(d) A second clarinet player was standing beside Ballot as the train approached. She produced the same frequency (137 Hz) as the clarinet player on the train, causing Ballot to hear beats. Explain why the sound wave reaching Ballot from the clarinet on the train did not have a frequency of 137 Hz, AND explain why Ballot heard beats, AND calculate the beat frequency

Excellence question
The clarinet on the train is moving towards the waves that it has already produced
This causes the wavelength to decrease and the frequency of the wave to increase (the wave speed is constant)
This is called the Doppler effect and is why the sound waves from the clarinet on the train reach Ballot with an increased frequency of 139 Hz instead of 137 Hz.
Beats are caused when the two waves of slightly different frequencies to arrive at Ballot with slightly different frequencies.
they sometimes arrive in phase interfering constructively causing a loud sound to be heard
when they arrive 180' out of phase they interfere deconstructively causing a quite sound to be heard
The beat frequency is the difference between the two frequencies, so the beat frequency is f1-f2=139-137= 2 Hz

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NCEA Level 3 Physics exam Notiz am Waves past exams modelling, erstellt von Shannon Ruscoe am 08/10/2018.