Making sound — answer key

1. SQ1 · Practice

Model answer. The vibrating string is the source. It pushes and pulls on nearby air, so a disturbance travels through the air after the finger has left.

A full answer includes

• The string vibrates backwards and forwards.
• Its movement pushes and pulls nearby air, starting a sound wave.

Also accept

• The string vibrates backwards and forwards.
• Its movement pushes and pulls nearby air, starting a sound wave.

Watch for

• “The finger must keep moving to make the continuing sound.” — The finger starts the vibration; the string continues to vibrate afterwards.

2. SQ2 · Practice

Model answer. The sound travels upwards, so the air particles vibrate up and down: backwards and forwards along the direction of travel. A vibration parallel to the direction of travel is what makes a wave longitudinal. Each particle moves about one place; the disturbance, not the particle, reaches the ceiling.

A full answer includes

• I chose “Up and down”.
• I wrote that the particle vibrates along the line the sound travels (parallel to it).
• I wrote that a vibration parallel to the direction of travel makes a longitudinal wave.
• I did not write that the particle travels all the way to the ceiling.

Also accept

• I chose “Up and down”.
• I wrote that the particle vibrates along the line the sound travels (parallel to it).
• I wrote that a vibration parallel to the direction of travel makes a longitudinal wave.
• I did not write that the particle travels all the way to the ceiling.

Watch for

• “Air particles in sound always vibrate left and right, as in a drawing of a speaker facing sideways.” — The vibration follows the direction of travel, whichever way that is. Here the sound goes up, so left and right would be at right angles to it: that is a transverse vibration.
• “The particle moves upwards only, all the way to the ceiling.” — The disturbance travels to the ceiling, but each particle moves up and back down about its own place. Only the vibration is passed on.

3. SQ3 · Practice

Options as numbered on the pupil sheet: 1 — Rarefaction; 2 — Compression.

Model answer. Undisturbed air has 20 particles in a box. K has 31, so its particles are closer together: K is a compression, with higher pressure and density than undisturbed air. J (12) and L (14) have fewer, so their particles are farther apart: they are rarefactions, with lower pressure and density. J and L still contain particles; a rarefaction is not empty.

A full answer includes

• I matched K to compression.
• I matched J and L to rarefaction.
• I wrote that K has more particles in the same space than undisturbed air, so they are closer together.
• I wrote that the pressure in K is higher than in undisturbed air.
• I wrote that J and L have fewer particles in the same space, so they are farther apart.
• I wrote that the pressure in J and L is lower than in undisturbed air.
• I did not write that J or L is empty or a vacuum.

Also accept

• I matched K to compression.
• I matched J and L to rarefaction.
• I wrote that K has more particles in the same space than undisturbed air, so they are closer together.
• I wrote that the pressure in K is higher than in undisturbed air.
• I wrote that J and L have fewer particles in the same space, so they are farther apart.
• I wrote that the pressure in J and L is lower than in undisturbed air.
• I did not write that J or L is empty or a vacuum.

Watch for

• “A rarefaction is an empty gap.” — J and L still have 12 and 14 particles; they are more widely spaced, not absent.
• “The box with the most particles has the lowest pressure, because the particles have no room to move.” — Crowding more particles into the same space raises the pressure. K, the most crowded box, has the highest pressure.

4. SQ4 · Practice

Model answer. The approximate human audible range is 20 Hz to 20 000 Hz. Individual limits vary, and a sound also has to be strong enough to hear.

A full answer includes

• I wrote a lower limit of about 20 Hz.
• I wrote an upper limit of about 20 000 Hz (20 kHz).
• I did not write 2000 Hz or 20 Hz as the upper limit.

Also accept

• I wrote a lower limit of about 20 Hz.
• I wrote an upper limit of about 20 000 Hz (20 kHz).
• I did not write 2000 Hz or 20 Hz as the upper limit.

Watch for

• “The range is 20 Hz to 20 Hz.” — The upper end is 20 kHz, and 1 kHz is 1000 Hz, so it is 20 000 Hz.

5. SQ5 · Practice

Model answer. U and V both have 5 cycles in 10 ms, but V rises only 1.1 divisions from the centre line instead of 2.2, so V has the smaller amplitude and is the quieter sound. U and W have the same height, but W has 3 cycles in 10 ms instead of 5, so W has the lower frequency and the lower pitch.

A full answer includes

• I wrote that V is quieter than U.
• I gave the reason: V has a smaller height from the centre line (amplitude).
• I wrote that W has a lower pitch than U.
• I gave the reason: W has fewer cycles in the same 10 ms (lower frequency).
• I did not use the number of cycles to compare loudness.

Also accept

• I wrote that V is quieter than U.
• I gave the reason: V has a smaller height from the centre line (amplitude).
• I wrote that W has a lower pitch than U.
• I gave the reason: W has fewer cycles in the same 10 ms (lower frequency).
• I did not use the number of cycles to compare loudness.

Watch for

• “W is quieter than U because it has fewer cycles.” — Fewer cycles in the same time means a lower frequency and a lower pitch. W and U have the same height, so the same loudness.
• “V has a lower pitch than U because its trace is shorter.” — Pitch comes from the number of cycles in the same time, and U and V both have 5. The height tells you about loudness.

6. SQ6 · Practice

Model answer. 4 kHz = 4000 Hz. Both 4000 Hz and 800 Hz lie between 20 Hz and 20 000 Hz. 4000 Hz is the higher frequency, so the bird’s call has the higher pitch. The frequencies say nothing about amplitude, so they cannot tell you which call is louder.

A full answer includes

• I chose “Both calls lie within the approximate human audible range”.
• I chose “The 4 kHz call has the higher pitch”.
• I changed 4 kHz to 4000 Hz before comparing.
• I did not choose “The 4 kHz call must be louder”: a frequency gives no amplitude.
• I did not choose “The 800 Hz call has the higher pitch”.

Also accept

• I chose “Both calls lie within the approximate human audible range”.
• I chose “The 4 kHz call has the higher pitch”.
• I changed 4 kHz to 4000 Hz before comparing.
• I did not choose “The 4 kHz call must be louder”: a frequency gives no amplitude.
• I did not choose “The 800 Hz call has the higher pitch”.

Watch for

• “The 4 kHz call must be louder because its number is larger.” — A frequency compares pitch, not loudness. Loudness depends on amplitude, which is not given.
• “The 800 Hz call has the higher pitch, because 800 is more than 4.” — The k in kHz means one thousand: 4 kHz = 4000 Hz, which is five times 800 Hz.

7. SQ7 · Practice

Model answer. The cone vibrates rapidly through a small distance, so it can look still. Its backwards-and-forwards movement pushes and pulls on nearby air and produces sound waves.

A full answer includes

• The cone vibrates too rapidly or by too small a distance to see.
• It pushes and pulls nearby air to start sound waves.

Also accept

• The cone vibrates too rapidly or by too small a distance to see.
• It pushes and pulls nearby air to start sound waves.

Watch for

• “A source has to show large visible movements to make sound.” — Vibrations can be small or fast enough to be invisible without magnification or slow motion.

8. SQ8 · Practice

Model answer. The air particles vibrate locally parallel to the direction of sound travel. They pass on the disturbance rather than travelling with it all the way from the bell to you.

A full answer includes

• I chose local vibration parallel to sound travel.
• I explained that each air particle vibrates along the line the sound travels.
• I explained that the particles pass on the disturbance and do not travel all the way to the listener.

Also accept

• I chose local vibration parallel to sound travel.
• I explained that each air particle vibrates along the line the sound travels.
• I explained that the particles pass on the disturbance and do not travel all the way to the listener.

Watch for

• “Air particles must move from the doorbell all the way to the listener.” — Local vibration passes the disturbance from region to region; a bulk flow of air is not required.
• “Air particles vibrate at right angles to the sound, like a rope moving up and down.” — A vibration at right angles to the travel makes a transverse wave. In sound the air vibrates along the line from the bell to you.

9. SQ9 · Practice

Model answer. X is a rarefaction with lower density and pressure, while Y is a compression with greater density and pressure. X contains fewer particles in the same volume, not zero particles, so it is not a vacuum.

A full answer includes

• X is a rarefaction.
• X has a lower pressure than undisturbed air.
• Y is a compression.
• Y has a higher pressure than undisturbed air.
• X still contains air particles and is not a vacuum.

Also accept

• X is a rarefaction.
• X has a lower pressure than undisturbed air.
• Y is a compression.
• Y has a higher pressure than undisturbed air.
• X still contains air particles and is not a vacuum.

Watch for

• “Any region with fewer particles is a vacuum.” — A rarefaction has air at lower pressure and density; an ideal vacuum has no matter.

10. SQ10 · Practice

Model answer. Only 250 Hz lies between approximately 20 Hz and 20 000 Hz. The 15 Hz and 22 000 Hz sources are outside the range; the 250 Hz sound must also be strong enough, and individual hearing limits vary.

A full answer includes

• 250 Hz is inside the range.
• 15 Hz is below the lower limit of about 20 Hz.
• 22 000 Hz is above the upper limit of about 20 000 Hz.
• Being inside is not a guarantee that every person hears a faint sound.

Also accept

• 250 Hz is inside the range.
• 15 Hz is below the lower limit of about 20 Hz.
• 22 000 Hz is above the upper limit of about 20 000 Hz.
• Being inside is not a guarantee that every person hears a faint sound.

Watch for

• “Anything that exists as sound must be audible to humans.” — Sources can vibrate outside the human hearing range; our hearing limits do not decide whether sound exists.

11. SQ11 · Practice

Model answer. E has two cycles rather than four in 10 ms, so E has the lowest frequency and pitch. F and D both have four cycles, so their pitch is the same; F has greater amplitude, so F represents a louder sound.

A full answer includes

• E has the lowest pitch: fewest cycles in the same time.
• F and D have the same pitch: equal cycle counts.
• F is louder than D: greater amplitude.

Also accept

• E has the lowest pitch: fewest cycles in the same time.
• F and D have the same pitch: equal cycle counts.
• F is louder than D: greater amplitude.

Watch for

• “A taller trace necessarily has higher pitch.” — Height represents signal amplitude; frequency comes from the number of cycles per unit time.

12. SQ12 · Practice

Model answer. Increasing gain enlarges the displayed microphone signal, so the taller trace is caused by a display setting rather than greater sound amplitude. The horizontal axis is time; the trace is a signal at one place, not a picture of the wave in air.

A full answer includes

• Gain enlarges the displayed electrical signal without changing the sound.
• I did not write that the sound became louder.
• The horizontal axis represents time, not distance through air.

Also accept

• Gain enlarges the displayed electrical signal without changing the sound.
• I did not write that the sound became louder.
• The horizontal axis represents time, not distance through air.

Watch for

• “The peaks show air particles moving upwards.” — The vertical direction represents microphone signal, not a physical direction of particle motion.

13. ST1 · Apply

Model answer. The vibrating plate pushes and pulls the air and starts a travelling disturbance. Air particles vibrate locally along the travel direction, making the sound longitudinal. Supplement: the crowded region is a compression with higher pressure and density; the spread-out region is a rarefaction with lower pressure and density.

A full answer includes

• (a) I wrote that the plate vibrates towards and away from the listener.
• (a) I wrote that the plate pushes and pulls the nearby air.
• (b) I wrote that the air particles vibrate parallel to the direction the wave travels.
• (b) I wrote that the wave is longitudinal.
• (b) I did not write that the air particles travel to the listener.
• (c) Supplement: I identified the crowded region as a compression.
• (c) Supplement: I wrote that its pressure is higher than in undisturbed air.
• (c) Supplement: I identified the spread-out region as a rarefaction.
• (c) Supplement: I wrote that its pressure is lower than in undisturbed air.

Also accept

• (a) I wrote that the plate vibrates towards and away from the listener.
• (a) I wrote that the plate pushes and pulls the nearby air.
• (b) I wrote that the air particles vibrate parallel to the direction the wave travels.
• (b) I wrote that the wave is longitudinal.
• (b) I did not write that the air particles travel to the listener.
• (c) Supplement: I identified the crowded region as a compression.
• (c) Supplement: I wrote that its pressure is higher than in undisturbed air.
• (c) Supplement: I identified the spread-out region as a rarefaction.
• (c) Supplement: I wrote that its pressure is lower than in undisturbed air.

Watch for

• “The metal plate must send its own particles to the listener.” — The plate drives nearby air; the disturbance travels through local air vibration.

14. ST2 · Apply

Model answer. For example, choose 800 Hz, above 400 Hz and within the approximate 20–20 000 Hz range, and reduce amplitude. At fixed microphone position, gain and timebase, the trace has more cycles across the screen and a smaller amplitude.

A full answer includes

• Any frequency greater than 400 Hz up to approximately 20 000 Hz, for example 800 Hz.
• Reduce amplitude.
• Trace has more cycles in the same time.
• Trace has a smaller height from the centre line.

Also accept

• Any frequency greater than 400 Hz up to approximately 20 000 Hz, for example 800 Hz.
• Reduce amplitude.
• Trace has more cycles in the same time.
• Trace has a smaller height from the centre line.

Watch for

• “Increase frequency to make the note quieter.” — Higher frequency raises pitch; lower amplitude makes it quieter. Change both independently.