Sound as a wave

1. Sound as a wave Interference of sound Diffraction of sound Other wave properties of sound 1 2 3

2. Close all windows. Block your ears with cotton. Use something to cover you up. Others: ________. One day some construction work is proceeding outside your home. How to reduce the noise?    What is the principle behind this/these method(s)?

3. Interference of sound a ‘Seeing’ sounds ‘Seeing’ sound with a CRO & microphone. • Microphone • connects to CRO • changes the vibration of sound to electrical vibrations shown on CRO screen

4. signal generator loudspeaker Interference of sound Set the signal generator at 2 kHz. Covering one ear, walk across in front of the loudspeakers. Detect any changes in the loudspeakers.

5. Interference of sound Connect a microphone to a CRO and move it across in front of the two loudspeakers. CRO metre rule microphone Detect any changes in the amplitude of the waveform on the CRO.

6. Interference of sound b Interference pattern of sound In Expt. 10 h, alternate loud and soft sounds are heard while walking across in front of the loudspeakers. The sounds from the loudspeakers reinforce each other at some positions... The sounds from the loudspeakers cancel each other at some positions...

7. Interference of sound b Interference pattern of sound v = f Wave formula can also apply to sound waves.

8. Interference of sound c Coherent sound sources for interference Two loudspeakers are connected to a signal generator. The sounds emitted from the loudspeakers are in phase and same frequency They are two coherent sources.

9. Diffraction of sound ... !!! a Diffraction of sound Sound can bend around corners (diffract) like water waves and light waves - hear others’ conversation behind a doorway Examples: - hear a plane flying when staying indoor

10. Diffraction of sound b Degree of diffraction If the wavelengths of sound waves are comparable to the size of the gap a remarkable diffraction pattern appears.

11. Diffraction of sound ... !!! b Degree of diffraction Since the width of the doorway is comparable to the wavelength of most speech and music... such sound can bend around the corner.

12. ballon filled with CO2 CRO loud-speaker signal generator micro-phone Other wave properties of sound • Reflection (e.g. echo) • Refraction Waves are refracted and focused.

13. Other wave properties of sound • Diffraction (e.g. hearing people talking around corner) • Interference

14. Q1 Two loudspeakers are… Two loudspeakers are connected to a signal generator. signal generator P R S Q Ris in the midway of the loudspeakers.

15. Q1 Two loudspeakers are… If alternate loud and soft sounds are heard along PQ, which is INCORRECT? A When the loudspeakers are put close together, fewer soft sounds are heard along PQ. B Alternate loud and soft sounds can also be heard along RS. C The loudness of the sound at S decreases when one of the loudspeakers is disconnected.

16. Q2 Why can we hear the... Why can we hear the conversation of people around a corner? Oh, my students like me! Yes, she is very kind... Have you met the new teacher Ms Chan?

17. Q2 Why can we hear the... Since sound waves of most speech have _________ wavelengths, they ________ around corners. long bend  we can hear people talking around the corner.

18. Example 6 How far apart should loudspeakers be separated? To produce interference, a signal of frequency 2 kHz is fed into two loudspeakers. How far apart should the two loudspeakers be placed? (they are separated by 3) Speed of sound in air = 340 m s-1.

19. Example 6  = = 0.17m 340 2000 How far apart should loudspeakers be separated? From the wave equation v = f,  3 = 0.51 m The two loudspeakers should be placed about 0.5 m apart.