Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme C — Wave Behaviour

C.2.3 Sound Waves — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 37 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

C.2.3 Sound Waves — IB PHYSICS SL notes: Explain sound as a longitudinal pressure wave, relate pitch and loudness to frequency and amplitude, and use v = fλ for sound.Nature — sound is a longitudinal mechanical wave that needs a mediumSource — a vibrating source such as a loudspe…

  • Particle motion
  • Audible range
  • Ultrasound
  • Typical values
  • Order of media
  • Temperature

Section 1 of 10Production of sound

Production of sound
Nature
  • Nature sound is a longitudinal mechanical wave that needs a medium
Source
  • Source a vibrating source such as a loudspeaker cone pushes the air to form a compression, then pulls back to form a rarefaction
  • The pressure changes pass outward layer by layer through collisions between molecules.
Vacuum
  • Vacuum sound cannot travel through a vacuum
  • A bell in an evacuated jar becomes inaudible as air is removed, though it still visibly vibrates.
Particle motion
  • Particle motion air molecules oscillate a tiny distance about equilibrium parallel to the wave motion
  • The pressure swings very slightly above and below atmospheric pressure even for loud sounds.

Section 2 of 10Audible range

Spectrum
  • Spectrum sound waves form a continuous spectrum of frequencies
Audible range
  • Audible range humans hear about 20 Hz to 20 kHz
  • Sounds below or above this range cannot be detected by the ear.
Ultrasound
  • Ultrasound sound above about 20 kHz

Section 3 of 10Speed of sound

Typical values
  • Typical values about 340 m s−1 in air at 20 °C, 1500 m s−1 in water and 5000 m s−1 in steel
Order of media
  • Order of media sound is fastest in solids, then liquids, then gases
  • Solid particles are closely packed and pass oscillations to neighbours faster.
  • Gas particles are spread out and transfer oscillations less efficiently.
  • The bonding effect outweighs the effect of greater density.
Temperature
  • Temperature in a gas the speed rises with temperature because the particles have more kinetic energy
Pressure
  • Pressure at fixed temperature the speed does not depend on pressure
Frequency
  • Frequency it does not affect the speed in a given medium, so low and high notes arrive together
Wave equation
  • Wave equation v = fλ,
    • so raising the pitch shortens the wavelength
      What the symbols mean: v = wave speed (m s−1) · f = frequency (Hz) · λ = wavelength (m)

Section 4 of 10Pitch, loudness and quality

Pitch
  • Pitch it is determined by frequency
  • A high pitch means a high frequency and short wavelength.
  • A low pitch means a low frequency and long wavelength.
Loudness (volume)
  • Loudness (volume) it is related to the amplitude of the pressure variation and hence to the intensity
  • Larger amplitude gives greater intensity, I ∝ amplitude2, and a louder sound.
Quality (timbre)
  • Quality (timbre) it depends on the mixture of frequencies present
  • A violin and a flute playing the same note share a fundamental but differ in overtones.
Intensity
  • Intensity power per unit area in W m−2
  • Sound levels use a logarithmic decibel scale because the ear responds over a huge range.

Section 5 of 10Reflection, echoes and ultrasound

Echoes
  • Echoes sound reflects from hard surfaces, and an echo is distinct if the delay exceeds about 0.1 s
Echo timing
  • Echo timing the sound travels there and back, so v = 2d/t
Sonar
  • Sonar it uses the echo principle to find distances under water
Medical ultrasound
  • Medical ultrasound it uses frequencies above 20 kHz
  • Different tissues reflect part of the pulse.
  • The delay of each reflected pulse locates the boundary.
  • The short wavelength lets fine detail be resolved.

Section 6 of 10Refraction and diffraction of sound

Diffraction
  • Diffraction low-frequency sound with long wavelength diffracts around doorways and corners much more than high-frequency sound
  • This is why speech can be heard around a corner when the speaker cannot be seen.
Refraction
  • Refraction sound also refracts when its speed changes

Section 7 of 10Measuring the speed of sound

1
Place two microphones a known distance apart, both connected to an oscilloscope.
2
Make a short clap or click to trigger both microphones.
3
Read the time delay between the two traces.
4
Calculate v = distance / time.
Reducing uncertainty
  • Reducing uncertainty use a longer separation and repeat the readings to average out random error
Temperature
  • Temperature record the air temperature because the speed depends on it
Alternative
  • Alternative a resonance tube can also be used (C.4.4)

Section 8 of 10More from the specification

More from the specification
Longitudinal Nature of Sound Waves
  • Sound propagates through elastic media as longitudinal pressure waves where particle displacement occurs parallel to the direction of wave energy propagation,
  • forming compressions and rarefactions.
Sound Waves versus Transverse Electromagnetic Waves
  • Unlike electromagnetic waves which are transverse oscillations of electric and magnetic fields that travel through a vacuum at speed c, sound requires a material medium to propagate.
Wave Period and Oscillatory Frequency
  • The period T of a sound wave is defined as the time taken for one full pressure oscillation cycle, measured in seconds and inversely related to frequency by T = 1/f.
Wavelength and Particle Displacement in Metres
  • Particle displacement measures deviation from mean equilibrium position;
    • the wavelength lambda is the physical distance between consecutive compressions, measured in metres (m) with wave speed v = f lambda.
Period
  • Time for one revolution.
Displacement
  • Change in position.
Position
  • Location relative to a reference point.
Electromagnetic wave
  • Transverse wave that transfers energy from a source to an absorber
Transverse wave
  • Wave whose vibration is at right angles to the direction of propagation
Equilibrium position
  • Position where the net force is zero.
Electromagnetic spectrum
  • Continuous family of waves from radio waves to gamma rays
Matter wave
  • The wave associated with a moving particle such as an electron.

Section 9 of 10Key terms

TermMeaning
PitchPerceived highness of a note, set by its frequency.
LoudnessPerceived strength of a sound, linked to amplitude and intensity.
EchoReflected sound heard after a time delay.
UltrasoundSound with frequency above the human hearing limit of about 20 kHz.
TimbreQuality of a note given by its mixture of harmonics.
IntensityPower per unit area at right angles to the wave.

Section 10 of 10Exam technique

1
Drawing sound as transverse — it is longitudinal with compressions and rarefactions.
2
Saying sound is faster in air than in water because air is less dense — sound is generally faster in stiffer media.
3
Using d = vt for an echo without halving — the time covers the journey there and back.
4
Confusing pitch with loudness — pitch is frequency, loudness is amplitude.
5
Assuming the speed of sound changes with frequency in a given medium.

Last updated:

\n