2. 11.6 Limitations of our ears

3. p.96 - Human ears cannot hear all sounds in the surroundings. We can only hear sounds within a certain range of frequencies. I cannot hear the sound of the dog whistle. Why?

4. p.96 => per second => The number of vibrations per second - e.g. the tuning fork vibrates 100 times per second, Frequency ( 頻率 ) - Unit: hertz ( 赫茲 ), written as Hz. Its frequency = 100 Hz

5. frequency - Each tuning fork has a number which indicates its frequency. 512 Hz => This tuning fork can produce sounds of frequency at 512 Hz. => How many times does it vibrate per second per second?

6. - The faster an object vibrates, => the higher the frequency, => the higher the pitch of the sound it produces. vibrations of air particles when using a tuning fork with higher frequency vibrations of air particles when using a tuning fork with lower frequency

7. Which of the following cases will produce higher pitch of sound? 呀 唉!唉!唉!唉! Ah! Oh!

8. in a musical scale The table shows the frequencies of eight notes in a musical scale: C doh 256 NoteFrequency D ray 288 E me 320 F fah 340 G soh 384 A lah 427 B te 480 C’ doh’ 512

9. The unit of frequency is named after a German scientist, Hertz. He first invented the method to produce and receive radio waves in 1888.

10. Radio wave ( 無線電波 ) i ii is now commonly used in transmitting sound and information of images. The unit, hertz, is also used in radio broadcast.

11. Different people have different ranges of hearing. Our hearing range will become narrower when we are getting older.?? p.97 old people. e.g. The upper limit of hearing of young people is usually higher than that of old people.

12. ?? can magnify sound waves, help the elders to hear clearly Many elders cannot hear low voice clearly. Hearing aid (助聽器 ) can be used to help them. What problems do the elders have in their ears?

13. cochlea - Sensory cells inside cochlea deteriorate. The ear bonesThey - The ear bones deteriorate. They cannot amplify the sound waves to the hearing level. Why does the hearing of the elders become weaker?

14. they hear This is because they hear some sounds which are outside our hearing range. the dogs the cats severely Sometimes we can hear the dogs bark or the cats mew severely without a reason.

15. p.98 ranges of hearing - Different animals have different ranges of hearing. a wider Those animals having a wider hearing range have a bigger chance to survive in the wild. frequency (in hertz) humandogcatdolphingrass- hopper bat

16. ultrasound? Do you know which animals can produce ultrasound? Sound which have frequencies too high for the human ear to hear is called ultrasound. bat dolphin

17. Ultrasound Ultrasound has many applications. check e.g.- check the status of a foetus in the mother’s uterus of the foetus When an ultrasound transmitter is moved over the mother’s body, ultrasound signals reflected from different parts of the foetus are processed which then generate an image.

18. reflected ultrasound signals the - Ships have devices to detect reflected ultrasound signals in order to measure the depth of the seathe position depth of the sea, or to locate the position of fish groups of fish groups. ultrasound fish group reflected signal fishing net

19. - Ultrasound can be used to clean s ss small and sophisticated or e ee easily broken objects ( e.g. jewelleries and eyeglasses). Put the objects into the ultrasonic bath. Dirt on the objects can be washed away by the vibrations of liquid inside the bath.