CDS Physics · Free notes
Sound: CDS Physics
Sound has 18 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (I): 8 EASY, 10 MODERATE and none HARD. The 2023 papers alone asked five of them. Five are short sums on the wave equation or the echo rule, each two lines of working. The rest ask what kind of wave sound is, what it travels fastest in, and what pitch depends on. Start with waves in general, because sound is one case of them, then learn the speed table and the three characteristics of a sound.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Waves and the Speed of Sound
10 PYQsLongitudinal and transverse waves, why sound needs a medium, the wave equation v = f × λ, and how fast sound travels in solids, liquids and gases.
Pitch, Echo and Uses of Sound
8 PYQsWhat decides pitch and loudness, how the ear responds, the octave, how to find a distance from an echo, and how ultrasound is made and used.
Formula & revision sheet
2 formulas · 4 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
2 formulas · 4 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (2)
Longitudinal and transverse waves5 rows
| Wave | Type and medium |
|---|---|
| Sound in air | Longitudinal: compressions and rarefactions; needs a mediumQ CDS 2023 (I): sound and light in air are longitudinal and transverse respectively. |
| Light | Transverse electromagnetic wave; travels through a vacuum |
| Wave along a stretched string | Transverse; needs the string |
| Ripples on water | Transverse at the surface |
| Sound in a vacuum | Cannot travel: there is no mediumQ CDS 2018 (I): sound cannot pass through a hollow pipe kept in a vacuum. |
Speed of sound in different media8 rows
| Medium | Speed of sound (m/s, NCERT values) |
|---|---|
| Aluminium | 6420 |
| Steel | 5960Q CDS 2023 (II): of glass, stainless steel, water and oxygen, sound is fastest in steel. |
| Glass (flint) | 3980 |
| Sea water | 1531Q CDS 2024 (II): sea water is faster than distilled water, drinking water or methanol. |
| Distilled water | 1498 |
| Methanol | 1103 |
| Air at 25 °C | 346 |
| Oxygen | 316 |
Watch out for (6)
- Sound and light are opposite kinds of wave→ Longitudinal and transverse waves
- A solid inside a vacuum still carries sound; a vacuum does not→ Longitudinal and transverse waves
- A crest and a trough together make one wave→ The wave equation: speed, frequency and wavelength
- Convert to metres and seconds first→ The wave equation: speed, frequency and wavelength
- Denser is not the rule; stiffer is→ Speed of sound in different media
- A sonic boom needs a speed above the speed of sound, not light→ Speed of sound in different media
Reference tables (2)
Pitch, loudness and the human ear5 rows
| Characteristic of sound | Depends on |
|---|---|
| Pitch | FrequencyQ Asked twice: CDS 2020 (I) and 2022 (II). The answer is frequency, never amplitude or speed. |
| Loudness | Amplitude |
| Quality (timbre) | Waveform |
| One octave higher | Double the frequencyQ CDS 2019 (II): the note an octave above 500 Hz is 1000 Hz. |
| Sensitivity of the ear | Varies with frequency; best near 2 to 5 kHzQ CDS 2016 (II): the statement that the ear's sensitivity does not vary with frequency is the false one. |
Ultrasound, SONAR and transducers5 rows
| Device or term | What it does |
|---|---|
| Piezoelectric crystal | Produces ultrasound when an alternating voltage is appliedQ CDS 2017 (II): ultrasonic waves are produced using piezoelectric material. |
| SONAR | Measures distance under water from the echo of ultrasound |
| Ultrasound scanner | Forms images of organs inside the body |
| Microphone | Sound to electrical signal |
| Loudspeaker | Electrical signal to sound |
Watch out for (6)
- Pitch is frequency; loudness is amplitude→ Pitch, loudness and the human ear
- An octave doubles the frequency; it does not add to it→ Pitch, loudness and the human ear
- Halve speed × time→ Echo: finding a distance from the time
- Watch the units in the options→ Echo: finding a distance from the time
- Piezoelectric, not pyroelectric or ferromagnetic→ Ultrasound, SONAR and transducers
- Microphone and speaker both deal in electrical signals→ Ultrasound, SONAR and transducers
PYQ weightage by concept
6 concepts · 18 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
6 concepts · 18 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Speed of sound in different media | 4 | 22% |
| Longitudinal and transverse waves | 3 | 17% |
| The wave equation: speed, frequency and wavelength | 3 | 17% |
| Concept | PYQs | Share |
|---|---|---|
| Pitch, loudness and the human ear | 4 | 22% |
| Echo: finding a distance from the time | 2 | 11% |
| Ultrasound, SONAR and transducers | 2 | 11% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.