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Concept module

Sound Waves and Longitudinal Motion

See sound as a longitudinal wave by keeping parcel motion, compression and rarefaction, probe timing, and energy transfer tied to one compact medium-first bench.

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Starter track

Step 1 of 50 / 5 complete

Sound and Acoustics

Next after this: Pitch, Frequency, and Loudness / Intensity.

1. Sound Waves and Longitudinal Motion2. Pitch, Frequency, and Loudness / Intensity3. Beats4. Doppler Effect+1 more steps

This concept is the track start.

Also in Waves.

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Why it behaves this way

Explanation

Sound in a gas or liquid is a longitudinal wave. The disturbance moves through the medium, but the parcels in the medium do not ride across the whole tube with it. They oscillate back and forth around local resting positions while compressions and rarefactions move onward.

This bench keeps one source piston, one particle train, one pressure ribbon, and one movable probe on the same stage. The source and probe timing, the local compression cue, and the right-moving disturbance all come from the same displacement field, so sound stays visually honest without turning into a full acoustics engine.

Key ideas

01In a longitudinal wave, parcel motion is parallel to the direction the disturbance travels, not perpendicular to it.

02A compression is a region where neighboring parcels are crowded more tightly than usual, while a rarefaction is a region where they are spread farther apart.

03Sound transfers energy through the medium because each parcel pushes on its neighbor, so the disturbance and its energy move onward even though each parcel only oscillates locally.

Frozen walkthrough

Step through the frozen example

Frozen walkthrough

These checks read the same source, probe, and compression state that the stage and graphs already show, so the algebra never disconnects from the longitudinal wave.

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Frozen valuesUsing frozen parameters

For the current sound wave with $v_{w a v e} = 2.4 m/s$ and $λ = 1.8 m$ , what source frequency does that require, and how long does the disturbance take to reach the probe at $x_{p} = 2.25 m$ ?

v_{w a v e}

Wave speed

2.4 m/s

λ

Wavelength

1.8 m

x_{p}

Probe position

2.25 m

f

Frequency

1.33 Hz

Δ t

Travel delay

0.94 s

1. Start from the wave timing relations

Use

v_{w a v e} = f λ

, so

f = \frac{v _{w a v e}}{λ}

, and use

Δ t = \frac{x _{p}}{v _{w a v e}}

for the probe delay.

2. Substitute the live speed, spacing, and probe position

f = \frac{2}{.} 41.8 = 1.33 Hz

and

Δ t = \frac{2}{.} 252.4 = 0.94 s

3. Interpret what the source and probe must do

That means the source launches 1.33 cycles each second, each cycle lasts 0.75 s, and the probe parcel repeats that motion after a delay of 0.94 s.

Current sound timing

f = 1.33 Hz, Δ t = 0.94 s

The probe is close enough to the source that the travel delay is short, but the same right-moving disturbance still has to propagate through the medium before the parcel responds.

Compression checkpoint

The medium stays the same, so the wave speed is fixed. What single change makes compressions pack closer together while the particle motion stays longitudinal?

Make a prediction before you reveal the next step.

Decide whether you should increase amplitude, shorten wavelength, or move the probe farther right.

Check your reasoning against the live bench.

Shorten the wavelength.

Compression spacing follows the wavelength. Shortening

λ

packs more compressions and rarefactions into the same distance without changing the fact that each parcel still oscillates back and forth along the tube.

Common misconception

If the sound wave moves to the right, each air parcel must also drift steadily to the right with it.

The disturbance travels to the right, but each parcel mainly oscillates back and forth around its own equilibrium position.

What moves onward are the compression and rarefaction pattern and the energy transfer through the medium, not one chunk of matter riding all the way across the stage.

Quick test

Misconception check

Question 1 of 4

Use the linked particle and pressure views, not memory alone. These checks focus on what longitudinal sound waves mean physically.

Which statement best describes the difference between parcel motion and wave propagation in this sound model?

Use the live bench to test the result before moving on.

Accessibility

The simulation shows a horizontal tube with a source piston on the left, a row of parcel markers inside the tube, and a movable probe parcel. A colored ribbon inside the tube marks local compression and rarefaction, while optional overlays label parcel-motion direction, compression regions, and the rightward energy-transfer cue.

Changing particle amplitude, wave speed, wavelength, or probe position updates the same stage, probe graphs, and readout card so parcel motion, compression state, and travel delay stay synchronized.

Graph summary

The first graph compares source and probe parcel displacement over time so the downstream delay stays tied to the live sound bench.

The second graph compares the probe parcel's normalized shift with the local compression cue, which helps the learner separate one parcel's position from the local crowding of the medium.

Carry sound into later wave ideas

Keep this idea moving

Open the next concept, route, or track only when you want the current model to widen into a larger branch.

Pitch vs loudnessOscillations

Sound Waves and Longitudinal Motion

Sound and Acoustics

Explanation

Step through the frozen example

For the current sound wave with vwave​=2.4m/s and λ=1.8m, what source frequency does that require, and how long does the disturbance take to reach the probe at xp​=2.25m?

Which statement best describes the difference between parcel motion and wave propagation in this sound model?

Keep this idea moving

Pitch, Frequency, and Loudness / Intensity

Beats

Doppler Effect

For the current sound wave with $v_{w a v e} = 2.4 m/s$ and $λ = 1.8 m$ , what source frequency does that require, and how long does the disturbance take to reach the probe at $x_{p} = 2.25 m$ ?