Sound waves — return questions key

1. SR1 · Return

Options as numbered on the pupil sheet: 1 — travels with the sound all the way to your ear; 2 — vibrates at right angles to the line the sound travels; 3 — keeps vibrating after the strike; 4 — a rarefaction, at lower pressure than still air; 5 — vibrates along the line the sound travels; 6 — is pushed and pulled by the prongs; 7 — an empty gap with no air particles; 8 — a compression, at higher pressure than still air.

Model answer. The prongs keep vibrating after the strike and push and pull the air touching them. Further along, each air particle vibrates backwards and forwards along the line the sound travels: the wave is longitudinal, and no particle travels all the way to your ear. Supplement: where particles are crowded there is a compression, at higher pressure than still air; where they are spread out there is a rarefaction, at lower pressure. A rarefaction still contains air.

A full answer includes

• I matched each prong to “keeps vibrating after the strike”.
• I matched the air touching a prong to “is pushed and pulled by the prongs”.
• I matched the particle halfway to your ear to “vibrates along the line the sound travels”.
• I did not match any air to “travels with the sound all the way to your ear”.
• Supplement: I matched the crowded region to “a compression, at higher pressure than still air”.
• Supplement: I matched the spread-out region to “a rarefaction, at lower pressure than still air”.
• Supplement: I did not use “an empty gap with no air particles”.

Also accept

• I matched each prong to “keeps vibrating after the strike”.
• I matched the air touching a prong to “is pushed and pulled by the prongs”.
• I matched the particle halfway to your ear to “vibrates along the line the sound travels”.
• I did not match any air to “travels with the sound all the way to your ear”.
• Supplement: I matched the crowded region to “a compression, at higher pressure than still air”.
• Supplement: I matched the spread-out region to “a rarefaction, at lower pressure than still air”.
• Supplement: I did not use “an empty gap with no air particles”.

Watch for

• “A moving wave means the same particles move all the way to your ear.” — Energy and the disturbance travel; particles vibrate locally.
• “The air particles vibrate at right angles to the line the sound travels.” — That describes a transverse wave. Sound in air is longitudinal: the vibration is along the line of travel.
• “Supplement: a rarefaction is an empty gap with no air particles.” — A rarefaction has fewer particles in the same space, at lower pressure; it is not a vacuum.

2. SR2 · Return

Model answer. 1.4 kHz = 1400 Hz, so the frequency rises and the pitch is higher. The amplitude falls, so the sound is quieter. On the trace there are more cycles in the same time and a smaller height from the centre line: row B. The approximate audible range is 20 Hz to 20 000 Hz, and both 700 Hz and 1400 Hz lie inside it.

A full answer includes

• I chose row B.
• I did not choose row C: 1.4 kHz = 1400 Hz, which is higher than 700 Hz.
• I did not choose row A: a higher note is not louder by itself.
• I wrote that the range is about 20 Hz to 20 000 Hz.
• I wrote that both 700 Hz and 1400 Hz lie inside the range.

Also accept

• I chose row B.
• I did not choose row C: 1.4 kHz = 1400 Hz, which is higher than 700 Hz.
• I did not choose row A: a higher note is not louder by itself.
• I wrote that the range is about 20 Hz to 20 000 Hz.
• I wrote that both 700 Hz and 1400 Hz lie inside the range.

Watch for

• “Row A: a higher note must also be louder.” — Pitch depends on frequency and loudness on amplitude. Here the amplitude falls, so the note is quieter.
• “Row C: 1.4 is less than 700, so the pitch falls.” — The k means one thousand: 1.4 kHz = 1400 Hz, twice 700 Hz. The pitch rises.
• “Row D: a higher pitch spreads the cycles out.” — A higher frequency means more cycles in the same time, so the cycles are closer together on a fixed timebase.

3. SR3 · Return

Model answer. Removing air weakens the medium carrying sound through the jar, while visible motion shows that the source remains active. Faint sound can still travel through residual air or the supports and base, so it does not prove transmission through a vacuum.

A full answer includes

• I wrote that the bell keeps vibrating, so the source has not stopped.
• I wrote that less air remains to carry the sound.
• I wrote “No”: the faint sound can travel through the air that is left.
• I wrote that the faint sound can also travel through the supports and the base.
• I did not write that a faint sound proves sound crosses a vacuum.

Also accept

• I wrote that the bell keeps vibrating, so the source has not stopped.
• I wrote that less air remains to carry the sound.
• I wrote “No”: the faint sound can travel through the air that is left.
• I wrote that the faint sound can also travel through the supports and the base.
• I did not write that a faint sound proves sound crosses a vacuum.

Watch for

• “Any faint remaining sound proves sound can cross a vacuum.” — A real apparatus can retain air and provide solid paths; neither is an ideal vacuum path.

4. SR4 · Return

Model answer. Sound in air travels at approximately 330–350 m/s. The light from the axe arrives almost at once, so the sound takes 0.60 s: distance = 340 m/s × 0.60 s = 204 m, about 200 m. Supplement: in general the fastest-to-slowest order is solids, liquids, gases.

A full answer includes

• (a) I wrote about 330–350 m/s.
• (a) I used distance = speed × time.
• (a) I wrote 340 × 0.60 = 204 m (about 200 m).
• (a) I did not divide 340 by 0.60.
• (b) Supplement: I wrote solids, then liquids, then gases.
• (b) Supplement: I wrote that this is true in general.

Also accept

• (a) I wrote about 330–350 m/s.
• (a) I used distance = speed × time.
• (a) I wrote 340 × 0.60 = 204 m (about 200 m).
• (a) I did not divide 340 by 0.60.
• (b) Supplement: I wrote solids, then liquids, then gases.
• (b) Supplement: I wrote that this is true in general.

Watch for

• “The speed in air is thousands of metres per second.” — The air value is only about 340 m/s; speeds of thousands of metres per second belong to solids.