11 6 Limitations of our ears Limitations of

11. 6 Limitations of our ears

Limitations of our ears 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?

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

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

- 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

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

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

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

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

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

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

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

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

- Different animals have different p. 98 ranges of hearing frequency (in hertz) human dog cat dolphin grasshopper bat Those animals having a wider hearing range have a bigger chance to survive in the wild.

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

e. g. - check the status of a foetus in the Ultrasound has many applications. mother’s uterus 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.

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

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