P.1 Book 3B Section 8.2 Wave nature of sound Anti-noise headphones Sound is produced by vibrations Wave properties of sound Sound as a longitudinal wave Check-point 3 8.2Wave nature of sound

P.2 Book 3B Section 8.2 Wave nature of sound Anti-noise headphones Crew members on helicopters are equipped with anti-noise headphones. Inside the headphones, there are speakers which help reduce noise. But how can a speaker reduce noise?

P.3 Book 3B Section 8.2 Wave nature of sound 1 Sound is produced by vibrations All sound sources (e.g. tuning fork, loudspeaker) have parts that vibrate. The vibrating part causes nearby air particles to vibrate and produces sound waves. 8.2 Chirping ruler Video

P.4 Book 3B Section 8.2 Wave nature of sound 2 Wave properties of sound a Reflection of sound Sound is reflected when it hits a hard surface. Reflected sound or echoes can be heard if you shout towards a cliff or a high wall at some distance away. Some animals, like bats and whales, use this wave property for navigation and hunting.

P.5 Book 3B Section 8.2 Wave nature of sound 2 Wave properties of sound b Refraction of sound Sound is refracted when it travels from air to another medium. Sound from a loudspeaker is refracted and focused by a balloon filled with CO 2.

P.6 Book 3B Section 8.2 Wave nature of sound 2 Wave properties of sound c Diffraction of sound i ‘Seeing’ sound Use a microphone to send sound signals to a computer (with suitable sound processing software) or a cathode-ray oscilloscope (CRO).

P.7 Book 3B Section 8.2 Wave nature of sound i ‘Seeing’ sound Sound makes a tiny metal plate inside the microphone vibrate  vibrations changed into electrical vibrations  shown as a waveform on the computer monitor / the screen of CRO loud sound soft sound

P.8 Book 3B Section 8.2 Wave nature of sound c Diffraction of sound ii Diffraction of sound Diffraction of sound Expt 8b

P.9 Book 3B Section 8.2 Wave nature of sound Experiment 8b Diffraction of sound 1Set up the apparatus. Adjust the signal generator at 1 kHz. 2Move a microphone connected to a CRO across in front of the speaker. Note the amplitude of the waveform. 3Change to 7 kHz and repeat the expt. 8.3 Expt 8b – Diffraction of sound Video

P.10 Book 3B Section 8.2 Wave nature of sound ii Diffraction of sound In Expt 8b, sound can be heard in the shadow region of the paper tube.  Sound can diffract around corners (like water waves and light waves). of sound waves comparable to or larger than the size of the gap  prominent diffraction pattern

P.11 Book 3B Section 8.2 Wave nature of sound ii Diffraction of sound of 1-kHz sound  0.3 m comparable to the diameter of the paper tube The sound can bend & cover a large area. of 7-kHz sound  0.05 m smaller than the size of the paper tube The sound bends less.

P.12 Book 3B Section 8.2 Wave nature of sound ii Diffraction of sound We hear people talking behind a doorway.  sound diffracts ∴ The sound diffracts noticeably through a doorway and around corners. Typical of human voice = 1 m comparable to the width of a doorway

P.13 Book 3B Section 8.2 Wave nature of sound 2 Wave properties of sound d Interference of sound Interference of sound Expt 8c Interference: another important criterion for wave Does sound show interference?

P.14 Book 3B Section 8.2 Wave nature of sound Experiment 8c Interference of sound 1Connect two loudspeakers in parallel to a signal generator which is set at 2 kHz. Covering one ear, walk across in front of the speakers. Detect any change in loudness of the sound.

P.15 Book 3B Section 8.2 Wave nature of sound Experiment 8c Interference of sound 2Connect a microphone to a CRO and move it across in front of the two speakers. Detect any change in amplitude of the waveform. 8.4 Expt 8c – Interference of sound Video

P.16 Book 3B Section 8.2 Wave nature of sound d Interference of sound Sounds from the two loudspeakers reinforce each other at some positions (constructive) and cancel each other at some other positions (destructive). ∴ Alternate loud and soft sounds are heard.

P.17 Book 3B Section 8.2 Wave nature of sound d Interference of sound  The sounds emitted are in phase and have the same frequency.  coherent sources 8.5 Interference of sound The two loudspeakers are connected to the same signal generator. Anti-noise headphones Video Example 6

P.18 Book 3B Section 8.2 Wave nature of sound Example 6 Anti-noise headphones The internal structure of an anti-noise headphone: The microphones detects the external sound waves. The speakers will then create sound waves in antiphase. incoming sound

P.19 Book 3B Section 8.2 Wave nature of sound Example 6 Anti-noise headphones Draw the sound wave emitted by the speaker and the resultant wave. Then explain why such headphones can reduce the external noise. Sound emitted by the speaker and the incoming sound undergo complete destructive interference.  external noise 

P.20 Book 3B Section 8.2 Wave nature of sound d Interference of sound Interference of sound Example 7

P.21 Book 3B Section 8.2 Wave nature of sound Example 7 Anti-noise headphones The signal generator is set at 1 kHz. Consecutive loud and soft sounds are heard along AB. Speed of sound in air = 340 m s –1 signal generator loudspeaker P loudspeaker Q XP = 1.87 m XQ = 2.04 m

P.22 Book 3B Section 8.2 Wave nature of sound Example 7 Anti-noise headphones (a) of the 1-kHz sound = ? By v = f, = 0.34 m = vfvf = (b) A loud or a soft sound is heard at X ? Path difference at X = 2.04 – 1.87 = 0.17 m = 0.5  destructive interference at X  soft sound at X 2.04 m 1.87 m

P.23 Book 3B Section 8.2 Wave nature of sound Example 7 Anti-noise headphones (c) How will the separation between two consecutive loud sound change if frequency of the sound  ? Frequency   wavelength   separation between two consecutive loud sound (antinodal lines) 

P.24 Book 3B Section 8.2 Wave nature of sound 3 Sound as a longitudinal wave The flame moves backwards and forwards as the loudspeaker sounds.  Air vibrates along the same direction as the flame (direction of travel of the sound).  Sound is a longitudinal wave. 8.6 Sound wave through air Video

P.25 Book 3B Section 8.2 Wave nature of sound 3 Sound as a longitudinal wave Loudspeaker cone vibrates  stretches and compresses the air in front  series of rarefactions and compressions produced  the eardrum vibrates and the sound is heard Sound is a longitudinal wave. It shows all properties of wave.

P.26 Book 3B Section 8.2 Wave nature of sound Check-point 3 – Q1 If the loudspeaker begins to emit sound waves, what will happen to the bead? A It moves away from the loudspeaker. B It vibrates to and fro. C It vibrates sideways. D It does not move. small polystyrene bead

P.27 Book 3B Section 8.2 Wave nature of sound A Loudspeakers put closer together  fewer soft sounds heard along PQ B Alternate loud and soft sounds heard along RS C Loudness of the sound at S  when a loudspeaker is disconnected Check-point 3 – Q2 R : midway between the loudspeakers Which of the following is incorrect? loudspeaker Alternate loud and soft sounds heard along PQ ; loud sound heard at S

P.28 Book 3B Section 8.2 Wave nature of sound Check-point 3 – Q3 Why can we hear the conversation of people around a corner? Since the sound waves of most speech have _______ wavelengths, they can __________ around corners. Therefore, we can hear people talking around a corner. long diffract

P.29 Book 3B Section 8.2 Wave nature of sound The End