Physics notes · Chapter 7 of 11
Waves & Sound
11 sections, 62 flashcards. Open a card to check your answer, or revise them in the app with spaced revision.
What is a Wave?
- A wave is a disturbance that travels and carries energy, not matter. The medium is the material it passes through (air, water, steel).
- An oscillation (vibration) is the to-and-fro motion of a particle.
- Mechanical waves need a medium and cannot cross a vacuum (sound); electromagnetic waves need none (light). So we see the Sun but cannot hear it.
Check yourself
Define a wave
A disturbance that travels and carries energy, not matter, from place to place.
Define the medium
The material a wave passes through (air, water, steel).
Do particles of the medium travel with the wave?
No — they only vibrate about their rest position.
Mechanical vs electromagnetic wave
Mechanical needs a medium and cannot cross a vacuum (sound); EM needs no medium and travels through vacuum (light).
Why can we see the Sun but not hear it?
Light is an EM wave (no medium needed); sound is mechanical and cannot cross the vacuum of space.
What is an oscillation?
The to-and-fro motion of a particle — also called vibration.
Transverse vs Longitudinal Waves
- Transverse wave: particles vibrate perpendicular to the wave's direction, making crests and troughs. As mechanical waves they travel only in solids and on liquid surfaces.
- Longitudinal wave: particles vibrate parallel to the wave's direction, making compressions and rarefactions; it travels in solids, liquids and gases.
- Transverse vs Longitudinal
| Feature | Transverse | Longitudinal |
|---|---|---|
| Particle motion | Perpendicular to wave | Parallel to wave |
| Parts | Crests & troughs | Compressions & rarefactions |
| Travels in | Solids, liquid surfaces | Solids, liquids, gases |
| Example | Light, rope wave | Sound in air |
Check yourself
Define a transverse wave
Particles vibrate perpendicular (90°) to the wave's direction.
Parts of a transverse wave
High points are crests, low points are troughs.
Define a longitudinal wave
Particles vibrate parallel to the wave's direction.
Parts of a longitudinal wave
Squeezed regions are compressions (C); stretched regions are rarefactions (R).
Where can transverse mechanical waves travel?
Only in solids and on liquid surfaces; longitudinal waves travel in solids, liquids and gases.
Sound in air vs light — which is which?
Sound in air is longitudinal; light is a transverse EM wave.
Measuring a Wave: Key Quantities
- Wavelength (λ): the length of one full wave, crest to crest, in metres. Amplitude (A): the maximum displacement from rest, which shows the wave's energy.
- Frequency: waves passing a point per second, in hertz (Hz). Time period T = 1/f, in seconds.
- v = f × λ: speed = frequency × wavelength.
- Wave Quantities and Units
| Quantity | Symbol | SI Unit |
|---|---|---|
| Wavelength | λ | metre (m) |
| Amplitude | A | metre (m) |
| Frequency | f | hertz (Hz) |
| Time period | T | second (s) |
| Wave speed | v | metre/second (m/s) |
Check yourself
Define wavelength (λ)
The length of one full wave (crest to crest); unit metre (m).
Define amplitude (A)
The maximum displacement from rest; it shows the wave's energy.
Define time period (T)
The time for one wave; T = 1/f, unit second.
Master wave formula
v = f × λ (speed = frequency × wavelength).
Does wave speed depend on the source's frequency?
No — wave speed depends only on the medium.
Define frequency and its unit
The number of waves passing a point per second; unit hertz (Hz). (Hertz)
Properties of Sound: Loudness, Pitch, Quality
- Sound always comes from a vibrating object (a drum, the vocal cords, a string).
- Loudness depends on amplitude, and falls with distance as the amplitude decreases.
- Pitch (shrillness) depends on frequency: a whistle is high-pitched, a drum or a man's voice low-pitched. Tuning a sitar changes the string's tension, and so its frequency and pitch.
- Quality (timbre) tells a sitar from a flute playing the same note.
- What Each Sound Property Depends On
| Property | Depends on |
|---|---|
| Loudness | Amplitude |
| Pitch (shrillness) | Frequency |
| Quality (timbre) | Waveform/harmonics |
Check yourself
What produces sound?
Always a vibrating object (drum, vocal cords, string).
Loudness depends on…
The amplitude of the wave; it falls with distance as amplitude decreases.
Pitch (shrillness) depends on…
The frequency — high frequency = high pitch.
Which sounds are high vs low pitched?
A whistle is high-pitched; a drum or man's voice is low-pitched.
Define quality (timbre)
Lets us tell a sitar from a flute playing the same note.
Tuning a sitar adjusts what?
Tension, to change the string's frequency/pitch.
Audible Range, Infrasound and Ultrasound
- Human hearing: 20 Hz to 20,000 Hz (20 kHz); older ears lose the high-frequency end first.
- Infrasound (below 20 Hz) is heard by whales, elephants and rhinoceroses; earthquakes emit it, which may warn animals before a shock.
- Ultrasound (above 20 kHz) is made by bats, dolphins and porpoises; it is used in sonography, SONAR, breaking kidney stones and detecting flaws.
- Infrasound, Audible, Ultrasound
| Type | Frequency range | Example |
|---|---|---|
| Infrasound | Below 20 Hz | Whale, elephant |
| Audible | 20 Hz – 20 kHz | Human hearing |
| Ultrasound | Above 20 kHz | Bat, dolphin |
Check yourself
Human audible range
20 Hz to 20,000 Hz (20 kHz).
What do older ears lose first?
Sensitivity to the high-frequency end.
Define infrasound
Below 20 Hz — heard by whales, elephants, rhinoceroses.
Define ultrasound
Above 20 kHz — produced by bats, dolphins, porpoises.
What do earthquakes emit?
Infrasound, which may warn animals before a shock.
Uses of ultrasound
Sonography, SONAR, kidney-stone breaking and flaw detection.
Sound Needs a Medium
- In the bell-in-a-jar experiment the bell grows silent as the air is pumped out, though it is still seen ringing. Astronauts talk by radio because space has no medium for sound.
- Sound travels best where particles are closely packed: fastest in solids, slowest in gases.
Check yourself
Can sound travel through a vacuum?
No — sound is a mechanical wave.
Bell-in-a-jar experiment
A ringing bell grows silent as air is pumped out, though still seen ringing.
Why do astronauts talk by radio in space?
Sound has no medium there.
Where does sound travel best?
Where particles are closely packed — so it is fastest in solids and slowest in gases.
Speed of Sound in Different Media
- Speed of sound: solid > liquid > gas; denser, stiffer media carry it faster.
- In air at room temperature (about 22°C) sound travels about 344 m/s; the speed increases with temperature and humidity.
- Speed of Sound by Medium
| Medium | Approx. speed (m/s) |
|---|---|
| Air (22°C) | 344 |
| Water | 1480 |
| Steel/Iron | 5960 |
| Vacuum | 0 (cannot travel) |
Check yourself
Order of sound speed by state
solid > liquid > gas — denser/stiffer media carry sound faster.
Speed of sound in air at room temperature
About 344 m/s (around 22°C).
How do temperature and humidity affect sound speed in air?
Speed increases with both.
Sound vs light speed
Sound is far less than the speed of light (3 × 10⁸ m/s).
Reflection of Sound and Echo
- Sound obeys the laws of reflection, bouncing off hard surfaces.
- An echo is a sound heard again after reflection; the ear needs a gap of at least 0.1 second to tell it apart.
- Reverberation: repeated reflection that makes sound persist in a hall; sound-absorbing materials on walls and ceilings cut it.
Check yourself
Does sound obey the laws of reflection?
Yes — it bounces off hard surfaces following them.
Define an echo
A sound heard again after reflection from a distant surface.
Minimum gap for the ear to distinguish an echo
0.1 second.
Minimum distance of the reflector for an echo
About 17.2 metres.
Define reverberation
Repeated reflection causing sound to persist in a hall — cut by sound-absorbing materials on walls and ceilings.
SONAR and Practical Uses of Sound
- SONAR = Sound Navigation And Ranging: it uses ultrasound echoes to find distance and direction underwater, measuring the depth of the sea and locating submarines, rocks and shipwrecks.
- Echo method: distance = (speed of sound × echo time) ÷ 2.
- Ultrasound in medicine: sonography, imaging organs and the fetus. In industry it cleans parts, breaks kidney stones and detects cracks in metal.
Check yourself
SONAR stands for…
Sound Navigation And Ranging.
How does SONAR work?
Uses ultrasound echoes to find distance and direction underwater.
What does SONAR measure/locate?
Depth of the sea; it locates submarines, rocks and shipwrecks.
Echo method formula
Distance = (speed of sound × echo time) / 2.
Medical use of ultrasound
Sonography — imaging organs and the fetus.
Industrial uses of ultrasound
Cleans parts, breaks kidney stones and detects metal cracks.
Noise, Decibels and Permissible Levels
- Loudness is measured in decibels (dB). Normal conversation ≈ 60 dB; busy traffic near 80 dB.
- Above about 80 dB sound becomes harmful noise pollution; prolonged loud noise causes hearing loss and stress.
- Permissible limits are higher in industrial areas than residential ones.
- Permissible Noise Limits (Daytime)
| Area (daytime) | Permissible limit (dB) |
|---|---|
| Industrial | 75 |
| Commercial | 65 |
| Residential | 55 |
| Silence zone | 50 |
Check yourself
Unit of loudness
The decibel (dB).
Decibel level of rustling leaves
About 10–20 dB.
Decibel level of conversation and traffic
Normal conversation ≈ 60 dB; busy traffic near 80 dB.
When does sound become noise pollution?
Above about 80 dB it becomes harmful.
Effects of prolonged loud noise
Hearing loss and stress.
Which areas have higher permissible limits?
Industrial areas, compared to residential.
The Electromagnetic (EM) Spectrum
- EM waves are transverse, need no medium, and travel at the speed of light.
- In order of increasing frequency: radio, microwave, infrared, visible, UV, X-ray, gamma. Higher frequency means shorter wavelength and more energy: gamma rays have the highest frequency, radio waves the lowest.
- Satellites are tracked with radio waves and microwaves, not sound.
- EM Waves by Frequency
| Wave | Frequency |
|---|---|
| Gamma rays | Highest |
| X-rays | Very high |
| Ultraviolet | High |
| Radio waves | Lowest |
Check yourself
Three properties of EM waves
They are transverse, need no medium, and travel at the speed of light.
EM spectrum in order of increasing frequency
Radio, microwave, infrared, visible, UV, X-ray, gamma.
Which has the highest frequency and shortest wavelength?
Gamma rays.
Which has the lowest frequency and longest wavelength?
Radio waves.
Higher frequency means…
Shorter wavelength and more energy.
What carries dish-antenna signals, and what weakens them?
Microwaves — scattered and absorbed by heavy rain.
How are satellites tracked?
Using radio/microwaves, not sound.
All 11 chapters of Physics notes
- Units, Measurement & Physical Quantities11 sections
- Motion, Laws of Motion & Forces13 sections
- Gravitation11 sections
- Work, Power, Energy & Conservation13 sections
- Properties of Matter & Fluids12 sections
- Heat & Thermodynamics12 sections
- Waves & Sound11 sections
- Light & Optics15 sections
- Current Electricity13 sections
- Magnetism & Electromagnetism15 sections
- Modern & Nuclear Physics16 sections