Sound & Doppler Effect — A/L Physics

What must I remember about Sound & Doppler Effect?

• Characteristics of Sound: Pitch (determined by fundamental frequency f), Loudness (determined by sound intensity I and amplitude, measured on logarithmic decibel scale β = 10 log10(I / I0)), Quality/Timbre (determined by number and relative strengths of harmonic overtones).
• Beats Phenomenon: Periodic waxing and waning of sound intensity produced by the superposition of two sound waves of slightly different frequencies f1 and f2: Beat Frequency f_beat = |f1 - f2|.
• Doppler Effect Definition: The apparent shift in observed frequency when there is relative motion between a wave source and an observer along the line joining them.
• General Doppler Formula: Observed frequency f' = f * [(v ± v_o) / (v ∓ v_s)], where v is speed of sound in medium, v_o is speed of observer, and v_s is speed of source.

Test yourself (1 minute)

1. Sign conventions in Doppler formula

   Numerator: Use (+) when observer approaches source, (-) when moving away; Denominator: Use (-) when source approaches observer, (+) when moving away.

2. Doppler Shift in Wavelength

   When source moves at speed v_s toward observer, the emitted wavelength is physically compressed: λ' = (v - v_s) / f; when observer moves, emitted wavelength λ is unchanged.

3. Reflection from moving boundary

   If sound reflects off a moving reflector (e.g., bat echolocation or radar gun), Doppler shift occurs twice: once upon reception at moving reflector, and once upon emission back to observer.

Practise Sound & Doppler Effect questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 3: Waves & Sound

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Where does Sound & Doppler Effect fit in the syllabus?

Unit 3.5 · Year 1 · Term 2