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Class 9–10 · Physics
Waves and sound
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Waves and sound
Lessons in this chapter
Lessons
1Oscillations
Period is time for one complete oscillation; frequency is oscillations per second. In simple harmonic motion, restoring acceleration is proportional to displacement and opposite in direction. A small-angle ideal pendulum approximates this behaviour.
Key relationship
f = 1/T; a ∝ −x; T = 2π√(L/g)
Worked example
20 oscillations in 40 s ⇒ T = 2 s, f = 0.5 Hz
Next lesson →2Waves and their types
Waves transfer energy without transporting the medium bodily along the entire path. Oscillations are perpendicular to travel in transverse waves and parallel in longitudinal waves. Mechanical waves need a medium; electromagnetic waves do not.
Key relationship
transverse: oscillation ⟂ travel; longitudinal: oscillation ∥ travel
Worked example
Rope wave: transverse; sound in air: longitudinal.
Next lesson →3Amplitude, wavelength and speed
Amplitude is maximum displacement from equilibrium. Wavelength separates successive points in the same phase. A wave travels one wavelength per period, giving v = fλ. At fixed wave speed, frequency and wavelength are inversely related.
Key relationship
v = fλ = λ/T
Worked example
v = 340 m/s, f = 170 Hz ⇒ λ = 2 m
Next lesson →4Sound production and propagation
A vibrating source creates pressure disturbances transmitted through a medium. Sound cannot travel in vacuum. Its speed depends on the medium and temperature; it is generally faster in solids than gases under ordinary conditions.
Key relationship
in air approximately v = (331+0.6θ) m/s, θ in °C
Worked example
At 20°C, v ≈ 343 m/s; thunder delay 3 s ⇒ distance ≈ 1029 m
Next lesson →5Echoes, reverberation and resonance
An echo is a separately heard reflected sound. Reverberation prolongs sound through many reflections. Resonance increases oscillation amplitude when driving frequency is near a natural frequency; it is a different mechanism.
Key relationship
echo distance d = vt/2
Worked example
v = 340 m/s, return time 0.4 s ⇒ d = 68 m
Next lesson →6Intensity, pitch and timbre
Intensity is power per area. Perceived loudness also depends on hearing sensitivity. Pitch is mainly related to frequency, while timbre depends on the mix of harmonics. Decibels describe a logarithmic relative level.
Key relationship
I = energy/(area × time); I ∝ amplitude²
Worked example
Double amplitude ⇒ approximately 4 times intensity; unchanged frequency ⇒ unchanged pitch
Next lesson →7Hearing range and ultrasound
Typical human hearing spans roughly 20 Hz–20 kHz, varying by person and age. Ultrasound is above this range; infrasound is below. Sonar uses reflected pulses and the appropriate medium's sound speed. Loud or prolonged exposure can damage hearing.
Key relationship
sonar d = vt/2
Worked example
Water v = 1500 m/s, t = 0.2 s ⇒ depth 150 m
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. f = 50 Hz and λ = 4 m: speed?
v = 50 × 4 = 200 m/s
2. Which quantity mainly increases pitch?
Frequency, not amplitude
3. Echo returns in 0.2 s at 340 m/s: distance?
d = 340 × 0.2/2 = 34 m
4. Particle oscillation direction for sound in air?
Parallel to propagation: longitudinal
5. Double frequency at fixed speed: wavelength?
λ = v/f ⇒ half
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.