1.  Sound

2. What is sound?  Any object vibrating in air creates sound waves by moving air molecules back and forth.  Sound waves are longitudinal and are composed of compressions and rarefactions.

3. Speakers  An alternating current is supplied to the coil.  This induces an alternating magnetic field which causes a force on the magnet.  The moving magnet causes the cone to vibrate and make sound.

4. Speakers  If we increase the frequency of the alternating current, we increase the frequency of the sound.

5. Properties of Sound  Sound waves travel through solids, liquids and gases. They cannot travel in a vacuum.  Sound waves travel faster through solids because the molecules are closer together.

6. Echoes  A sound echo occurs when a sound wave is reflected off of a hard, smooth surface.  To prevent echoes you can cover walls with a soft material that is uneven (like in music studios).

7. Summary  Sound waves are produced when a vibrating surface pushes and pulls on the surrounding substance.  Sound waves are longitudinal with alternating compressions and rarefactions.  Sound travels through solids, liquids and gases but not vacuums.  An echo is due to sound reflecting off of a smooth, hard surface.

8.  Speed of Sound

9.  The speed of sound depends on the substance it is traveling through.  In general, sound travels faster in solids and liquids.

10. Speed of Sound  The speed of sound in air is approximately 340 m/s.  The speed of sound in water is approximately 1400 m/s.

11. Example 1  A person is standing in front of a large wall. They clap their hands and hear an echo 2 seconds later. How far away is the wall?

12. Example 2  Dolphins use echo-location to find fish. A dolphin sends a sound wave and hears the echo 1.3 seconds later. How far away is the fish?

13. Summary  The speed of sounds depends on the substance and the temperature of the substance.  The speed of sound in air is about 340 m/s.  The speed of sound in water is about 1400 m/s.

14.  Musical Sounds

15.  Musical notes are sound waves that change smoothly and have a pattern that repeats.  Noise is sound waves that change randomly without any pattern.

16. Instruments  Musical instruments, including your voice, create sound waves by vibrating air.  At certain frequencies, the vibrations become stronger. The instrument resonates at these frequencies.

17. Waveforms  Sound waves are longitudinal but can be converted into transverse waveforms.

18. Waveforms  The loudness of a sound changes the amplitude of the waveform. A large amplitude means a loud sound.  The pitch

19. Summary  Instruments produce sound by vibrating the air around it.  The loudness of a note increases the amplitude of the wave.  The pitch of the note increases the frequency.