11.5 How do we hear?

Put two fingers gently on your throat while you speak. What do you feel?

How is sound produced? Sound is produced by vibrations ( 振動 ). p.90

- All sound sources, like loudspeakers, tuning forks, guitars and flutes, have the parts that vibrate.

The loudspeaker vibrates,so the balls move up and down. The tuning fork vibrates, so ripples form.

- When the tuning fork vibrates, => => the surrounding air particles vibrate. p.91 => => the vibrations are transmitted from one place to another. sound wave => => the relevant parts of the ear vibrate. - When the vibrations reach the ear, => => sound is heard. How is sound transmitted?

Musical instruments produce vibrations - Musical instruments produce vibrations in the air. These vibrations are in the air. These vibrations are transmitted to our ears. transmitted to our ears.

p.94 Ear - divided into three parts outer ear middle ear inner ear tube leading to throat with a tube leading to throat full of fluid

Structure of ear outer ear ear canal ear flap eardrum cochleacochlea auditory nerve inner ear ear bones middle ear What are the functions of different parts?

Activity Corner H p.94 How does the ear work? Read the sentences on the next page. Arrange them in an order which best describes the working of the ear.

A The messages are sent along the auditory nerve to the brain. The brain interprets these messages as sounds. B The three ear bones magnify the vibrations in the eardrum and pass them to the inner ear. C The ear flap collects sound vibrations in the air. D The eardrum vibrates when sound vibrations hit it. F The sound vibrations travel along the ear canal to the eardrum. E The liquid in the inner ear vibrates. The sensory cells inside the cochlea detect the vibrations and change them into messages. Order : C  __  __  __  __  A FDBE

Ear bones - Magnify the vibrations from the eardrum by 50 times. - The smallest bones in human body. Among them, the smallest one is 3 mm long. only 3 mm long.

Have you experienced fullness in the ears when descending in an aircraft or a lift? p.95 This is because of the increasing pressure pushing the eardrum inwards.

NormalNormalDescendingDescending P A = atmospheric pressure P M = pressure inside middle ear You can ease the fullness simply by swallowing. This equalizes the air pressure at both sides of the eardrum.

Put a burning candle in front of the loudspeaker, what will happen to the candle flame?

The candle flame moves back and forth.

The paper disc of the loudspeaker vibrates, so the candle flame moves in the same direction as the surrounding air. - Sound cannot be transmitted if there is no medium. e.g. air

Why can we hear the noise coming from construction works in the flat works in the flatnearby?

p.92 How can sound be transmitted? - Sound can travel through solids, liquids and gases.

In ancient times, a soldier placed his leather arrow sheath on the ground to hear the sounds of horses of the enemies. Shen Kuo ( 沈括 ) in the Northern Song period gave a correct scientific explanation of this.

- Divers under water can hear the sounds of motorboats. - It is believed that dolphins make sounds and ‘talk’ to each other under water.

Can sound travel through a vacuum? to electricity supply electric bell to vacuum

- There is no air in outer space to transmit the sound. Astronauts can only talk to each other using radio waves.

Sound travels at different speeds in different media. air 330 water 1430 medium speed (m/s) steel 5030 Can you explain this using the particle theory? p.93

Lightning always come before thunderstorm. Do you know why?

The best toy telephone p.93 Lily is not satisfied with the sound quality of the toy telephone she used in Experiment Centre She wants to make a better one! Little Scientist

Below are some of her questions: 1 1 Is it better to use a thick or thin string? 2 2 What material should be used to make the cups? 3 3 Does the size of the cups make a difference? 4 4 Does the length of the string joining the cups make a difference?

What are your answers to Lily’s questions? What are your hypotheses? Design your investigations. Carry them out and use your findings to make the best toy telephone. Design your investigations. Carry them out and use your findings to make the best toy telephone.