Exam definitions and explanations

Question 1

What is a beat?

Answer 1

A beat is a periodic increase and decrease in wave amplitude.

Question 2

What causes a beat?

Answer 2

Beats are caused when waves of different frequencies interfere, producing moments of low/zero amplitude, and moments of high amplitude.

These are caused by destructive interference; waves being out of phase, and constructive interference; waves in phase.

Question 3

(Doppler Effect) What happens to the frequency of the string and the speed of the sound in the air for an approaching object?

Answer 3

The frequency of the string and the speed of the sound in the air do not change.

Question 4

(Doppler Effect) How is the wavelength of the sound in the air in front of and behind the violin affected?

Answer 4

There is a decreased wavelength going in front, and an increased wavelength behind.

This is because as the violin moves, it produces wavefronts closer to the previous and further behind.

Question 5

(Doppler Effect) How are the frequencies (detected by microphones A and B) affected by the moving trailer?

Answer 5

v = fλ (c = constant)

The mic in front of the trailer experiences decrease in wavelength and increase in frequency.

The mic behind the trailer experiences increase in wavelength and decrease in frequency.

Question 6

What are the components of the doppler effect apparent frequency equation (approaching)?

Answer 6

f’ = f x Vw / Vw - Vs

f = frequency
Vw = velocity of the wave
Vs = velocity of the source

Question 7

In the doppler effect frequency equation, what does the Vw usually represent? (For exams)

Answer 7

Vw usually is represented by the speed of sound (look out for that in exams - it will probably be needed for this equation).

Question 8

How is the fundamental frequency of the string affected when the string gets longer (ceterus paribus)?

Answer 8

When the length increases, then the wavelength increases. V is constant.

V = fλ
If wavelength increases, the frequency will decrease, therefore the frequency of the string will decrease.

Question 9

How do you calculate the possible frequencies at which a string is vibrating when you hear a beat of e.g. 2.1Hz

Answer 9

You take the original frequency and + and - the original frequency by the beat (e.g. original f = 100 –> 100+2.1 // 100-2.1)