# Electricity, Sound and Light Chapter Nine: Waves and Sound 9.1 Harmonic Motion 9.2 Waves 9.3 Sound.

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Electricity, Sound and Light

Chapter Nine: Waves and Sound 9.1 Harmonic Motion 9.2 Waves 9.3 Sound

9.3 The frequency of sound The pitch of a sound is how you hear and interpret its frequency. A low-frequency sound has a low pitch. A high-frequency sound has a high pitch. Each person is saying “Hello”.

9.3 The frequency of sound Humans can generally hear frequencies between 20 Hz and 20,000 Hz. Most sound has more than one frequency. Almost all the sounds you hear contain many frequencies at the same time.

9.3 The loudness of sound The loudness of a sound is measured in decibels (dB). The decibel is a unit used to express relative differences in the loudness of sounds.

9.3 The loudness of sound Most sounds fall between 0 and 100 on the decibel scale, making it a very convenient number to understand and use.

9.3 The frequency of sound Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. According to this curve, a 40 dB sound at 2,000 Hz sounds just as loud as an 80 dB sound at 50 Hz.

9.3 The speed of sound The speed of sound in normal air is 343 meters per second (660 miles per hour). Sound travels through most liquids and solids faster than through air. Sound travels about five times faster in water, and about 18 times faster in steel.

9.3 The speed of sound Objects that move faster than sound are called supersonic. If you were on the ground watching a supersonic plane fly toward you, there would be silence. The sound would be behind the plane, racing to catch up.

9.3 The speed of sound A supersonic jet “squishes” the sound waves so that a cone-shaped shock wave forms where the waves “pile up” ahead of the plane. In front of the shock wave there is total silence. Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr.

9.3 The Doppler effect When the object is moving, the frequency will not be the same to all listeners. The shift in frequency caused by motion is called the Doppler effect. You hear the Doppler effect when you hear a police or fire siren coming toward you, then going away from you.

9.3 What is a sound wave? Sound waves are pressure waves with alternating high and low pressure regions. When they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure.

9.3 What is a sound wave? At the same temperature, higher pressure contains more molecules per unit of volume than lower pressure.

9.3 The wavelength of sound The wavelength of sound in air is similar to the size of everyday objects.

9.3 The wavelength of sound We usually think about different sounds in terms of frequency, but wavelength is also important. Musical instruments use the wavelength of a sound to create different frequencies.

9.3 How we hear sound The parts of the ear work together: 1.When the eardrum vibrates, three small bones transmit the vibrations to the cochlea. 2.The vibrations make waves inside the cochlea, which vibrates nerves in the spiral. 3.Each part of the spiral is sensitive to a different frequency.

Technology Connection Sound All Around From experience you know that a cafeteria and gym are often loud places, and libraries are quiet. How should we design these spaces and what types of materials should we use in them?

Activity Almost all speakers contain magnets and coils of wire that interact to create mechanical vibrations out of pulsating electric currents. You can make your own speaker to hear music. Make Your Own Speaker

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