- Stationary sources radiate sound in concentric circles around the source.
- Moving sources radiate sound in spheres around successive positions.
- At the speed of sound, all the compressions pile up in one spot to produce very dense air.
- Sound Barrier â The compression of very dense air that forms when objects move at the speed of sound.
- Improperly designed air craft would be buffeted about by this compression.
- If properly designed, the aircraft will cut through this barrier without problem and leave the sound behind.
- When travelling faster than sound, the spheres interfere to produce large compressions and rarefactions, which sweep behind the aircraft in a cone shape.
- This cone sweeps across the ground to produce a sonic boom (2 sharp cracks, one for each side of the cone as it passes over).
- The cone is usually five times as wide as the aircraft is high.
- Sonic booms usually do not cause damage but they could affect ecosystems if it occurs frequently.
- When a car speeds by you notice that the pitch of the horn decreases as it passes.
- Wave fronts in front of the car are close together and behind are far apart.
- C hears a higher than normal pitch.
- A hears a lower than normal pitch.
- B hears a pitch that changes from high to low.
- The driver hears the normal pitch.
- Doppler effect â The apparent change in frequency of a wave due to relative motion between source and observer.
- If the relative motion is towards, the pitch increases.
- If the relative motion is away, the pitch decreases.
- The equation used to determine the apparent observed frequency is
- f1 = true frequency of the wave
- f2 = apparent frequency of the wave
- v = Speed of sound
- vo = Speed of the observer
- vs = speed of the source
- The rule of signs
- The Doppler effect occurs for any object emitting waves. The equation for the Doppler effect is different for light because light does not have a medium that it moves through. For light we have
- For source and observer approaching each other
- For source and observer separating from each other
- fâ = observed frequency.
- f = actual frequency.
- v = relative velocity between observer and source.
- Applications of the Doppler effect include determining the speed of cars by the police, and the speed of stars with respect to the Earth.
Home
November 18, 2013