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Beats and the Doppler Effect

conceptmedium~40 min study9 MCQ

Two slightly different frequencies produce beats at f1 - f2; relative motion between source and observer shifts the observed frequency, f' = f (v + vo)/(v - vs).

Practice 9 questionsBack to syllabus~14 min

What is Beats and the Doppler Effect?

Two slightly different frequencies produce beats at f1 - f2; relative motion between source and observer shifts the observed frequency, f' = f (v + vo)/(v - vs).

Key formula / rule: Beat frequency = |f1 - f2|, audible up to about 10 Hz.

Key points

  • Compute beat frequency and identify the unknown tuning fork frequency.
  • Apply the Doppler formula with a consistent sign convention.

Common exam trap

Mixing up the signs in the numerator and denominator.

Definitions

Term

Beats and the Doppler Effect

Meaning

Two slightly different frequencies produce beats at f1 - f2; relative motion between source and observer shifts the observed frequency, f' = f (v + vo)/(v - vs).

Term

Beats and the Doppler Effect — explanation

Meaning

The Doppler formula demands a sign convention taken along the source-to-observer direction. For sound the medium matters, so source motion and observer motion are not equivalent.

Learning objectives

  • Compute beat frequency and identify the unknown tuning fork frequency.

  • Apply the Doppler formula with a consistent sign convention.

Formulae

Key point

Beat frequency = |f1 - f2|, audible up to about 10 Hz.

Key point

Doppler: f' = f (v + vo)/(v - vs) with a fixed sign convention.

Key point

Source and observer motion give different results for sound.

Prerequisites

  • PHY-U4-WAV-T1-S1-C2

Common mistakes

  • Mixing up the signs in the numerator and denominator.

  • Applying the sound formula to light without relativistic correction.

Keywords

  • Beats

  • Doppler

  • Effect

Practice preview

  • Two sound waves with frequencies 256 Hz and 260 Hz are superimposed. What is the beat frequency produced?…

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  • A tuning fork A of frequency 384 Hz produces 6 beats per second with another tuning fork B. When a small piece of wax is attached to B, the beat frequency reduces to 4 beats per second. What is the frequency of tuning fo…

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  • A source of sound of frequency 400 Hz moves towards a stationary wall with a speed of 10 m/s. An observer is standing stationary behind the source. The speed of sound in air is 340 m/s. What is the beat frequency heard b…

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Practice 9 questions