Wave Motion & Sound
Mechanical and sinusoidal waves, interference and standing waves, and sound, speed, propagation, intensity, pitch and the Doppler effect.
Wave motion & sound, summary notes
- Waves transfer energy without transferring matter. Mechanical waves (sound, water waves) need a medium and can be transverse or longitudinal.
- Every wave has a wavelength (λ), frequency (f), period (T) and amplitude.
- Simple harmonic motion (SHM) describes periodic motion like a pendulum, the basis for understanding vibration and resonance in rotating machinery.
- Resonance: when a forcing frequency matches a system's natural frequency, amplitude grows dramatically, a real maintenance concern for propellers, turbine blades and airframe panels, where uncontrolled resonance causes fatigue failure.
- Sound is a longitudinal pressure wave; its speed in air depends on temperature, not pressure.
- Wave speed
- v = f λ
- Period
- T = 1 / f
- Pendulum period
- T = 2π√(L / g)
- Speed of sound in air
- ≈ 331 + 0.6 × T(°C) m/s (≈343 m/s at 20°C)
A sound wave has a frequency of 500 Hz and travels at 340 m/s. Find its wavelength.
λ = v / f = 340 / 500 = 0.68 m.
Find the period of a pendulum of length 1 m (g = 9.81 m/s²).
T = 2π√(L/g) = 2π√(1/9.81) = 2.01 s. Any structure with a natural frequency close to an engine's operating frequency risks resonance, why propeller and rotor blade natural frequencies are checked so carefully.
Waves & resonance, standing waves
Wave properties map
Wave Properties
Wave motion & sound quiz
Wave Motion & Sound
1. A sound wave of frequency 500 Hz has a wavelength of 0.68 m. Its speed is:
340 m/s735 m/s0.0014 m/s2. A wave with a frequency of 50 Hz has a period of:
0.02 s20 s0.05 s3. Resonance occurs when a system is driven at its:
Natural frequencyHalf frequencyDamping ratio4. Frequency is measured in:
HertzMetresSeconds5. The period of a wave equals 1 divided by:
FrequencyWavelengthAmplitude6. Wave speed equals frequency multiplied by:
WavelengthAmplitudePeriod7. A wave with f = 10 Hz and λ = 2 m travels at:
20 m/s5 m/s12 m/s8. In a transverse wave, particles move ___ to the wave direction.
PerpendicularParallelIn circles9. In a longitudinal wave, particles move ___ to the wave direction.
ParallelPerpendicularAt 45°10. Sound is an example of a ___ wave.
LongitudinalTransverseElectromagnetic11. Light is an example of a ___ wave.
TransverseLongitudinalSound12. Amplitude is the maximum ___ from the rest position.
DisplacementFrequencyWavelength13. Wavelength is the distance between successive:
CrestsParticlesFrequencies14. An example of simple harmonic motion is:
A swinging pendulumA car brakingA falling rock only15. The time for one complete oscillation is the:
PeriodFrequencyAmplitude