Electricity, Sound and Light

2. Electricity, Sound and Light

3. Chapter Nine: Waves and Sound • 9.1 Harmonic Motion • 9.2 Waves • 9.3 Sound

4. 9.3 The frequency of sound • The pitchof 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”.

5. 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.

6. 9.3 The loudness of sound • The loudnessof a sound is measured in decibels (dB). • The decibelis a unit used to express relative differences in the loudness of sounds.

7. 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.

8. 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. 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.

10. 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.

11. 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 subsonicbecause they travel at speeds from 400 to 500 mi/hr.

12. 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.

14. 9.3 What is a sound wave? • Sound waves are pressure waveswith 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.

16. 9.3 What is a sound wave? • At the same temperature, higher pressure contains more molecules per unit of volume than lower pressure.

17. 9.3 The wavelength of sound • The wavelength of sound in air is similar to the size of everyday objects.

18. 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.

19. 9.3 How we hear sound • The parts of the ear work together: • When the eardrum vibrates, three small bones transmit the vibrations to the cochlea. • The vibrations make waves inside the cochlea, which vibrates nerves in the spiral. • Each part of the spiral is sensitive to a different frequency.

21. 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?

22. Activity Make Your Own Speaker • 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.