11.611.6

Limitations ofLimitations of

our earsour ears

p.96p.96Limitations of our earsLimitations of our ears

- Human ears cannot hear all sounds in the surroundings. We can only hear sounds within a certain range of frequencies.

I I cannot hear cannot hear the sound the sound ofof the the

dog whistledog whistle. . WhyWhy??

p.96p.96

=> => The number of vibrations per second per second

- e.g. the tuning fork vibrates 100 times per second,

FrequencyFrequency ((頻率頻率 ))FrequencyFrequency ((頻率頻率 ))

- Unit: hertz (赫茲 ), written as Hz.

Its frequency = Its frequency = 100 Hz100 HzIts frequency = Its frequency = 100 Hz100 Hz

- Each tuning fork has a number which indicates its frequencyfrequency.

This tuning fork can produce sounds of frequency at 512 Hz512 Hz.=>=> How many times does it vibrate per secondper second?

- The faster an object vibrates,=> the higher the frequency,=> the higher the pitch of the sound it produces.

vibrations of air vibrations of air particles particles when usingwhen using

a tuning fork with a tuning fork with higher frequency higher frequency

vibrations of air vibrations of air particles particles when usingwhen using

a tuning fork with a tuning fork with higher frequency higher frequency

vibrations of air vibrations of air particles particles when using when using

a tuning fork with a tuning fork with lower frequency lower frequency

vibrations of air vibrations of air particles particles when using when using

a tuning fork with a tuning fork with lower frequency lower frequency

Which of the following cases will produce higher pitch of sound?

呀呀 唉唉 !!AhAh!! OhOh

!!

The table shows the frequencies of eight notes in a in a

musical scalemusical scale:

C doh 256

Note Frequency

D ray 288

E me 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 is now commonly used in used in transmitting transmitting sound sound and

information of imagesinformation of images.

The unit, hertz, is also usedin radio broadcast.

Different people have different ranges of hearing.

Our hearing range will become narrower when we are getting older.

????

p.97p.97

e.g. The upper limit of hearing of young people is usually higher than that of old peopleold people..

????cancan magnify magnify sound sound

waveswaves, , help help the the elders to hear elders to hear

clearlyclearly

cancan magnify magnify sound sound waveswaves, , help help the the elders to hear elders to hear

clearlyclearly

Many elders cannot hear low voice clearly.Hearing aidHearing aid ( 助聽器助聽器 ) can be used

to help them.

What What problemsproblems do dothethe elders elders havehavein in their earstheir ears??

What What problemsproblems do dothethe elders elders havehavein in their earstheir ears??

- Sensory cells inside cochlea cochlea deteriorate.

- The ear bonesThe ear bones deteriorate. They They cannot amplify the sound waves to the hearing level.

Why does Why does the hearing ofthe hearing of the elders the elders become weakerbecome weaker??Why does Why does the hearing ofthe hearing of the elders the elders become weakerbecome weaker??

This is because they hearthey hear some sounds which are outside our hearing range.

Sometimes we can hear the dogs the dogs bark or the cats the cats mew severelyseverely without a reason.

p.98p.98- Different animals have different ranges of hearingranges of hearing.

Those animals having aa wider wider hearingrange have a bigger chance to survivein the wild.

frequency (in hertz)

human

dog cat dolphin

grass-hoppe

r

bat

Do you know which animals can produceultrasoundultrasound??

Sound which have frequencies too high for the human ear to hear is called ultrasound.

batbat dolphindolphin

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

When an ultrasound transmitter is movedover the mother’s body, ultrasound signals

reflected from different partsof the foetusof the foetus are processed which

then generate an image.

- Ships have devices to detect reflected reflected ultrasound signals ultrasound signals in order to measure thethe depth of the seadepth of the sea, or to locate the positionthe position of fish groupsof fish groups.

ultrasoundultrasound

fishfishgroupgroup

reflectedreflectedsignalsignal

fishingfishingnetnet

- Ultrasound can be used to clean small ansmall and d sophisticated sophisticated or easily brokeneasily broken objects ((e.g. jewelleries and eyeglasses)).

Put the objects into the Put the objects into the ultrasonic ultrasonic bathbath. . DirtDirt on the objects can be on the objects can be washed awaywashed away by the by the vibrations of vibrations of liquidliquid inside the bath. inside the bath.