SOUND

1. SOUND

2. Chapter Twenty-Four: Sound • 24.1 Properties of Sound • 24.2 Sound Waves • 24.3 Sound Perception and Music

3. Chapter 24.1 Learning Goals • Describe properties of sound. • Explain how sound waves are created and recorded. • Discuss examples of the Doppler effect.

4. Key Question: Does sound behave like other waves? Investigation 24A Properties of Sound

5. 24.1 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”.

6. 24.1 The frequency of sound • Almost all the sounds you hear contain many frequencies at the same time. • Humans can generally hear frequencies between 20 Hz and 20,000 Hz.

7. 24.1 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.

8. 24.1 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. 24.1 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 25 dB sound at 1,000 Hz sounds just as loud as an 40 dB sound at 100 Hz.

10. 24.1 Amplitude and decibels • The amplitude of a sound increases ten times every 20-decibels.

12. 24.1 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.

13. 24.1 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.

14. 24.1 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.

15. 24.1 The speed of sound • Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr.

16. 24.1 The Doppler effect • When the object is moving, the frequency will notbe 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.

18. 24.1 Recording sound To record a sound you must store the pattern of vibrations in a way that can be replayed and be true to the original sound. • A microphone transforms a sound wave into an electrical signal with the same pattern of vibration.

19. 24.1 Recording sound • An “analog to digital converter” converts the electrical signal to digital values between 0 and 65,536.

20. 24.1 Recording sound • One second of compact-disc-quality sound is a list of 44,100 numbers which represents the amplitudes converted sounds.

21. 24.1 Recording sound • To play the sound back, the string of numbers is read by a laser and converted into electrical signals again by a second circuit which reverses the process of the previous circuit.

22. 24.1 Recording sound • The playback circuit converts the string of numbers back into an electrical signal. • The electrical signal is amplified to move the coil in a speaker and reproduce the sound.

23. 24.1 Recording sound • Most of the music you listen to has been recorded in stereo. • The slight differences in how sound reaches your ears lets you know where sound is coming from.

24. Chapter Twenty-Four: Sound • 24.1 Properties of Sound • 24.2 Sound Waves • 24.3 Sound Perception and Music

25. Chapter 24.2 Learning Goals • Justify the classification of sound as a wave. • Analyze sound interactions at boundaries. • Explain how factors like temperature and pressure affect the behavior of sound waves.

26. Key Question: How can resonance be controled to make the sounds we want? Investigation 24B Resonance in Other Systems

27. 24.2 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.

28. 24.2 What is a sound wave? • At the same temperature and volume, higher pressure contains more molecules than lower pressure.

30. 24.2 The wavelength of sound • The wavelength of sound in air is similar to the size of everyday objects.

32. 24.2 The wavelength of sound • Wavelength is also important to sound. • Musical instruments use the wavelength of a sound to create different frequencies.

33. 24.2 Standing waves • A wave that is confined in a space is called a standing wave. • A string with a standing wave is a kind of oscillator.

34. 24.2 Standing waves • The lowest natural frequency is called the fundamental. • A vibrating string also has other natural frequencies called harmonics.

35. 24.2 Standing waves • The place on a harmonic with the greatest amplitude is the antinode. • The place where the string does not move (least amplitude) is called a node.

36. 24.2 Standing waves • It is easy to measure the wavelength of a standing wave on a string. • Two harmonics equals one wave!

37. 24.2 Standing waves in pipes • A panpipe makes music as sound resonates in tubes of different lengths. • The natural frequency of a pipe is proportional to its length.

38. 24.2 Standing waves in pipes • Because frequency and wavelength are inversely related, longer pipes have lower natural frequencies because they resonate at longer wavelengths. • A pipe that must vibrate at a frequency 2 times higher than another pipe must be 1/2 as long. If the long pipe has a frequency of 528 Hz, what is the frequency of the short pipe?

39. 24.2 Standing waves in pipes • Blowing across the open end of a tube creates a standing wave inside the tube. • If we blow at just the right angle and we match the natural frequency of the material and the sound resonates (spreads).

40. 24.2 Standing waves in pipes • The open end of a pipe is an open boundary to a standing wave and makes an antinode. • The pipe resonates to a certain frequency when its length is one-fourth the wavelength of that frequency.

42. 24.2 Sound wave interactions • Like other waves, sound waves can be reflected by hard surfaces and refracted as they pass from one material to another. • Diffraction causes sound waves to spread out through small openings. • Carpet and soft materials can absorb sound waves.

44. 24.2 Reverberation • The reflected sound and direct sound from the musicians together create a multiple echo called reverberation. • The right amount of reverberation makes the sound seem livelier and richer.

45. Chapter Twenty-Four: Sound • 24.1 Properties of Sound • 24.2 Sound Waves • 24.3 Sound Perception and Music

46. Chapter 24.3 Learning Goals • Explore how the brain makes meaning of sounds. • Describe how humans hear sounds. • Explain the sound is used to create music.

47. 24.3 Sound perception and music • When you hear a sound, the nerves in your ear respond to more than 15,000 different frequencies at once. • The brain makes sense of complex sound because the ear separates the sound into different frequencies.

48. 24.3 Sound perception and music • A frequency spectrum shows the amplitudes of different frequencies present in a sound.

49. 24.3 Sonograms • More information is found in a sonogram which combines three sound variables: • frequency, • time, and • amplitude (loudness).

50. 24.3 Sonograms Which letter represents a soft sound lasting 5 seconds? What is it’s frequency?