation

longitudinal

Sound particles vibrate in the same direction the wave travels, making it a longitudinal wave.

Problem 3

Increasing the frequency of a sound wave makes the pitch…
  1. higher
  2. lower
  3. louder only
  4. quieter only
Answer and explanation

higher

Higher frequency corresponds to higher pitch.

Problem 4

Sound CANNOT travel through…
  1. water
  2. steel
  3. a vacuum (empty space)
  4. air
Answer and explanation

a vacuum (empty space)

Sound needs a medium of particles to travel through, so it cannot travel through a vacuum.

Problem 5

Using the words compression and rarefaction, explain how a speaker creates sound.
Answer and explanation

The speaker cone vibrates back and forth, pushing air molecules together to create compressions and pulling them apart to create rarefactions. This repeating pattern travels outward through the air as a sound wave.

Problem 6

A sound wave travels at 340 m/s and has a wavelength of 0.85 m. Find its frequency.
Answer and explanation

400 Hz

f = v ÷ λ = 340 ÷ 0.85 = 400 Hz.

Problem 7

An echo returns after 0.4 s. Using sound speed 340 m/s, find the distance to the reflecting wall. Explain the reasoning in your own words.
Answer and explanation

68 m to the wall in this model.

Total path=340×0.4=136 m.

The path goes to the wall and back, so wall distance=136/2=68 m.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

A sound has frequency 200 Hz and another 400 Hz. Which is higher pitched under the simple comparison? Show the reasoning, not just the conclusion.
Hints

Remember the return path; the echo delay is not one-way time.

Answer and explanation

The 400 Hz sound, because its frequency is higher.

Problem 9

Why cannot ordinary sound travel through a vacuum between two objects?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Sound needs particles of a medium to transmit mechanical disturbances; a vacuum lacks that medium.

Problem 10

Sound as compressions and rarefactionsSound is a pressure variation that needs a mediumcompressionrarefactiontravels through the mediumIn a vacuum there are no particles to compress, so sound does not travel
A sound wave is a pattern of crowding and spreading passing through matter. Drawing it as particles rather than as a curve makes the requirement obvious: remove the particles and there is nothing left to carry the pattern.
For this unit’s illustrated case, what travels from a speaker to the listener in this model?
Answer and explanation

The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.

Draw a line of particles with alternating close and less-close spacing. Show travel and local motion along the same axis in the longitudinal model. Do not show one particle traveling all the way from a speaker to a listener.

The pressure disturbance and energy travel through the air. Individual particles move locally rather than carrying themselves from speaker to listener.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

An echo returns after 0.3 s in a supplied air model with sound speed 340 m/s. Find the reflector distance and explain why using the full 102 m as the one-way distance is wrong.
Answer and explanation

Round-trip length is 340×0.3 = 102 m, so reflector distance is 51 m. The measured time includes travel out and back.

Draw both travel legs.

Divide total path by two only under the stated same-path out-and-back comparison.

Round-trip length is 340×0.3 = 102 m, so reflector distance is 51 m. The measured time includes travel out and back.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

OpenSciEd graph A of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd 8.2 Sound Waves, Lesson 5: actual position-versus-time record A. Read the labeled axes and estimate a cycle interval before opening the worked guidance. The vertical value is the vibrating point’s position relative to a coordinate origin, not air pressure or sound wavelength. Sound-source vibration graph A · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Read OpenSciEd record A. Estimate a peak-to-peak cycle interval, calculate cycles per second and measure displacement amplitude about the midline. Explain why its mean position near 20 mm is not its amplitude. Approximate graph readings are expected.
Answer and explanation

One cycle takes about 0.0033 s, giving frequency about 300 Hz using cycles per second = 1/period. The midline is near 20 mm and extrema are about 19.2 and 20.8 mm, so displacement amplitude is about 0.8 mm. Amplitude measures departure from the midline, not distance from the coordinate origin. Allow reasonable approximate readings from this source graph.

Compare consecutive peaks: about 0.0033 s per cycle.

Compute 1/period, approximately 300 cycles per second, or Hz.

Amplitude is about 20.8−20 = 0.8 mm (half the peak-to-peak displacement).

Response criteria

  • Read time on the horizontal axis; do not call it wavelength.
  • Estimate frequency from repeats per second.
  • Measure amplitude from the midline and distinguish coordinate origin from equilibrium.

Problem 13

OpenSciEd graph B of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd 8.2 Sound Waves, Lesson 5: actual position-versus-time record B. Read the labeled axes and estimate a cycle interval before opening the worked guidance. The vertical value is the vibrating point’s position relative to a coordinate origin, not air pressure or sound wavelength. Sound-source vibration graph B · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Read OpenSciEd record B. Estimate its period, frequency and amplitude. Compare it with record A, whose frequency is about 300 Hz and amplitude about 0.8 mm. Which vibration property has changed by a factor of two, and which has stayed approximately the same?
Answer and explanation

One cycle takes about 0.0017 s, giving frequency about 600 Hz using cycles per second = 1/period. The midline is near 20 mm and extrema are about 19.2 and 20.8 mm, so displacement amplitude is about 0.8 mm. Amplitude measures departure from the midline, not distance from the coordinate origin. Allow reasonable approximate readings from this source graph. B has about twice A’s frequency and half its period, while displacement amplitude stays approximately 0.8 mm. Faster repetition does not automatically mean a larger vibration.

Compare consecutive peaks: about 0.0017 s per cycle.

Compute 1/period, approximately 600 cycles per second, or Hz.

Amplitude is about 20.8−20 = 0.8 mm (half the peak-to-peak displacement).

Response criteria

  • Read time on the horizontal axis; do not call it wavelength.
  • Estimate frequency from repeats per second.
  • Measure amplitude from the midline and distinguish coordinate origin from equilibrium.

Problem 14

OpenSciEd graph C of source position in millimetres against time from 0 to 0.01 seconds, showing repeated vibrations around a mean position of about 20 millimetres.
OpenSciEd 8.2 Sound Waves, Lesson 5: actual position-versus-time record C. Read the labeled axes and estimate a cycle interval before opening the worked guidance. The vertical value is the vibrating point’s position relative to a coordinate origin, not air pressure or sound wavelength. Sound-source vibration graph C · OpenSciEd · 8.2 Sound Waves, Lesson 5, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Read OpenSciEd record C. Estimate period, frequency and amplitude. The 0.010 s window contains about one and a half cycles. Why would counting only its one fully completed cycle underestimate the frequency? Use the peak-to-peak interval or include the partial cycle.
Answer and explanation

One cycle takes about 0.0067 s, giving frequency about 150 Hz using cycles per second = 1/period. The midline is near 20 mm and extrema are about 19.2 and 20.8 mm, so displacement amplitude is about 0.8 mm. Amplitude measures departure from the midline, not distance from the coordinate origin. Allow reasonable approximate readings from this source graph. About 1.5 cycles in 0.010 s gives 150 Hz. Counting only one ignores the half-cycle and would incorrectly give 100 Hz; use the measured period or account for the partial cycle.

Compare consecutive peaks: about 0.0067 s per cycle.

Compute 1/period, approximately 150 cycles per second, or Hz.

Amplitude is about 20.8−20 = 0.8 mm (half the peak-to-peak displacement).

Response criteria

  • Read time on the horizontal axis; do not call it wavelength.
  • Estimate frequency from repeats per second.
  • Measure amplitude from the midline and distinguish coordinate origin from equilibrium.

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