NDA Physics · Teaching notes
Sound — NDA Physics
Sound is NDA Physics's lowest-HARD chapter — 34 PYQs across 2017–2025, almost entirely EASY and MODERATE. The chapter teaches in four progressive movements: (1) Foundations — what sound IS (mechanical, longitudinal, needs medium), how we PERCEIVE it (pitch, loudness, quality), and the ear chain that does the conversion (cochlea = biological mic); (2) Wave equation, speed, and bands — v = fλ, why speed depends on the medium alone, and the named frequency bands (infrasonic, audible, ultrasonic) plus the Mach scale; (3) Sound behaviours — reflection (echo + reverberation), interference (beats), and the canonical properties checklist with the polarization trap; (4) Applications — SONAR + bats + medical imaging, electronic transducers (microphone, loudspeaker, piezoelectric), and musical instruments. 13 concepts, every PYQ tagged — drill the table, drill the formula, walk out with the marks.
Subtopic notes
Foundations: What Sound Is and How We Hear It
11 PYQsSound is a mechanical longitudinal wave; we perceive it via three independent attributes (pitch, loudness, quality) extracted by a four-stage ear chain (pinna → eardrum → ossicles → cochlea).
Open note
How We Measure Sound — v = fλ, Speed, and the Frequency Bands
13 PYQsThree quantities describe any sound — frequency, wavelength, speed — tied by v = fλ. Speed is set by the medium (not by frequency, not by pressure at constant T). The named regions on the frequency axis (infrasonic, audible, ultrasonic) are just bands.
Open note
What Sound DOES — Reflection, Echo, Reverberation, Beats
5 PYQsBecause sound is a wave, it does what waves do — reflect, refract, diffract, interfere — except polarize. Reflection gives echo (single) and reverberation (multiple); interference of two close frequencies gives beats.
Open note
How We USE Sound — SONAR, Transducers, Musical Instruments
5 PYQsOnce you know what sound is, how to measure it, and what it does, the applications follow. SONAR + bats use ultrasonic reflection; microphones convert acoustic to electrical; musical instruments produce notes by vibrating air columns, strings, or membranes.
Open note
PYQ weightage by concept
13 concepts · 34 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
13 concepts · 34 PYQs — where the marks actually sit, so you know what to drill first
Foundations: What Sound Is and How We Hear It11 PYQs · 32%
| Concept | PYQs | Share |
|---|---|---|
| Sound is a mechanical longitudinal wave | 6 | 18% |
| Pitch, loudness, and quality — the perceptual triad | 4 | 12% |
| The human ear — anatomy chain that converts pressure to nerve impulses | 1 | 3% |
How We Measure Sound — v = fλ, Speed, and the Frequency Bands13 PYQs · 38%
| Concept | PYQs | Share |
|---|---|---|
| Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter | 6 | 18% |
| Frequency, period, wavelength — and v = fλ | 4 | 12% |
| Speed of sound depends on the MEDIUM — not on f, not on P (at constant T) | 3 | 9% |
What Sound DOES — Reflection, Echo, Reverberation, Beats5 PYQs · 15%
| Concept | PYQs | Share |
|---|---|---|
| Reverberation — sustained sound from many reflections | 2 | 6% |
| What sound CAN and CANNOT do — the properties checklist | 1 | 3% |
| Echo — a single distinct reflection | 1 | 3% |
| Beats — periodic loud/soft from two close frequencies (interference) | 1 | 3% |
How We USE Sound — SONAR, Transducers, Musical Instruments5 PYQs · 15%
| Concept | PYQs | Share |
|---|---|---|
| SONAR, bats, medical imaging — applications of ultrasonic | 3 | 9% |
| Microphone, loudspeaker — converting between acoustic and electrical | 1 | 3% |
| Musical instruments — how wind, string, and percussion produce notes | 1 | 3% |
Formula & revision sheet
4 formulas · 6 reference tables · 25 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
4 formulas · 6 reference tables · 25 gotchas across all subtopics — the exam-eve cheat-sheet
Foundations: What Sound Is and How We Hear It
Reference tables (1)
The human ear — anatomy chain that converts pressure to nerve impulses5 rows
| Part | Function / mechanism | Note |
|---|---|---|
| Pinna (outer ear) | Funnels sound into the ear canal | Acoustic collector — no signal conversion |
| Eardrum (tympanic membrane) | Sound waves mechanical vibration | Thin membrane at the end of the ear canal |
| Ossicles (malleus, incus, stapes) | Mechanical amplification & impedance matching | Three tiny bones in the middle ear |
| Cochlea | Mechanical pressure electrical (nerve impulses) | Fluid-filled spiral in the inner ear — the biological mic NDA 2022 Sep — the pressure electrical converter IS the cochlea (not the eardrum, ossicles, or auditory nerve). |
| Auditory nerve | Carries nerve signals from cochlea to brain | Transmission, not conversion |
Each stage performs a distinct physical conversion. Distractors swap the cochlea (the converter) with the eardrum (mechanical-only) or the auditory nerve (transmission-only).
Watch out for (5)
- Sound is longitudinal — and that's exactly why it CANNOT polarize→ Sound is a mechanical longitudinal wave
- Sound vs light — both waves, but VERY different→ Sound is a mechanical longitudinal wave
- Amplitude is measured in pressure (Pa), NOT decibels→ Pitch, loudness, and quality — the perceptual triad
- Loudness depends on amplitude, NOT frequency→ Pitch, loudness, and quality — the perceptual triad
- Cochlea, not eardrum, is the mechanical electrical converter→ The human ear — anatomy chain that converts pressure to nerve impulses
How We Measure Sound — v = fλ, Speed, and the Frequency Bands
Formulas (2)
Reference tables (1)
Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter14 rows
| What | Value / range | Note |
|---|---|---|
| Audible frequency range (human ear) | 20 Hz to 20 000 Hz | Drilled most years — memorise both endpoints |
| Infrasonic | < 20 Hz | Below the lower limit of human hearing — whales, earthquakes |
| Ultrasonic | > 20 000 Hz (> 20 kHz) | Bats, SONAR, medical imaging — applications in Subtopic 4 |
| Ultrasonic vs audible (same medium) | Same speed, higher f, shorter | From : is medium-set; higher shorter Distractors pair higher frequency with higher SPEED — wrong; speed is set by the medium. |
| Speed of sound in air (C) | m/s | Standard round number — memorise |
| Speed of sound in water (C) | m/s | Tested in 2019: distractors at 330 / 800 / 5000 |
| Speed of sound in steel | m/s | Solid > liquid > gas |
| Mach number | object speed / sound speed | Compares object's speed to local sound speed |
| Mach < 1 | Subsonic | Most everyday motion (cars, propeller aircraft) |
| Mach = 1 | Sonic / transonic | At the speed of sound — sonic boom region |
| Mach > 1 | Supersonic | Faster than sound (fighter jets, Concorde) NDA 2017 tested exactly this — Mach > 1 means supersonic. |
| Mach > 5 | Hypersonic | Re-entry vehicles, scramjets |
| Decibel (dB) | log scale of intensity ratio | Unit of intensity LEVEL — NOT a unit of frequency or amplitude |
| Richter scale | log scale of earthquake energy | Devised 1935 by C.F. Richter; no upper limit (though > 9.5 is rare) |
The audible-range endpoints (20 Hz, 20 kHz) and the speed-in-water number ( m/s) are the most-tested rows — they appear almost yearly.
Watch out for (9)
- dB measures intensity LEVEL — not frequency, not amplitude→ Frequency, period, wavelength — and v = fλ
- 3 Hz means 3 cycles per second — full cycles, not half or quarter→ Frequency, period, wavelength — and v = fλ
- Pressure dependence trap — P only matters via T→ Speed of sound depends on the MEDIUM — not on f, not on P (at constant T)
- Frequency does NOT change the speed of sound→ Speed of sound depends on the MEDIUM — not on f, not on P (at constant T)
- Solid > Liquid > Gas (elasticity wins over density)→ Speed of sound depends on the MEDIUM — not on f, not on P (at constant T)
- Audible range: 20 Hz to 20 kHz — NOT 0 Hz to 20 kHz→ Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter
- Speed of sound in water m/s, NOT 5000 m/s (that's steel)→ Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter
- Mach > 1 is SUPERsonic, not SUBsonic→ Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter
- Ultrasonic does NOT travel faster than audible sound→ Bands and scales — audible / infra / ultrasonic, Mach, sound speeds, decibel, Richter
What Sound DOES — Reflection, Echo, Reverberation, Beats
Formulas (2)
Reference tables (1)
What sound CAN and CANNOT do — the properties checklist9 rows
| Property / behaviour | Sound? | Why |
|---|---|---|
| Reflection (echoes) | Yes | All waves reflect off a hard boundary |
| Refraction | Yes | Speed changes between media wave bends |
| Diffraction | Yes | Bends around obstacles when obstacle size |
| Interference (beats) | Yes | Two waves superpose — alternating loud/soft |
| Resonance | Yes | Forced oscillation at the natural frequency |
| Doppler effect | Yes | Observed pitch shifts with source/observer motion |
| Polarization | NO | Polarization requires a TRANSVERSE wave; sound is longitudinal The single most-tested NDA trap — "polarization applies to sound" is always WRONG. |
| Travel through vacuum | NO | No medium no molecular collisions no propagation |
| Ultrasonic obeys all the above the same way | Yes | Ultrasonic = sound above 20 kHz, otherwise identical behaviour |
Rows 7 (polarization) and 8 (vacuum) account for the bulk of the bank's "which is NOT correct" distractors. Row 9 catches the "ultrasonic cannot reflect / refract / be absorbed" trap.
Watch out for (7)
- Ultrasonic obeys the same property rules as audible sound→ What sound CAN and CANNOT do — the properties checklist
- Echo is REFLECTION — not refraction, diffraction, or resonance→ Echo — a single distinct reflection
- Round-trip / 2 — sound goes there AND comes back→ Echo — a single distinct reflection
- Reverberation is MULTIPLE reflections — not refraction, not diffraction→ Reverberation — sustained sound from many reflections
- Echo vs reverberation — single vs many, distinct vs sustained→ Reverberation — sustained sound from many reflections
- Beats need NEARLY equal frequencies — not equal, not far apart→ Beats — periodic loud/soft from two close frequencies (interference)
- Beat formula gives MAGNITUDE — the sign is ambiguous→ Beats — periodic loud/soft from two close frequencies (interference)
How We USE Sound — SONAR, Transducers, Musical Instruments
Reference tables (3)
SONAR, bats, medical imaging — applications of ultrasonic7 rows
| Acronym / use | Wave type | Application / setting |
|---|---|---|
| SONAR | Ultrasonic (sound) | Sound Navigation And Ranging — underwater distance / submarine / sea-depth SONAR uses ultrasonic, NOT audible sound — easy distractor. |
| RADAR | Radio waves (EM) | RAdio Detection And Ranging — aircraft / weather, works through air |
| LIDAR | Light / laser (EM) | LIght Detection And Ranging — surveying, autonomous vehicles, atmospheric science |
| Bats / dolphins | Ultrasonic | Echolocation — emit ultrasonic, receive reflected echo, infer obstacle position |
| Medical sonography / ultrasound imaging | Ultrasonic | Pulse + echo through soft tissue — pregnancy scans, organ imaging |
| Industrial: defect detection, drilling | Ultrasonic | Reflections inside metal reveal cracks; high-frequency vibration drills hard materials |
| Ultrasonic cleaning | Ultrasonic | High-frequency vibrations in a liquid bath dislodge contaminants from delicate parts |
All rows 4–7 work by the same principle as SONAR: emit pulse, measure echo, infer geometry. The only difference is medium.
Microphone, loudspeaker — converting between acoustic and electrical3 rows
| Device | Input | Output |
|---|---|---|
| Microphone | Sound waves (mechanical pressure) | Electrical signal NDA 2022 Sep tested exactly this — distractor swaps sound microwaves. |
| Loudspeaker | Electrical signal | Sound waves (mechanical pressure) |
| Piezoelectric crystal | Electrical signal (or mechanical stress) | Mechanical vibration (or electrical signal) |
The microphone and loudspeaker are essentially the same device run in opposite directions. The piezoelectric crystal works both ways — it's how SONAR and medical-imaging probes generate ultrasonic pulses.
Musical instruments — how wind, string, and percussion produce notes3 rows
| Instrument family | Vibrating element | Pitch determined by |
|---|---|---|
| Wind (flute, clarinet, etc.) | Vibrating air column inside (and outside) the tube | Tube length + open holes (sets the standing-wave wavelength) NDA 2023 Apr trap — loudness comes from AMPLITUDE / intensity of the air column's oscillation, NOT from "momentum of waves on the blowing jet". |
| Stringed (guitar, violin) | Vibrating string coupled to a resonance box | String length / tension / mass per unit length |
| Percussion (drum, tabla) | Vibrating membrane or solid body | Membrane tension + size |
In all three families, loudness is set by the AMPLITUDE of the vibrating element — bigger displacement = louder. Trap-aware row is the flute.
Watch out for (4)
- SONAR uses ULTRASONIC, not audible sound→ SONAR, bats, medical imaging — applications of ultrasonic
- Bats use ULTRASONIC, not radio waves or microwaves→ SONAR, bats, medical imaging — applications of ultrasonic
- Microphone is sound electrical, NOT the other way around→ Microphone, loudspeaker — converting between acoustic and electrical
- Flute loudness comes from amplitude — NOT momentum, NOT arrival time→ Musical instruments — how wind, string, and percussion produce notes