Physics 1 · Unit 7: Simple Harmonic Motion and Waves · 14 min read · Updated 2026-05-11
AP Physics 1 Sound Waves — AP Physics 1
AP Physics 1 · Unit 7: Simple Harmonic Motion and Waves · 14 min read
1. Nature of Sound★☆☆☆☆⏱ 2 min
Sound is a mechanical longitudinal pressure wave that requires a material medium to propagate, meaning it cannot travel through a vacuum. This fact is frequently tested on AP Physics 1 multiple choice questions.
Unlike transverse waves (e.g., waves on a string), particle motion is parallel to wave direction, not perpendicular. The standard speed of sound in dry room-temperature air is $343\ \text{m/s}$, which you can assume in problems unless stated otherwise. Sound makes up 4-6% of the total AP Physics 1 exam weight.
2. Speed of Sound and Fundamental Wave Relation★★☆☆☆⏱ 4 min
The fundamental wave relation $v = f\lambda$ holds for all waves, including sound. Critically, the speed of sound depends only on the medium, not the frequency or wavelength of the source. When sound travels between media, frequency (set by the source) stays constant, while speed and wavelength change.
Exam tip: Always remember frequency is determined by the source, and speed is determined by the medium — never flip these two properties when solving for wavelength across different media.
3. Beat Frequency★★☆☆☆⏱ 3 min
Beats are periodic variations in loudness caused by the superposition of two sound waves with slightly different frequencies. Constructive interference produces loud pulses, destructive interference produces quiet pulses, and the rate of these pulses is the beat frequency.
f_b = |f_1 - f_2|
Exam tip: When solving beat frequency tuning problems, always test both possible values of the unknown frequency against the change in beat frequency after adjustment — AP problems almost always require eliminating the wrong candidate.
4. Standing Sound Waves in Open and Closed Tubes★★★☆☆⏱ 4 min
When sound reflects off the ends of a tube, incident and reflected waves interfere to form standing waves, the basis of most wind instruments. The allowed harmonics depend on boundary conditions at the ends:
For a tube of length $L$, the allowed harmonic frequencies are:
Exam tip: Always confirm boundary conditions first: remember closed ends are displacement nodes, open ends are displacement antinodes, and only odd harmonics exist for tubes closed at one end — this is the most commonly tested fact about standing sound waves.
5. Doppler Effect for Sound★★★☆☆⏱ 3 min
The Doppler effect is the shift in observed frequency caused by relative motion between a sound source and an observer. When source and observer move towards each other, observed frequency is higher than the source frequency; when they move away, observed frequency is lower.
f_o = f_s \frac{v \pm v_o}{v \pm v_s}
The sign rule is simple: any motion towards the other increases $f_o$: add $v_o$ if observer moves towards source, subtract $v_s$ if source moves towards observer. Reverse signs for motion away.
Exam tip: To get the sign right every time, after calculating, confirm that motion towards gives higher frequency and motion away gives lower frequency. If your result contradicts this, flip the signs.
Common Pitfalls
Why: Students confuse which property is set by the source vs the medium, incorrectly assuming frequency changes with medium
Why: Students forget there are two possible solutions, and often default to the lower frequency
Why: Students mix up standing waves on strings (fixed-fixed, same as open tube) with closed tubes, forgetting the boundary condition difference
Why: Students memorize sign rules backwards instead of using physical intuition
Why: Students confuse mechanical waves (sound) with electromagnetic waves (light), which do not need a medium