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Doppler effect in air

IntroductionEducational data logger

In this experiment we will learn how the observed frequencies of a sound change with:

  • velocity of the source
  • velocity of observer
  • velocity of medium carrying the waves

We will use an air stream generated by a fan to simulate the conditions required to show how the observed frequency of a sound is changed by the Doppler effect.

The frequency is generated by hitting a pipe or musical instrument (triangle). Picoscope software was used to log the spectrum of the generated frequencies. The shift in frequency is observed when we perform the experiment inside the air stream generated by a fan, hit the same pipe or music instrument (inside the air stream) and compare the observed frequencies before and after the fan is switched on.

Before you perform the experiment we recommend you know the theory of Doppler effect and consider the applications of this principle (measurement of speed of cars in highways and police, velocity and expansion of universe). In this case there is no velocity for the source or the observer of the frequency, but there is a change in the velocity of the carrying air when the fan is switched on. Depending on the set up of experiment, you can observe a positive or negative change in source frequency. You can observe a positive change if the air stream comes from source frequency (it simulates source frequency speed) and a negative change when the air stream comes from the observer (it simulates DrDAQ speed or observer speed).

This experiment is suitable for children in Secondary level or High School level.

Doppler effect as a powerful scientific tool

The Doppler effect is a very useful and powerful tool used by many workers in science and applied technology. The Doppler effect is applied for the study of:

  • The speed of cars in highways and
  • The big bang theory of the Universe
  • How fast the stars and galaxies are running away from us

This is applied for the study of far away stars also and for the measurement velocity of fluids in pipes with out interfering the motion of studied liquids (fluid meters).

Equipment required.

Experiment setup

Install the fan 3 feet away from the DrDAQ data logger. Using the wire, hang the pipe or triangle 2.5 ft away from the DrDAQ (The pipe should be between the fan and DrDAQ and at the same level). Check the air flow is strong enough and uniform from the fan to the DrDAQ and there is not interference of air stream.

Setting up Picoscope: You should run Picoscope software and use the trigger mode for Single and Falling with 0%. Select the spectrum mode. Use the Trigger mode in high resolution (4800 frequency bands) and dB.

set up

Carrying out the experiment

Step 1. Take spectrum without air stream.

After the experiment is set up, run Picoscope in Trigger mode and spectrum. Hit the pipe with the metallic bar to take a spectrum of the generated sound. Make several runs of the same hit and check you have a repetitive spectrum and clear and strong frequencies in the range of 1800 to 2500 Hz. Mark the strongest frequency you observe in this range and save the file as


and as


You should observe in the range of 1800 to 2500 Hz a shift in the strongest frequency of 5 to 25 Hz. This shift is the Doppler effect introduced by air stream. Take several spectrums in this conditions to observe the effect.

Questions and discussion of results.

  1. Why do we see the change in the selected frequency?
  2. Why is the shift in frequency positive?
  3. If we change the position of the fan and send the air stream from the sound sensor to the frequency source, how does the observed frequency change?
  4. What happens with frequency if a boy hitting the pipe is running towards the sound sensor? (fan off)
  5. What happens with frequency if a boy hitting the pipe is running away from the sound sensor? (fan off)
  6. According to the theory of Doppler Effect, how can we measure the air velocity using the shift in frequency?