2. 1© Manhattan Press (H.K.) Ltd. Loudness, pitch and quality Intensity level and loudness Intensity level and loudness 11.2 Properties of sound

3. 2 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 172) Loudness, pitch and quality sound human ear responded: 20 Hz – 20 000 Hz ultrasonics: above this range infrasonics: below this range (sources: earthquakes, thunder, volcanoes, heavy vibrating machinery) Go to More to Know 3 More to Know 3 Go to More to Know 4 More to Know 4

4. 3 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 173) Loudness, pitch and quality (a) Loudness: - subjective sensation - determined by intensity of sound, sensitivity of ear a – amplitude of sound r – distance from source (b) Pitch: - higher frequency, higher pitch

5. 4 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 173) Loudness, pitch and quality (c) Quality: - depends on waveform of sound Audio- frequency generator PianoGuitar

6. 5 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 174) Intensity level and loudness 1. Intensity in decibels 10 -12 W m -2 : threshold of hearing for sound of 1 000 Hz I: sound intensity Unit: dB Note: Intensity level increase of 3 dB corresponds to about doubling the sound intensity. Go to More to Know 5 More to Know 5

7. 6 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 175) Intensity level and loudness 1. Intensity in decibels Intensity  W m -2 dB Threshold of pain10 6 120 Thunder10 5 110 Jet engine10 4 100 Vacuum cleaner160 Conversation10 -2 40 Rusting leaves10 -4 20 Threshold of hearing at 1 000 Hz 10 -6 0

8. 7 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 175) Intensity level and loudness 1. Intensity in decibels Threshold of hearing - minimum intensity of sound that normal human ear can hear Threshold of pain - sound of intensity 1 W m -2 or higher which causes pain to our ears Go to Common Error

9. 8 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 175) Intensity level and loudness 2. Loudness of sound - Human ear responds to sound over 120 dB - Ear is not equally sensitive to all frequencies Go to Example 2 Example 2 Go to Example 3 Example 3

10. 9 © Manhattan Press (H.K.) Ltd. End

11. 10 © Manhattan Press (H.K.) Ltd. Ultrasonics in nature Although ultrasonics is out of the audible range of human beings, it can be heard by some animals. For example, bats and dolphins use ultrasonics of frequencies 20 kHz and 150 kHz respectively for communicating. Return to Text 11.2 Properties of sound (SB p. 172)

12. 11 © Manhattan Press (H.K.) Ltd. Infrasonics in nature Some animals such as elephants, rhinos and whales can hear infrasonics. It is believed that this ability helps them to have an early signal about earthquake and other weather disturbances. Return to Text 11.2 Properties of sound (SB p. 173)

13. 12 © Manhattan Press (H.K.) Ltd. Audible intensity range Different people have different ranges of audible frequencies. The range usually decreases with age. Return to Text 11.2 Properties of sound (SB p. 174)

14. 13 © Manhattan Press (H.K.) Ltd. 11.2 Properties of sound (SB p. 175) A sound of 0 dB does not mean that there is no sound anymore. It is the minimum sound that the human ears can detect and is equivalent to the threshold of hearing. Return to Text

15. 14 © Manhattan Press (H.K.) Ltd. Q: Q: A small source of sound emits energy uniformly in all directions. For a particular frequency, the intensity of sound at a distance of 1.0 m from the source is 1.0 × 10 −5 W m −2, and corresponds to an amplitude of oscillation of the air molecules of 70  m. Assuming sound is propagated without any loss of energy, find (a) the intensity of sound, (b) the amplitude of oscillation of the air molecules at a distance of 5.0 m from the source. Solution 11.2 Properties of sound (SB p. 176)

16. 15 © Manhattan Press (H.K.) Ltd. Solution: Return to Text 11.2 Properties of sound (SB p. 176)

17. 16 © Manhattan Press (H.K.) Ltd. Q: Q: A ship’s siren vibrates with displacement y where y = A sin200  t (a) This sound causes the diaphragm of an eardrum of an observer 400 m away to vibrate. If the speed of sound is 340 m s −1, calculate (i) the frequency of the sound, (ii) the number of wavelengths of this sound between the siren and the eardrum, (iii) the phase difference between the vibrations of the siren and the eardrum, (iv) the maximum energy of the eardrum if its mass is 1.00 × 10 −5 kg and the amplitude of its vibration is 1.00 × 10 −8 m. (b) If the value of the speed and distance given were reliable only to 3 significant figures, comment on the confidence that could be placed in your answers to (a)(iii). Solution 11.2 Properties of sound (SB p. 177)

18. 17 © Manhattan Press (H.K.) Ltd. Solution: 11.2 Properties of sound (SB p. 177) (a) (i) Comparing the equation y = A sin200  t with the wave equation y = A sin  t = A sin2  ft 2  ft = 200  t ∴ Frequency (f ) = 100 Hz (b) If the value of the speed and distance were reliable to 3 significant figures, then the answer to (a)(ii) would be 118 and the phase difference in (a)(iii) would be zero. Return to Text