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

3. 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”.

4. 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.

5. 24.1 The loudness of sound
The loudness of a sound is measured in decibels (dB).

6. 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.

7. 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.

8. 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.

9. 24.1 The speed of sound
Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr.

10. 24.1 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.

12. 24.1 Recording sound
One second of compact-disc-quality sound is a list of 44,100 numbers which represents the amplitudes converted sounds.

13. 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.

14. 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.

15. 24.2 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.

16. 24.2 What is a sound wave?
At the same temperature and volume, higher pressure contains more molecules than lower pressure.

18. 24.2 The wavelength of sound
The wavelength of sound in air is similar to the size of everyday objects.

19. 24.2 The wavelength of sound
Wavelength is also important to sound.
Musical instruments use the wavelength of a sound to create different frequencies.

20. 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.

21. 24.2 Standing waves
The lowest natural frequency is called the fundamental.
A vibrating string also has other natural frequencies called harmonics.

22. 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.

23. 24.2 Standing waves
It is easy to measure the wavelength of a standing wave on a string.
Two harmonics equals one wave!

24. 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.

25. 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?

26. 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).

27. 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.

29. 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.

31. 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.

32. 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.

33. 24.3 Sound perception and music
A frequency spectrum shows the amplitudes of different frequencies present in a sound.

34. 24.3 Sonograms
More information is found in a sonogram which combines three sound variables:
frequency,
time, and
amplitude (loudness).

35. 24.3 Sonograms Which letter represents a soft sound lasting 5 seconds?
What is it’s frequency?

36. 24.3 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.

38. 24.3 Sound protection
Listening to loud sounds for a long time causes the hairs on the nerves in the cochlea to weaken or break off resulting in permanent damage.

39. 24.3 Music
The pitch of a sound is how high or low we hear its frequency.
Rhythm is a regular time pattern in a series of sounds.
Music is a combination of sound and rhythm that we find pleasant.

40. 24.3 The musical scale
Most of the music you listen to is created from a pattern of frequencies called a musical scale.

41. 24.3 Music and notes Each frequency in the scale is called a note.
The C major musical scale that starts on the note C (262 Hz).

42. 24.3 Music and harmony
Harmony is the study of how sounds work together to create effects desired by the composer.
The tense, dramatic sound track of a horror movie is a vital part of the audience’s experience.
Harmony is based on the frequency relationships of the musical scale.

43. 24.3 Superposition
The superposition principle states that when sound waves occur at the same time they combine to make a complex wave.
When two frequencies of sound are not exactly equal in value, the loudness of the total sound seems to oscillate or beat.

44. 24.3 Music and harmony
When we hear more than one frequency of sound and the combination sounds pleasant, we call it consonance.
When the combination sounds unsettling, we call it dissonance.

45. 24.3 Making sounds
The human voice is complex sound that starts in the larynx, at the top of your windpipe.
The sound is changed by passing over by expandable folds (vocal cords) and through openings in the throat and mouth.

46. 24.3 Making sounds
For a guitar in standard tuning, the heaviest string has a natural frequency of 82 Hz and the lightest a frequency of 330 Hz.
Tightening a string raises its natural frequency and loosening lowers it.

47. 24.3 Harmonics and music
The same note sounds different when played on different instruments.
Suppose you compare the note C (262 Hz) played on a guitar and the same note played on a piano.
The variation comes from the harmonics in complex sound.
A single C note from a grand piano might include 20 or more different harmonics.

48. 24.3 Harmonics and music
A tuning fork is a useful tool for tuning an instrument because it produces a single frequency