CIE A Level: Doppler Effect for Sound Waves

Doppler Effect

Frequency change due to relative motion between source and observer.

Stationary Source

Source not moving relative to the observer.

Doppler Effect

Frequency change due to relative motion between source and observer.

Stationary Source

Source not moving relative to the observer.

Frequency

Number of wave cycles per second, measured in Hz.

Wavelength

Distance between successive wave peaks.

Higher Frequency

Perceived when source approaches the observer.

Lower Frequency

Perceived when source moves away from the observer.

Wave Equation

v = fλ, relates wave speed, frequency, and wavelength.

Observed Frequency (fo)

Frequency detected by the observer.

Source Frequency (fs)

Frequency emitted by the source.

Wave Velocity (v)

Speed of sound waves in air, 340 m/s.

Source Velocity (vs)

Speed of the source relative to the observer.

Doppler Shift Equation

fo = fs * (v ± vs) / v.

Moving Source

Source that changes position relative to observer.

Longer Wavelength

Occurs when source moves away, indicating lower frequency.

Shorter Wavelength

Occurs when source moves towards, indicating higher frequency.

Pitch

Perceived frequency of sound, higher or lower.

Cyclist Example

Cyclist moving away causes longer wavelength and lower pitch.

Police Car Example

Frequency heard is 397 Hz when moving away.

Examiner Tip

Identify source and observer correctly in calculations.

Frequency Decrease

Observed when source moves away from observer.

Frequency Increase

Observed when source moves towards observer.

Sound Waves

Mechanical waves that require a medium to travel.

Siren Example

Ambulance siren sounds lower as it moves away.