12 Feb, 2013
PRESENTATION: A tuning fork is placed at the end of a string that is then spun around in circles. The pulsating sound that is heard is different from the note emitted by the tuning fork when it at relative rest. The frequency of the sound emitted becomes higher when it approaches the observer and lower when it moves away.
- Doppler frequency shift of sound apparatus, Walter Schier, Phys. Teach. 49, 246 (2011)
- Noise Doppler-Shift Measurement of Airplane Speed, Ivan F. Costa and Alexandra Mocellin, Phys. Teach. 45, 356 (2007)
INTRODUCTION: The Doppler effect, so called after the Austrian Christian Andreas Doppler, is the apparent change in frequency of a wave produced by the relative movement between the source, the emitter and/or the medium. This effect can be observed continually in everyday life. A clear example is when an ambulance (with its siren on) approaches us, reaches us and then goes away.
OBJECTIVE: To produce the Doppler effect using a constant sound source that produces a square sound wave.
MATERIALS: loudspeaker, 9V batter, circuit board, oscillator, 3 resistors, electrolyte capacitor, ceramic capacitor, 555N chip, 1 clamp, 1 string.
SETUP: Mount the oscillator, the resistors, the chip and the capacitors on the circuit board. Use the battery to power them and connect the loudspeaker. This is our sound source. For the experiment, the clamp will be attached to the apparatus and then tied to the string.
EXPLANATION: Sound waves propagate in all directions. When the emitting source is static, the waves move outwards in concentric circles and reach anyone nearby with no changes in tone. However, if the source is moving, then the sound waves it gives off, for example those from our little device, will “pile up” in the direction of movement. This means that anyone standing at, say, 50 m in front of the moving object will receive a greater number of waves per unit of time and this means that the sound will have a higher-pitched tone. But the opposite is also true: when the movement is away from the person, the waves the device leaves behind going in their direction will become separated from each other, that is, fewer waves per unit of time, which means the sound becomes lower-pitched. We will make our sound source spin for this experiment.
CONCEPTS: Doppler effect, harmonic square wave, frequency, tone.
- Tipler P.A. Física. Barcelona: Reverté, 2010.
- De Juana J.M., Física General, Pearson, 2009.
- Serway R.A y J.W.Jewett. Física, Thomson-Paraninfo, 2010.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2010-2011: Marcos González, Andrea Gómez, Atanasio Gerassis.
LINK pdf STUDENTS (in Spanish):