An ambulance travelling at 25 ms\(^{-1}\) towards a stationary observer emits a siren with frequency 800 Hz. The speed of sound in air is 340 ms\(^{-1}\).
 

1. Calculate the frequency heard by the observer as the ambulance approaches.   (2 marks)

2. Calculate the frequency heard by the observer after the ambulance has passed.   (2 marks)

An ambulance is moving along a straight road with its siren turned on. An observer is also moving along the same road as seen below:
 

In which situation will the observer hear the highest pitch from the siren?

A stationary observer hears a siren mounted on a car. Initially, the car is stationary and produces a sound with speed \(v\) and wavelength \(\lambda_1\).

The car then begins to move away from the observer at a constant speed.

Which row of the table correctly shows the speed of the sound wave and the wavelength measured by the observer once the car is moving?

\begin{align*}
\begin{array}{l}
\rule{0pt}{2.5ex} \ \rule[-1ex]{0pt}{0pt}& \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|c|c|}
\hline
\rule{0pt}{2.5ex}\text{Wave Speed}\rule[-1ex]{0pt}{0pt}& \text{Wavelength }\lambda_2 \\
\hline
\rule{0pt}{2.5ex}\text{Equal to \(v\)}\rule[-1ex]{0pt}{0pt}&\text{Greater than } \lambda_1\\
\hline
\rule{0pt}{2.5ex}\text{Less than \(v\)}\rule[-1ex]{0pt}{0pt}& \text{Less than } \lambda_1\\
\hline
\rule{0pt}{2.5ex}\text{Equal to \(v\)}\rule[-1ex]{0pt}{0pt}& \text{Less than } \lambda_1\\
\hline
\rule{0pt}{2.5ex}\text{Greater than \(v\)}\rule[-1ex]{0pt}{0pt}& \text{Greater than } \lambda_1\\
\hline
\end{array}
\end{align*}

A fire truck is moving toward a stationary observer at a speed of 20 ms\(^{-1}\). The siren on the fire truck emits sound at a frequency of 600 Hz.
 

1. Calculate the observed frequency of the siren as heard by the observer.   (1 mark)

2. The flashing lights on the fire truck do not show a noticeable Doppler shift as it passes by. Explain why the Doppler effect for light is not observed in this situation.   (2 marks)

The horn of a stationary train emits sound at a frequency of 1400 Hz. What will be the apparent frequency heard by an observer if the train moves away at 30 m/s?

1. 1325 Hz
2. 1265 Hz
3. 1350 Hz
4. 1286 Hz

On a summer day, a train is moving at a speed of 25 m/s along a straight track. The train’s horn emits a steady tone at a frequency of 600 Hz. The speed of sound in air on that day is measured to be 343 m/s.
 

A stationary sound sensor is placed ahead of the train on the track.

1. What frequency will be detected by the sensor positioned in front of the moving train?   (2 marks)

2. The same experiment is repeated on a winter day, when the speed of sound in air is measured to be 320 m/s. Would the detected pitch at the same location be higher or lower than on the summer day? Justify your answer.   (2 marks)

Two ambulances, \(\text{A}\) and \(\text{B}\), are travelling along a straight road, both with the same constant velocity, \(v\). Both ambulances have their sirens on and the sounds produced are identical and have a constant frequency.

Ambulance \(\text{A}\) is travelling directly towards a stationary observer, while ambulance \(\text{B}\) is travelling directly away from the stationary observer, as shown in the diagram below.
 

Which one of the following best describes the frequency of each siren as measured by the stationary observer, compared to the frequency the observer would measure if the ambulances were stationary?

1. The observer measures each siren's frequency to be lower.
2. The observer measures each siren's frequency to be higher.
3. The observer measures the frequency of ambulance \(\text{A}\)'s siren to be lower and the frequency of ambulance \(\text{B}\)'s siren to be higher.
4. The observer measures the frequency of ambulance \(\text{A}\)'s siren to be higher and the frequency of ambulance \(\text{B}\)'s siren to be lower.

Lee listens while a police car with a loud siren comes towards her, travels past her and then continues on away from her.

Compared with the sound she would hear from the siren if the police car were stationary, the sound has.

1. a higher frequency as the car comes towards her and a lower frequency when the car moves away.
2. a lower frequency as the car comes towards her and a higher frequency when the car moves away.
3. a lower intensity as the car comes towards her and a greater intensity when the car moves away.
4. the same frequency at all times.

Alex hears the siren from a stationary fire engine.

Compared with the sound Alex hears from the stationary fire engine, the sound Alex will hear as the fire engine approaches him will have increased

1. speed.
2. period.
3. amplitude.
4. frequency.

A distant fire truck travelling at 20 ms\(^{-1}\) to a fire has its siren emitting sound at a constant frequency of 500 Hz.

Chris is standing on the edge of the road. Assume that the fire truck is travelling directly towards him as it approaches and directly away from him as it goes past. The arrangement is shown in the diagram.
 

1. On the diagram below, sketch the frequency that Chris will hear as the truck moves towards him and then moves away from him. The \(500 \text{ Hz}\) siren signal is shown as a dotted line for reference. No calculations are required.   (2 marks)
    

2. Name the physics principle involved in Chris’s experience.   (1 mark)
   

Which one of the following statements best describes an observation of the Doppler effect for sound?

1. a decrease in frequency received when a source of sound moves towards you
2. a decrease in frequency received when moving towards a stationary source of sound
3. an increase in frequency received when moving towards a stationary source of sound
4. a decrease in wavelength received when moving away from a stationary source of sound