CLASS 9 SCIENCE · CHAPTER 10 · STUDY NOTES
Sound Waves: Characteristics and Applications
Understand sound waves: characteristics and applications through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
A vibrating source starts the disturbance
Look for the part moving to and fro.A string moves about its resting line
- Sound source
- The vibrating object or material that produces a sound disturbance.
- Vibration
- To-and-fro motion about an equilibrium position; a steady tone involves repeated oscillation.
A plucked string, stretched membrane and air column can vibrate. A tuning fork’s prongs oscillate; a flute mainly uses vibrating air. Several parts of an instrument can interact.
Vocal folds in the larynx vibrate to produce voiced sound, while the tongue, lips and cavities shape speech. Not every speech sound needs vocal-fold vibration.
Take a moment. What do you think?
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Question
In a flute, is the air column or just the outside pipe the main vibrating sound source?
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EXPLORE
Sound needs material to carry it
Light and radio waves differ from sound here.A ringing bell needs a transmission route
- Medium
- The material through which a wave propagates.
- Mechanical wave
- A disturbance that needs a material medium.
Sound can travel through gases, liquids and solids. In a bell-jar demonstration, removing air reduces the sound heard even though the bell still vibrates. Some vibration can still pass through the support, so the apparatus is not a perfect isolation.
Sound does not cross empty space as ordinary acoustic waves. Astronauts use radio, which can travel through a vacuum; voices are converted to signals and back into sound inside their suits.
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Question
How can astronauts communicate across near-vacuum if sound needs a medium?
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EXPLORE
A pattern travels; particles oscillate locally
Do not imagine air flowing all the way from speaker to ear.Track a marked particle as the pattern shifts
- Compression / rarefaction
- The pointers follow a closely spaced region and a widely spaced region in each frame. The marked orange particle moves locally, not from one end of the medium to the other.
- Compression
- A region with pressure and density above their mean values.
- Rarefaction
- A region with pressure and density below their mean values.
- Longitudinal wave
- Particle displacement is parallel to wave propagation.
A source’s forward motion can compress nearby air; backward motion can create a rarefaction. Interactions between neighbours carry the disturbance onward.
The organised sound displacement is small to and fro about local mean positions, superimposed on ordinary thermal motion. Energy transfers through the medium without the source’s air particles travelling the whole route.
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Question
Does the coloured particle need to travel with the compression to the far end?
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EXPLORE
Compare the two directions
Wave direction and particle-motion direction are separate.Read the direction pair
- Transverse wave
- The medium’s displacement is perpendicular to the direction of propagation.
Sound in air and water is longitudinal. A transverse rope wave has up-down displacement while its pattern can travel sideways. Elastic solids can support different kinds of mechanical waves.
A wavy density graph of air sound does not make the actual sound wave transverse. Its vertical coordinate represents a change in density, not an air particle moving up and down.
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Question
Does a sinusoidal sound-density graph imply upward and downward particle motion?
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EXPLORE
A receiver can respond without touching the source
Vibration transfers energy through the medium.Signal information guides a powered output
- Sound energy transfer
- Transfer of mechanical energy through propagating pressure and density disturbances.
Sound can make a light diaphragm vibrate. This is the basis of many microphones: sound-related motion is converted to an electrical signal. A powered speaker converts an electrical signal into diaphragm motion and sound.
A microphone signal alone does not supply the energy for a powerful speaker; the amplifier’s power supply provides that. Avoid describing amplification as energy created from nothing.
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Question
What supplies the extra energy for a loud amplified speaker?
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A snapshot and a time record answer different questions
Read the horizontal axis before measuring a cycle.Density change at one instant
Density change from mean (schematic units)
Distance (cm)
- Spatial sound pattern · Mean density has been subtracted.
- 1. Compression · (0, 1)
- 2. Rarefaction · (1, -1)
- 3. Next compression · (2, 1)
Assumptions: Schematic density deviation: crests are compressions, troughs rarefactions. Distance axis; wavelength=2 cm. This is not an air-particle trajectory.
Read the plotted values
Spatial sound pattern
| Distance (cm) | Density change from mean (schematic units) |
|---|---|
| 0 | 1 |
| 0.08333333333333333 | 0.9659258262890683 |
| 0.16666666666666666 | 0.8660254037844387 |
| 0.25 | 0.7071067811865476 |
| 0.3333333333333333 | 0.5000000000000001 |
| 0.4166666666666667 | 0.25881904510252074 |
| 0.5 | 6.123233995736766e-17 |
| 0.5833333333333334 | -0.25881904510252085 |
| 0.6666666666666666 | -0.4999999999999998 |
| 0.75 | -0.7071067811865475 |
| 0.8333333333333334 | -0.8660254037844387 |
| 0.9166666666666666 | -0.9659258262890682 |
| 1 | -1 |
| 1.0833333333333333 | -0.9659258262890684 |
| 1.1666666666666667 | -0.8660254037844386 |
| 1.25 | -0.7071067811865477 |
| 1.3333333333333333 | -0.5000000000000004 |
| 1.4166666666666667 | -0.25881904510252063 |
| 1.5 | -1.8369701987210297e-16 |
| 1.5833333333333333 | 0.2588190451025203 |
| 1.6666666666666667 | 0.5000000000000001 |
| 1.75 | 0.7071067811865474 |
| 1.8333333333333333 | 0.8660254037844384 |
| 1.9166666666666667 | 0.9659258262890683 |
| 2 | 1 |
| 2.0833333333333335 | 0.9659258262890682 |
| 2.1666666666666665 | 0.866025403784439 |
| 2.25 | 0.7071067811865477 |
| 2.3333333333333335 | 0.4999999999999997 |
| 2.4166666666666665 | 0.2588190451025216 |
| 2.5 | 3.061616997868383e-16 |
| 2.5833333333333335 | -0.2588190451025202 |
| 2.6666666666666665 | -0.4999999999999992 |
| 2.75 | -0.7071067811865467 |
| 2.8333333333333335 | -0.8660254037844388 |
| 2.9166666666666665 | -0.9659258262890683 |
| 3 | -1 |
| 3.0833333333333335 | -0.9659258262890684 |
| 3.1666666666666665 | -0.8660254037844392 |
| 3.25 | -0.7071067811865471 |
| 3.3333333333333335 | -0.49999999999999983 |
| 3.4166666666666665 | -0.2588190451025209 |
| 3.5 | -4.286263797015736e-16 |
| 3.5833333333333335 | 0.2588190451025201 |
| 3.6666666666666665 | 0.4999999999999991 |
| 3.75 | 0.7071067811865466 |
| 3.8333333333333335 | 0.8660254037844387 |
| 3.9166666666666665 | 0.9659258262890682 |
| 4 | 1 |
Schematic density deviation: crests are compressions, troughs rarefactions. Distance axis; wavelength=2 cm. This is not an air-particle trajectory.
- Spatial snapshot
- Variation across position at one instant.
- Time record
- Variation with time at one fixed location.
On a density-change graph, positive values mean compression relative to the average; negative values mean rarefaction. The density itself does not become negative.
A distance graph gives wavelength between matching cycle positions. A time graph gives period between matching instants. A crest-to-adjacent-trough gap is only half a full cycle.
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Question
Which horizontal axis is needed to read the time period directly?
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EXPLORE
Count complete cycles
Frequency counts cycles per second; period times one cycle.Two cycles in four milliseconds
Density change from mean (schematic units)
Time (ms)
- Fixed-location record · 2 ms per cycle.
- 1. Cycle starts · (0, 1)
- 2. One period later · (2, 1)
Assumptions: Original schematic record: T=2 ms=0.002 s, hence f=500 Hz. Vertical density changes use relative units.
Read the plotted values
Fixed-location record
| Time (ms) | Density change from mean (schematic units) |
|---|---|
| 0 | 1 |
| 0.08333333333333333 | 0.9659258262890683 |
| 0.16666666666666666 | 0.8660254037844387 |
| 0.25 | 0.7071067811865476 |
| 0.3333333333333333 | 0.5000000000000001 |
| 0.4166666666666667 | 0.25881904510252074 |
| 0.5 | 6.123233995736766e-17 |
| 0.5833333333333334 | -0.25881904510252085 |
| 0.6666666666666666 | -0.4999999999999998 |
| 0.75 | -0.7071067811865475 |
| 0.8333333333333334 | -0.8660254037844387 |
| 0.9166666666666666 | -0.9659258262890682 |
| 1 | -1 |
| 1.0833333333333333 | -0.9659258262890684 |
| 1.1666666666666667 | -0.8660254037844386 |
| 1.25 | -0.7071067811865477 |
| 1.3333333333333333 | -0.5000000000000004 |
| 1.4166666666666667 | -0.25881904510252063 |
| 1.5 | -1.8369701987210297e-16 |
| 1.5833333333333333 | 0.2588190451025203 |
| 1.6666666666666667 | 0.5000000000000001 |
| 1.75 | 0.7071067811865474 |
| 1.8333333333333333 | 0.8660254037844384 |
| 1.9166666666666667 | 0.9659258262890683 |
| 2 | 1 |
| 2.0833333333333335 | 0.9659258262890682 |
| 2.1666666666666665 | 0.866025403784439 |
| 2.25 | 0.7071067811865477 |
| 2.3333333333333335 | 0.4999999999999997 |
| 2.4166666666666665 | 0.2588190451025216 |
| 2.5 | 3.061616997868383e-16 |
| 2.5833333333333335 | -0.2588190451025202 |
| 2.6666666666666665 | -0.4999999999999992 |
| 2.75 | -0.7071067811865467 |
| 2.8333333333333335 | -0.8660254037844388 |
| 2.9166666666666665 | -0.9659258262890683 |
| 3 | -1 |
| 3.0833333333333335 | -0.9659258262890684 |
| 3.1666666666666665 | -0.8660254037844392 |
| 3.25 | -0.7071067811865471 |
| 3.3333333333333335 | -0.49999999999999983 |
| 3.4166666666666665 | -0.2588190451025209 |
| 3.5 | -4.286263797015736e-16 |
| 3.5833333333333335 | 0.2588190451025201 |
| 3.6666666666666665 | 0.4999999999999991 |
| 3.75 | 0.7071067811865466 |
| 3.8333333333333335 | 0.8660254037844387 |
| 3.9166666666666665 | 0.9659258262890682 |
| 4 | 1 |
Original schematic record: T=2 ms=0.002 s, hence f=500 Hz. Vertical density changes use relative units.
- Wavelength λ
- Distance between consecutive matching phase points; unit metre.
- Frequency f (or ν)
- Number of complete cycles per second; unit hertz (Hz).
- Period T
- Time for one complete cycle; T=1/f.
An original source completing 600 cycles in 2 s has f=300 Hz and T=1/300 s≈3.33 ms. One cycle includes compression to rarefaction and back to compression.
Use seconds in f=1/T. A time-axis millisecond is 0.001 s, not one second. In the same medium, a higher-frequency sound usually has a shorter wavelength because wave speed is approximately fixed.
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Question
A period is 2 ms. What is the frequency?
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