Answers · Sound waves

Sound waves — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. SK1 · Final check

Model answer. (a) The vibrating wings push and pull the air next to them; each air particle on the way to you vibrates backwards and forwards along the direction of travel, so the wave is longitudinal: row C. (b) Each forward beat crowds the air into a compression, at higher pressure, and each backward beat leaves a rarefaction, at lower pressure. These pass your ear in turn, one compression for each beat, so the pressure there rises and falls about 500 times each second.

A full answer includes

(a) I chose row C.
(a) I did not choose row B: no air flows from the wings to my ear.
(a) I did not choose row D: a vibration along the direction of travel makes a longitudinal wave.
(b) Supplement: I wrote that compressions, at higher pressure, reach my ear.
(b) Supplement: I wrote that rarefactions, at lower pressure, arrive between them.
(b) Supplement: I wrote that each wingbeat sends one compression, so about 500 arrive each second.

Also accept

(a) I chose row C.
(a) I did not choose row B: no air flows from the wings to my ear.
(a) I did not choose row D: a vibration along the direction of travel makes a longitudinal wave.
(b) Supplement: I wrote that compressions, at higher pressure, reach my ear.
(b) Supplement: I wrote that rarefactions, at lower pressure, arrive between them.
(b) Supplement: I wrote that each wingbeat sends one compression, so about 500 arrive each second.

Watch for

“Row A: sound is transverse because a sound wave is drawn as a wavy line.” — A drawing or a trace that goes up and down does not show how the air moves. In sound the air vibrates along the direction of travel.
“Row B: air pushed by the wings flows all the way to your ear.” — Only the disturbance travels to you. Each air particle vibrates about its own place.
“Row D: the air moves backwards and forwards, but the wave is transverse.” — Vibration along the direction of travel is exactly what makes a wave longitudinal.

2. SK2 · Final check

Model answer. (a) M has more cycles than L in the same time and greater amplitude, so M is higher in pitch and louder. N has the same cycle count as L but greater amplitude, so N has the same pitch and is louder. (b) M completes 6 cycles in 4 ms = 0.004 s, so its frequency is 6 ÷ 0.004 s = 1500 Hz. (c) The approximate audible range is 20 Hz to 20 000 Hz; 1500 Hz lies inside it, so the sound could be heard if it is strong enough.

A full answer includes

(a) I wrote that M has a higher pitch than L: more cycles in the same time.
(a) I wrote that M is louder than L: greater amplitude.
(a) I wrote that N has the same pitch as L: the same number of cycles.
(a) I wrote that N is louder than L: greater amplitude.
(b) I counted 6 cycles of M in 4 ms.
(b) I changed 4 ms to 0.004 s.
(b) I wrote 6 ÷ 0.004 s = 1500 Hz.
(c) I wrote the limits 20 Hz and 20 000 Hz.
(c) I wrote that 1500 Hz lies inside, so a person could hear it.

Also accept

(a) I wrote that M has a higher pitch than L: more cycles in the same time.
(a) I wrote that M is louder than L: greater amplitude.
(a) I wrote that N has the same pitch as L: the same number of cycles.
(a) I wrote that N is louder than L: greater amplitude.
(b) I counted 6 cycles of M in 4 ms.
(b) I changed 4 ms to 0.004 s.
(b) I wrote 6 ÷ 0.004 s = 1500 Hz.
(c) I wrote the limits 20 Hz and 20 000 Hz.
(c) I wrote that 1500 Hz lies inside, so a person could hear it.

Watch for

“N has higher pitch than L because it is taller.” — N and L have equal cycles in the same time, hence equal frequency; the height difference affects loudness.

3. SK3 · Final check

Model answer. (a) The plate provides vibration, but transmission also needs a material medium. An ideal vacuum has no particles to pass on that disturbance. (b) The plate’s vibration passes through its stand into the steel base. Steel is a solid medium, so it carries the sound to the second microphone without the sound crossing the nearly empty jar.

A full answer includes

(a) I wrote that sound needs a medium to pass the vibration on.
(a) I wrote that an ideal vacuum has no particles.
(a) I did not write that a stronger vibration could cross the vacuum.
(b) I wrote that the vibration passes through the stand into the steel base.
(b) I wrote that steel, a solid, is a medium that carries sound.
(b) I did not write that the sound crosses the empty jar.

Also accept

(a) I wrote that sound needs a medium to pass the vibration on.
(a) I wrote that an ideal vacuum has no particles.
(a) I did not write that a stronger vibration could cross the vacuum.
(b) I wrote that the vibration passes through the stand into the steel base.
(b) I wrote that steel, a solid, is a medium that carries sound.
(b) I did not write that the sound crosses the empty jar.

Watch for

“A sufficiently strong vibration sends sound through an ideal vacuum.” — Increasing the source vibration cannot supply a missing medium across the vacuum.
“The second microphone proves that sound can cross a vacuum.” — Its sound comes through the stand and the steel base, a solid route. It never crosses the empty part of the jar.

4. SK4 · Final check

Model answer. (a) The light from the boards arrives almost at once, so the sound takes 2.05 s. Speed = distance ÷ time = 700 m ÷ 2.05 s = 341.46… m/s = 341 m/s to 3 significant figures. (b) The speed of sound in air is approximately 330–350 m/s; 341 m/s lies inside this range, so the value agrees.

Checked automatically

(a) Speed of sound: 341 m/s (3 significant figures)
- Common wrong answer 1435 : You multiplied the distance by the time. Speed = distance ÷ time.
- Common wrong answer 0.00293 : You divided the time by the distance. Speed = distance ÷ time = 700 ÷ 2.05.

A full answer includes

(a) I used speed = distance ÷ time.
(a) I wrote 700 ÷ 2.05 = 341 m/s, to 3 significant figures.
(b) I wrote that the speed of sound in air is about 330–350 m/s.
(b) I wrote that 341 m/s lies inside this range, so it agrees.

Also accept

(a) I used speed = distance ÷ time.
(a) I wrote 700 ÷ 2.05 = 341 m/s, to 3 significant figures.
(b) I wrote that the speed of sound in air is about 330–350 m/s.
(b) I wrote that 341 m/s lies inside this range, so it agrees.

Watch for

“Speed = 700 × 2.05.” — Speed is the distance covered in each second, so divide the distance by the time.

5. SK5 · Final check

Model answer. (a) Granite (solid, 6000 m/s) > glycerol (liquid, 1900 m/s) > oxygen (gas, 330 m/s): in general sound travels faster in solids than in liquids, and faster in liquids than in gases. (b) Granite has the greatest speed, so it takes the least time over the same distance. (c) Cork (500 m/s) is slower than glycerol, but the rule says “in general”: it describes most materials, not every solid and every liquid. Cork is an exception, so it does not disprove the general rule.

A full answer includes

(a) I wrote that, in general, sound travels faster in solids than in liquids.
(a) I wrote that, in general, it travels faster in liquids than in gases.
(b) I identified granite.
(b) I wrote that, over the same distance, the greatest speed gives the least time.
(c) I wrote “No”: the rule is a general one.
(c) I wrote that some solids, such as cork, are slower than some liquids.
(c) I did not write that every solid is faster than every liquid.

Also accept

(a) I wrote that, in general, sound travels faster in solids than in liquids.
(a) I wrote that, in general, it travels faster in liquids than in gases.
(b) I identified granite.
(b) I wrote that, over the same distance, the greatest speed gives the least time.
(c) I wrote “No”: the rule is a general one.
(c) I wrote that some solids, such as cork, are slower than some liquids.
(c) I did not write that every solid is faster than every liquid.

Watch for

“The general order is a universal rule with no exceptions.” — The syllabus comparison says “in general”; material properties and conditions affect the actual value.