Sound Sources, Vibrations & Receivers — Practice · Grade 8 Science

Chapter: Waves, Sound & Light

What this practice covers

By the end of Sound Sources, Vibrations & Receivers you should be able to:

15 questions, written answer, each with a worked explanation.

Vocabulary used: vibration, source, receiver, amplitude, frequency, model, evidence, system boundary, assumption.

Problem 1

Does no visible motion prove that a source is not vibrating?
Answer and explanation

No. The motion can be too fast or small to detect with the chosen method. A suitable measurement or record is needed.

No. The motion can be too fast or small to detect with the chosen method. A suitable measurement or record is needed.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

What is B’s amplitude and what quantity remains unchanged?
Answer and explanation

Amplitude is 3 mm. The cycle count per time, and hence frequency of 200 Hz, is unchanged in this comparison.

Amplitude is 3 mm. The cycle count per time, and hence frequency of 200 Hz, is unchanged in this comparison.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Why is counting only crests without an interval insufficient?
Answer and explanation

Frequency needs the number of complete cycles divided by a defined time. An unscaled drawing alone does not establish hertz.

Frequency needs the number of complete cycles divided by a defined time. An unscaled drawing alone does not establish hertz.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 4

What is missing from a drawing with only a source and an ear?
Answer and explanation

The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 5

Illustrative sources A/B/C have amplitudes 1/3/1 mm and complete 4/4/6 cycles respectively in 0.02 s. Compare frequency and amplitude and explain what a receiver prediction would need.
Answer and explanation

A and B are 200 Hz, while C is 300 Hz. B has three times A’s amplitude; C has A’s amplitude but higher frequency. Frequency and amplitude are distinct. Predicting received sound also needs distance, medium, losses and receiver response. The source records alone do not establish an exact perceived loudness.

A and B are 200 Hz, while C is 300 Hz. B has three times A’s amplitude; C has A’s amplitude but higher frequency. Frequency and amplitude are distinct. Predicting received sound also needs distance, medium, losses and receiver response. The source records alone do not establish an exact perceived loudness.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

Two sources share 250 Hz but have different amplitudes. Which sound property is expected to differ under comparable conditions?
Answer and explanation

Pitch is associated with the shared frequency, while loudness may differ with amplitude under comparable source, medium, distance and receiver conditions. Neither quantity is a universal perception guarantee.

Pitch is associated with the shared frequency, while loudness may differ with amplitude under comparable source, medium, distance and receiver conditions. Neither quantity is a universal perception guarantee.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 7

Evaluate and correct: A louder sound must always have a higher pitch.
Answer and explanation

Amplitude and frequency can vary independently. Compare graph height and cycles per stated time separately.

Amplitude and frequency can vary independently. Compare graph height and cycles per stated time separately.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

A simplified chain is ruler → nearby air → farther air → receiver. A paper membrane can be a model receiver, but detectable motion depends on many factors. Failure to see it move at quiet volume is not evidence that energy transfer never occurs. The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

A simplified chain is ruler → nearby air → farther air → receiver. A paper membrane can be a model receiver, but detectable motion depends on many factors. Failure to see it move at quiet volume is not evidence that energy transfer never occurs. The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

A source completes 8 cycles in 0.04 s at amplitude 2 mm. Another completes 8 cycles over the same time at 4 mm. Compare the changed quantity and a limit of predicting what a person hears.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

Both have frequency 200 Hz; amplitude doubles. Frequency-related pitch is comparable in the simple model, while received loudness also depends on distance, medium and receiver. The amplitude comparison alone does not fix a person’s perception.

Both have frequency 200 Hz; amplitude doubles. Frequency-related pitch is comparable in the simple model, while received loudness also depends on distance, medium and receiver. The amplitude comparison alone does not fix a person’s perception.

Problem 10

Two sources share 250 Hz but have different amplitudes. Which sound property is expected to differ under comparable conditions? Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

Pitch is associated with the shared frequency, while loudness may differ with amplitude under comparable source, medium, distance and receiver conditions. Neither quantity is a universal perception guarantee.

Pitch is associated with the shared frequency, while loudness may differ with amplitude under comparable source, medium, distance and receiver conditions. Neither quantity is a universal perception guarantee.

Problem 11

Change vibration size without changing cycle rateChange vibration size without changing cycle rateSmaller displacement amplitudeLarger displacement amplitudeSame time window; both complete two cycles.
Both source-position records show two cycles during the same stated time window, with different displacement amplitudes. Amplitude is measured from the middle position to a peak, not from peak to trough. The sketches compare patterns qualitatively; an actual amplitude and frequency require numerical axis scales, as in the OpenSciEd graphs used below.
For this unit’s illustrated case, which record has a higher frequency? What quantity differs, and what additional labels are needed for a numerical amplitude?
Answer and explanation

Neither has a higher frequency in this matched window. Displacement amplitude differs; a numerical vertical position scale and units are needed to measure it.

A cycle is one complete repeat, such as one peak to the next matching peak. A taller trace does not automatically have more cycles. Compare amplitude and frequency one at a time rather than using the word louder for both.

Neither has a higher frequency in this matched window. Displacement amplitude differs; a numerical vertical position scale and units are needed to measure it.

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

A source-position trace has peaks 0.2 s apart and swings from −3 to +3 mm around zero. Find period, frequency and amplitude, and distinguish amplitude from peak-to-trough span.
Answer and explanation

Period 0.2 s; frequency 5 Hz; amplitude 3 mm; peak-to-trough span 6 mm. The source moves locally rather than traveling across the sound path.

Use matching-peak separation for period and its reciprocal for frequency.

Measure amplitude from the middle position to a peak.

Period 0.2 s; frequency 5 Hz; amplitude 3 mm; peak-to-trough span 6 mm. The source moves locally rather than traveling across the sound path.

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 13

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 14

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 15

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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