1. Doppler Effect

2. What sound do you hear when a car races by at the race track? Or when an ambulance/police car speeds by?

3. Key Question: How does the sound from a moving object differ from that of a stationary object?

4. Doppler Effect When a source of sound approaches an observer, the observed frequency increases When the source moves away from the observer, the observed frequency decreases When the source is stationary there is no change

5. Explanation: Source is unmoving – all waves radiate evenly from centre

6. Observer as Train moves towards: the source of the sound waves are traveling towards crests are pushed closer together thus decreasing wavelength and increasing frequency

7. Observer as Train moves away: the source of the sound waves is moving away crests are stretched out behind thus increasing the wavelength and decreasing the frequency

8. Same thing happens to light

9. The same effect is experienced when the source of the sound is stationary and the observer moves towards or away from the source with enough speed

10. Equation for Frequency Change f obs = f source _v_ v  v source v = speed of sound in the medium (m/s) v s = speed of the source through medium(how fast the source is moving) v + v s – used when source is moving away from observer v – v s – used when source is moving towards observer

11. Sample Problem An ambulance races down Bramalea Road at 85 km/h with its siren blazing. The siren has a frequency of 456 Hz and the temperature of the air is 15 o C. What is the frequency of the sound waves as they reach the students at Bramalea S.S. a.) as the ambulance approaches? b.) after it has passed the students?

12. Thinking Question If Harsh is screaming and you are going to help him. As you run towards Harsh, will the frequency of his scream sound higher or lower?

13. Applications Astronomers: speed of stars Police radar Tracking satellites/weather systems Baseball pitch – measure speed Infrared detectors for home security

14. Supersonic Travel Subsonic speeds – speed less than that of sound in air Supersonic – speed greater than the speed of sound in air

15. Mach Number Mach 1 is the the speed of sound the ratio of the speed of an object to the speed of sound in air Mach number = speed of object speed of sound

16. Sample Problem If the temperature outside is - 20  C and a plane is flying at 900m/s, at what mach number is the plane flying?

17. Supersonic Speed When flying at the speed of sound, the waves in front of the object pile up, producing an area of very dense air, or intense compression – Sound Barrier extra thrust is required to "break through" this "wall" of air

18. sound waves left behind the object interfere constructively in a cone shape – creates a sonic boom an object must be specially designed to with stand the vibrations incurred when going through the sound barrier