Sound and Hearing Click to move on Mike Turner, Apr. 2004

How is sound made? Sound is made whenever anything vibrates. For example, the strings of a guitar are made to vibrate when they are plucked, and a drumskin vibrates when it is struck by a drumstick. The chords in our voicebox vibrate when we speak – you can feel the vibrations if you press your fingers to your throat as you talk. The cones of a loudspeaker are made to vibrate by an electric current. Click to move on Mike Turner, Apr. 2004

How does sound travel? Sound travels as a wave. Whenever an object vibrates, it makes the air around it vibrate. This spreads the sound out. These vibrations, which we call waves, travel very quickly in air - 330 m/s, which is about one kilometre every three seconds. This is faster than a jet airliner. Sound waves can also pass through liquids and solids. Evidence for this is that sound passes through walls, and whales and dolphins communicate under water using sound. The speed of sound increases with the density of the material it is passing through. It travels at about 1,500 m/s in water, and can reach a speed of 5,000 m/s in steel. Click to move on Mike Turner, Apr. 2004

Looking at Sound We can look at sounds if we use a microphone and an oscilloscope. This lets us see the shape of the sound wave. Musical sounds usually have quite a smooth shape. The distance from the centre to the top of the wave is called the amplitude. The wider the wave is, the lower its frequency. amplitude Click to move on Mike Turner, Apr. 2004

Controlling Waves - Frequency The pitch of a sound depends on its frequency. The frequency is a measure of how fast the wave is vibrating. The faster the vibration, the higher the frequency, and the higher the pitch. Frequency is measured in hertz (Hz). One hertz is one wave per second. Frequency is sometimes measured in kilohertz (kHz), where 1 kHz is 1,000 Hz, or in megahertz (MHz), where 1 M Hz is 1,000,000 Hz. The human ear can only hear frequencies from about 20 Hz up to about 20,000 Hz. This is called the audible range. As you get older, the highest frequency you can hear decreases. Hearing damage also often leads to a loss of higher frequency hearing. Low frequency, low pitch High frequency, high pitch Click to move on Mike Turner, Apr. 2004

Controlling Waves - Loudness The loudness, or volume, of a sound wave depends on the amplitude. The bigger the amplitude, the louder the sound. Large amplitude, loud sound Low amplitude, quiet sound Click to move on Mike Turner, Apr. 2004

Detecting Sounds – The Ear Our ears detect sounds. There are several parts to the ear. We usually talk about the outer ear, the middle ear and the inner ear. Outer Ear This is called the pinna, it collects sound waves and directs them into the ear. Inner Ear. This consists of the cochlea and the auditory nerves. The cochlea is a coiled tune filled with liquid. When the liquid vibrates, tiny hairs inside the cochlea vibrate and send electrical signals along the auditory nerves to the brain, which we then perceive as sound. Middle Ear This consists of the eardrum and the three smallest bones in the body. The eardrum detects the sound, and the bones pass on the vibration to the cochlea. Click to move on Mike Turner, Apr. 2004

How the Ear Works Click to move on Mike Turner, Apr. 2004

What can go wrong? The ear is a very delicate part of the body, and can be damaged if we are not careful. The most easily damaged parts are : The eardrum, which can be burst by sudden loud noises The hairs in the cochlea, which are damaged by loud sounds, or even by sounds that do not seem to loud, if we are exposed to them over a long period. Loudness is measured in decibels (dB). A quiet room has a sound level of 50 to 60 dB. Sounds above 90 dB start to have an effect on our ears. The louder a sound is, the shorter the time you are allowed to be exposed to it. In the UK, workers are not allowed to work in a sound level of 100 dB for more than 40 minutes. Some personal stereos can reach a level of 100 dB. Click to move on Mike Turner, Apr. 2004

Using Sound We use sound for many different things. One of the most useful is to find distances. If we see a flash of lightning, there is usually a delay before we hear the thunder. This is because light travels a lot faster than sound. In fact, light travels at 300,000,000 m/s, nearly 1 million times faster than sound. If there is a delay of 10 seconds between seeing the flash and hearing the thunder, how far away is the storm? (Speed of sound in air = 330 m/s). Distance = speed x time = 330 x 10 = 3,300 m The storm is 3,300 metres away. Click to move on Mike Turner, Apr. 2004

Using Sound (2) A ship sends out a sound signal. It receives an echo from the sea bed 0.2 s later. How deep is the sea? (speed of sound in water =1,500 m/s) Distance = speed x time = 1,500 x 0.2 = 300 m This is the distance to the sea bed and back again. So the depth of the sea is half this distance. The sea is 150 metres deep. Click to finish Mike Turner, Apr. 2004

Thank you for looking at this work, and good luck in your test. You have now reached the end of the revision presentation for the ‘Sound and Hearing’ topic. I hope you have found it useful. If you have any ideas to improve this presentation please let Mr. Turner know. Thank you for looking at this work, and good luck in your test. Click to finish Mike Turner, Apr. 2004