1© Manhattan Press (H.K.) Ltd Nature of sound waves

2 © Manhattan Press (H.K.) Ltd Nature of sound waves (SB p. 168) Nature of sound waves Sound - produced by vibrations of objects (e.g. vibrating strings in guitar, piano, violin) - frequency of sound waves = frequency of vibrating source

3 © Manhattan Press (H.K.) Ltd Nature of sound waves (SB p. 169) Nature of sound waves Sound - longitudinal wave (needs material medium) - air molecules are displaced from equilibrium positions during propagation of sound wave Go to More to Know 1 More to Know 1

4 © Manhattan Press (H.K.) Ltd Nature of sound waves (SB p. 169) Nature of sound waves Not displaced towards right towards left wavelength – between two rarefactions or compressions

5 © Manhattan Press (H.K.) Ltd Nature of sound waves (SB p. 170) Nature of sound waves Go to More to Know 2 More to Know 2

6 © Manhattan Press (H.K.) Ltd Nature of sound waves (SB p. 171) Nature of sound waves Go to Example 1 Example 1

7 © Manhattan Press (H.K.) Ltd. End

8 © Manhattan Press (H.K.) Ltd. Astronauts communicating in space Since sound wave is a mechanical wave, it cannot travel through a vacuum. Thus, the astronauts in space cannot talk to each other directly. Their helmets turn their voices into radio waves which can travel through space. Return to Text 11.1 Nature of sound waves (SB p. 169)

9 © Manhattan Press (H.K.) Ltd. The variation of pressure In fact, the variation of pressure of the air molecules in a sound wave is very small. In general, for a loud sound, the variation is only about 30 Pa, but the normal atmospheric pressure is of the order 10 5 Pa. However, our ears are very sensitive to the pressure change and able to detect a variation of 2 × 10 −5 Pa. Return to Text 11.1 Nature of sound waves (SB p. 170)

10 © Manhattan Press (H.K.) Ltd. Q: Q: The figure below shows a loudspeaker L emitting sound continuously at a frequency of 400 Hz along a straight line in the positive x-direction. The graph represents the displacement of the air particles from their respective equilibrium positions along the x-axis at one particular instant. (a) Among the points A, B, C and D, at which point is the instantaneous pressure maximum? Explain your answer. (b) Calculate the time taken by the sound to move a distance AD. Solution 11.1 Nature of sound waves (SB p. 171)

11 © Manhattan Press (H.K.) Ltd. Solution: 11.1 Nature of sound waves (SB p. 172) (a) The pressure is maximum at the point C because the displacement of air particles to the left of C is positive and the displacement of air particles to the right of C is negative. This causes the air at C to be compressed. Hence, the pressure at C is at a maximum. Note: The pressure at the points A and E is at a minimum because to the left of A (or E), the displacement is negative while to their right, the displacement is positive. Rarefaction occurs at A and E.

12 © Manhattan Press (H.K.) Ltd. Solution (cont’d): Return to Text 11.1 Nature of sound waves (SB p. 172)