NEET Physics — Waves and Sound Complete Chapter Guide

Chapter Overview & Weightage

Waves and Sound covers wave motion, superposition principle, standing waves, beats, and the Doppler effect. NEET tests formula-based problems and conceptual understanding of wave behaviour.

This unit carries 5-7% weightage in NEET with 2-3 questions. Standing waves (strings and pipes), beats, and the Doppler effect are the most tested topics.

Key Concepts You Must Know

Tier 1 (Core)

Wave equation: $v = f\lambda$ , speed of sound in air $\approx 330-340$ m/s
Transverse (perpendicular vibration) vs longitudinal (parallel vibration) waves
Standing waves on string: $f_n = \frac{nv}{2L}$ (both ends fixed), nodes = $n+1$ , antinodes = $n$
Open pipe: $f_n = \frac{nv}{2L}$ (all harmonics), closed pipe: $f_n = \frac{(2n-1)v}{4L}$ (odd harmonics only)
Beats: beat frequency = $|f_1 - f_2|$

Tier 2 (Frequently tested)

Doppler effect: $f' = f\left(\frac{v \pm v_o}{v \mp v_s}\right)$ (approach = increase, recede = decrease)
Resonance tube experiment: first resonance at $L_1 + e$ , second at $3(L_1 + e)$ , end correction $e$
Speed on string: $v = \sqrt{T/\mu}$ ( $T$ = tension, $\mu$ = mass per unit length)
Intensity: $I \propto A^2 f^2$

Important Formulas

String fixed at both ends: $f_n = \frac{n}{2L}\sqrt{\frac{T}{\mu}}$ ( $n = 1, 2, 3...$ , all harmonics)

Open pipe: $f_n = \frac{nv}{2L}$ ( $n = 1, 2, 3...$ , all harmonics)

Closed pipe: $f_n = \frac{(2n-1)v}{4L}$ ( $n = 1, 2, 3...$ , odd harmonics only: 1st, 3rd, 5th…)

Fundamental frequency ratio: for same length, open pipe has twice the fundamental frequency of closed pipe.

f' = f\left(\frac{v + v_o}{v - v_s}\right) \text{ (both approaching)}

Convention: all velocities measured from medium. Use $+$ for motion towards, $-$ for motion away.

Quick rules:

Source approaching → frequency increases (pitch goes up)
Source receding → frequency decreases (pitch goes down)
Same rules for observer approaching/receding

For closed pipe problems, remember: only odd harmonics (1st, 3rd, 5th). The first overtone of a closed pipe is the 3rd harmonic, not the 2nd. NEET tests this “overtone vs harmonic” distinction.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: Two tuning forks of frequencies 256 Hz and 260 Hz are sounded together. The beat frequency is:

Solution: Beat frequency = $|260 - 256| = \mathbf{4}$ Hz (4 beats per second)

PYQ 2 — NEET 2023

Problem: The fundamental frequency of an open pipe is 300 Hz. What is the fundamental frequency of a closed pipe of the same length?

Solution:

Open pipe: $f_{open} = v/(2L) = 300$ Hz

Closed pipe: $f_{closed} = v/(4L) = 300/2 = \mathbf{150}$ Hz

The closed pipe has half the fundamental frequency of an open pipe of the same length.

PYQ 3 — NEET 2022

Problem: A source of sound moves towards a stationary observer with velocity 30 m/s. If the frequency of the source is 500 Hz and speed of sound is 330 m/s, find the apparent frequency.

Solution:

f' = f\left(\frac{v}{v - v_s}\right) = 500 \times \frac{330}{330 - 30} = 500 \times \frac{330}{300} = \mathbf{550 \text{ Hz}}

Expert Strategy

Day 1: Standing waves — strings and pipes. Know the formulas and the distinction between open and closed pipes. Practice finding harmonics and overtones.

Day 2: Beats and Doppler effect. Beats are simple (difference of frequencies). Doppler needs sign convention practice — do 10 problems with different source/observer motion combinations.

Common Traps

Trap 1 — Closed pipe has ONLY odd harmonics. The 2nd harmonic doesn’t exist in a closed pipe. The first overtone = 3rd harmonic = $3f_1$ . Open pipes have all harmonics.

Trap 2 — “1st overtone” is NOT “1st harmonic.” The fundamental IS the 1st harmonic. The 1st overtone is the 2nd harmonic (for open pipe/string) or the 3rd harmonic (for closed pipe).

Trap 3 — In the Doppler effect formula, the sign depends on direction of approach/recession. If both source and observer approach each other, frequency increases maximally. If both recede, it decreases. Getting the signs wrong gives the opposite effect.

Trap 4 — Speed of sound depends on temperature, not pressure (at constant temperature). $v \propto \sqrt{T}$ . Doubling pressure at constant temperature does NOT change the speed of sound (density also doubles, effects cancel). NEET tests this.