Explain sound production by vibrating sources
Trace a vibrating source's effect on a surrounding material and distinguish frequency from how far the source moves.
Section P3.4 · Sound
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recognise a repeated oscillation.
- Distinguish energy transfer from material transport.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is one complete vibration?
What you will learn
- Describe sound production by vibrating sources.
- Identify repeated motion rather than transport of the source to the listener.
Find the moving source
A tuning fork, loudspeaker cone or instrument string can produce sound when it vibrates and disturbs a surrounding medium. Repeated source motion pushes on nearby material, transferring a changing disturbance outward. The whole source does not need to travel to the listener.
A tuning fork vibrates about its equilibrium position rather than steadily moving away. A speaker cone moves back and forth. In a real demonstration, whether a sound is audible also depends on frequency, strength, coupling and the listener; vibration alone does not guarantee a clearly heard sound.
Describe evidence and its limits
Source motion can be observed or detected separately from hearing the emitted sound. A visibly moving cone and an appropriate sound detector provide evidence for the linked process. Touching a vibrating object can alter its motion, so do not treat that interference as an unchanged measurement.
Frequency counts complete vibrations per second; amplitude describes displacement from equilibrium. These are different measurements. A source making 120 complete cycles in 0.50 s has frequency 240 Hz, regardless of the separately stated displacement amplitude.
Sources for this lesson
Pause and explain
What is the frequency of 120 complete cycles in 0.50 s?
Worked example
A source makes 120 complete vibrations in 0.50 s. Find its frequency and explain its sound production.
Show the worked solution
- Frequency = cycles/time = 120/0.50.
- The result is 240 Hz.
- The vibrating source disturbs a surrounding medium.
Answer 240 Hz; repeated source motion transfers a disturbance into the medium.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how a loudspeaker cone produces sound in air.
Give me a hint
Link source vibration to disturbance of nearby material.
Compare my reasoning
- The cone moves back and forth.
- It pushes on nearby air.
- A changing disturbance transfers energy outward.
A vibrating cone disturbs air and produces a travelling sound disturbance.
Look for these in your work
- Identify vibration.
- Include the medium.
A tuning fork completes 80 vibrations in 0.20 s. Calculate frequency.
Give me a hint
Use complete cycles per second.
Compare my reasoning
- Count = 80 complete cycles.
- Frequency = 80/0.20.
- Frequency = 400 Hz.
400 Hz.
Look for these in your work
- Divide by time.
- Give hertz.
A vibrating object is not heard by a particular observer. Does that prove it is stationary?
Give me a hint
Separate motion evidence from audibility conditions.
Compare my reasoning
- Motion may be observed independently.
- The sound may be weak or outside that listener's hearing range.
- No heard sound alone does not prove no vibration.
No; vibration and a listener's audibility are different observations.
Look for these in your work
- Identify an alternative explanation.
- Avoid overstating negative hearing evidence.
Common mistakes
- Count complete cycles.
- The source does not travel to the listener.
- Vibration frequency and amplitude are different quantities.
What can you explain now?
Must a loudspeaker cone move all the way to the listener?
Compare with the explanation
No.
Its local back-and-forth motion launches a disturbance in the surrounding air.
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Make it stick
Explain a vibrating source and calculate frequency from complete cycles without confusing amplitude or travel.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.