NEB Class 12 Physics Acoustic Phenomena Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Acoustic Phenomena Note Handwritten in PDF
Sound as a Pressure Wave
Sound is a longitudinal mechanical wave β particles vibrate parallel to the direction of wave travel. The wave consists of alternating compressions (high pressure) and rarefactions (low pressure).
Pressure amplitude (Ξp_max): The maximum variation in pressure from equilibrium. Related to displacement amplitude A by: Ξp_max = BAk, where B = bulk modulus, k = wave number.
Characteristics of Sound
Intensity (I):
- Physical quantity β power per unit area: I = P/A (W/mΒ²)
- Follows inverse square law: I β 1/rΒ²
- Doubles when distance reduces to 1/β2 of original
Loudness:
- Subjective perception of intensity
- Measured in decibels: L = 10 logββ(I/Iβ)
- Iβ = 10β»ΒΉΒ² W/mΒ² (threshold of human hearing)
- Normal conversation β 60 dB, jet engine β 140 dB
Pitch:
- Related to frequency high frequency = high pitch
- Not same as loudness (a loud bass note has high loudness, low pitch)
Quality (Timbre):
- Determined by the mixture of harmonics present
- Same note on piano vs violin sounds different because of different harmonic composition
- Related to the waveform shape
The Doppler Effect Concept
When there is relative motion between a sound source and observer, the observed frequency differs from the emitted frequency.
- Source approaching: More wavefronts per second reach observer β apparent frequency increases (higher pitch)
- Source receding: Fewer wavefronts per second β apparent frequency decreases (lower pitch)
The effect was proposed by Austrian physicist Christian Doppler in 1842.
Doppler Formula All Four Cases
General formula: f’ = f Γ (v Β± vβ) / (v β vβ)
Where v = speed of sound, vβ = speed of observer, vβ = speed of source
Sign rule:
- Numerator: +vβ when observer moves toward source; βvβ when moving away
- Denominator: βvβ when source moves toward observer; +vβ when moving away
Four specific cases:
| Case | Formula |
|---|---|
| Observer toward stationary source | f’ = f(v + vβ)/v |
| Observer away from stationary source | f’ = f(v β vβ)/v |
| Source toward stationary observer | f’ = fv/(v β vβ) |
| Source away from stationary observer | f’ = fv/(v + vβ) |
NEB exam tip: Case 3 and 4 (moving source) are the most frequently tested numerically. Always check which is moving β source or observer.
Applications of Doppler Effect
In daily life:
- Ambulance/police siren sounds higher pitched when approaching, lower when moving away
In technology:
- Radar speed guns: Police emit radio waves, measure frequency of reflection from moving car β shift gives car speed
- Doppler weather radar: Detects speed of rain clouds to forecast storms
In medicine:
- Doppler ultrasound: Measures blood flow velocity in arteries β detects blockages, measures fetal heart rate in pregnancy
In astronomy:
- Redshift: Light from distant galaxies shifted to lower frequency β they are moving away β proves universe is expanding (Hubble’s Law)
- Blueshift: Stars moving toward us shift light to higher frequency (shorter wavelength)
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Frequently Asked Questions
What is Doppler effect in Class 12 Physics?
Doppler effect is the apparent change in frequency when source and observer have relative motion. When approaching, observed frequency increases (higher pitch). When moving apart, frequency decreases (lower pitch). It applies to all waves including sound, light, and radio waves. Named after Austrian physicist Christian Doppler (1842).
What is the Doppler effect formula Class 12?
General Doppler formula: f’ = f(v Β± vβ)/(v β vβ). Use + in numerator when observer moves toward source, β when moving away. Use β in denominator when source moves toward observer, + when moving away. v = speed of sound, vβ = observer speed, vβ = source speed. Always identify who is moving first.
What are the four cases of Doppler effect NEB Physics?
Case 1: Observer toward stationary source β f’ = f(v+vβ)/v. Case 2: Observer away from source β f’ = f(vβvβ)/v. Case 3: Source toward stationary observer β f’ = fv/(vβvβ). Case 4: Source away from observer β f’ = fv/(v+vβ). Cases 3 and 4 (moving source) are most commonly tested in NEB exams.
What are real life applications of Doppler effect?
Radar speed guns measure car speed from frequency shift of reflected radio waves. Doppler ultrasound measures blood flow velocity detecting artery blockages. In astronomy, redshift of galaxy light (lower frequency) proves universe is expanding β Hubble’s Law. Doppler weather radar tracks storm cloud movement and speed.
What is the difference between intensity and loudness of sound?
Intensity (I) is physical β power per unit area in W/mΒ², follows inverse square law I β 1/rΒ². Loudness is subjective β measured in decibels: L = 10 log(I/Iβ) where Iβ = 10β»ΒΉΒ² W/mΒ². Same intensity sounds louder at 1000 Hz than at 100 Hz due to frequency-dependent sensitivity of human ear.
What is the difference between pitch and quality of sound?
Pitch is determined by frequency β higher frequency means higher pitch. It is objective and measurable. Quality (timbre) is determined by the mixture of harmonics present in the sound β explains why same note sounds different on piano versus violin. Both pitch and quality together describe what we hear as musical tone.
How to solve Doppler effect numericals in NEB exam?
Steps: identify clearly if source or observer (or both) are moving. Write Doppler formula with correct sign convention. Use v = 340 m/s for sound in air unless stated otherwise. Substitute values carefully. Common mistake: confusing which sign goes in numerator versus denominator. Practice all four cases separately before exams.
Which acoustic phenomena topics are most important for Exam?
Most important: Doppler formula with all four cases and numericals (4-6 marks), intensity and loudness difference (2 marks), characteristics of sound (2 marks). Doppler effect numericals β especially moving source cases β appear in almost every NEB Class 12 Physics board examination paper.
