2. Chapter 15 Sound 15.1 Properties of Sound 15.2 Sound Waves
15.3 Sound, Perception, and Music 2

3. Chapter 15 Objectives
Explain how the pitch, loudness, and speed of sound are related to properties of waves.
Describe how sound is created and recorded.
Give examples of refraction, diffraction, absorption, and reflection of sound waves.
Explain the Doppler effect.
Give a practical example of resonance with sound waves.
Explain the relationship between the superposition principle and Fourier’s theorem.
Describe how the meaning of sound is related to frequency and time.
Describe the musical scale, consonance, dissonance, and beats in terms of sound waves. 3

4. Chapter 15 Vocabulary Terms
acoustics
beats
cochlea
consonance
decibel
dissonance
Doppler effect
Fourier’s theorem
frequency
spectrum
microphone
musical scale
note
octave
pitch
pressure
reverberation
rhythm
shock wave
sonogram
speaker
stereo
subsonic
supersonic

5. Inv 15.1 Properties of Sound
Key Question:
What is sound and how do we hear it? 5

6. 15.1 Properties of Sound
If you could see atoms, the difference between high and low pressure is not as great.
The image below is exaggerated to show effect.

7. 15.2 The frequency of sound
We hear frequencies of sound as having different pitch.
A low frequency sound has a low pitch, like the rumble of a big truck.
A high-frequency sound has a high pitch, like a whistle or siren.
In speech, women have higher fundamental frequencies than men.

8. 15.1 Complex sound
When we hear complex sounds, the nerves in the ear respond separately to each
different frequency. The brain interprets the signals from the ear and creates a
“sonic image” from the frequencies. The meaning in different sounds is derived
from the patterns in how the different frequencies get louder and softer. 8

9. Common Sounds and their Loudness

10. 15.1 Loudness
Logarithmic scale
Linear scale
Decibels (dB)
Amplitude 1 20 10 40
100 60
1,000 80
10,000
100,000
120
1,000,000
Every increase of 20 dB, means the pressure wave is 10 times greater in amplitude.

11. 15.1 Sensitivity of the ear
How we hear the loudness of sound is affected by the frequency of the sound as well as by the amplitude.
The human ear is most sensitive to sounds between 300 and 3,000 Hz.
The ear is less sensitive to sounds outside this range.
Most of the frequencies that make up speech are between 300 and 3,000 Hz.
The Equal Loudness Curve on the right shows how sounds of different frequencies
compare. Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same
decibel level.
For example, the Equal Loudness Curve shows that a 40 dB sound at 2,000 Hz
sounds just as loud as an 80 dB sound at 50 Hz. 11

12. 15.1 How sound is created
The human voice is a complex sound that starts in the larynx, a small structure at the top of your windpipe.
The sound that starts in the larynx is changed by passing through openings in the throat and mouth.
Different sounds are made by changing both the vibrations in the larynx and the shape of the openings.
The Equal Loudness Curve on the right shows how sounds of different frequencies
compare. Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same
decibel level.
For example, the Equal Loudness Curve shows that a 40 dB sound at 2,000 Hz
sounds just as loud as an 80 dB sound at 50 Hz. 12

13. 15.1 How sound is created
A speaker is a device that is specially designed to reproduce sounds accurately.
The working parts of a typical speaker include a magnet, a coil of wire, and a cone.

14. 15.1 Recording sound
A common way to record sound starts with a microphone.
The microphone transforms a sound wave into an electrical signal with the same pattern of oscillation.
In modern digital recording, a sensitive circuit converts analog sounds to digital values between 0 and 65,536.

15. 15.1 Recording sound
Numbers correspond to the amplitude of the signal and are recorded as data. One second of compact- disk-quality sound is a list of 44,100 numbers.

16. 15.1 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.

17. 15.1 Recording sound
The electrical signal is amplified until it is powerful enough to move the coil in a speaker and reproduce the sound.

18. Chapter 15 Sound 15.1 Properties of Sound 15.2 Sound Waves
15.3 Sound, Perception, and Music 18

19. Inv 15.2 Sound Waves Investigation Key Question:
Does sound behave like other waves? 19

20. 15.2 Sound Waves We know sound is a wave because:
Sound has both frequency and wavelength.
The speed of sound is frequency times wavelength.
Resonance happens with sound.
Sound can be reflected, refracted, and absorbed and also shows evidence of interference and diffraction.

21. 15.2 Sound Waves
A sound wave is a wave of alternating high- pressure and low-pressure regions of air.

22. 15.2 Amplitude of sound The amplitude of a sound wave is very small.
Even a loud 80 dB noise creates a pressure variation of only a few millionths of an atmosphere.

23. 15.2 The wavelength of sound

24. 15.2 The Doppler effect
The shift in frequency caused by motion is called the Doppler effect.
It occurs when a sound source is moving at speeds less than the speed of sound.

25. 15.2 The speed of sound
The speed of sound in air is 343 meters per second (660 miles per hour) at one atmosphere of pressure and room temperature (21°C).
An object is subsonic when it is moving slower than sound.

26. 15.2 The speed of sound
We use the term supersonic to describe motion at speeds faster than the speed of sound.
A shock wave forms where the wave fronts pile up.
The pressure change across the shock wave is what causes a very loud sound known as a sonic boom.

27. 15.2 The speed of sound
The speed of a sound wave in air depends on how fast air molecules are moving.
The speed of sound in materials is often faster than in air.

28. 15.2 Standing waves and resonance
Spaces enclosed by boundaries can create resonance with sound waves.
The closed end of a pipe is a closed boundary.
An open boundary makes an antinode in the standing wave.
Sounds of different frequencies are made by standing waves.
A particular sound is selected by designing the length of a vibrating system to be resonant at the desired frequency.

30. 15.2 Sound waves and boundaries
Like other waves, sound waves can be reflected by surfaces and refracted as they pass from one material to another.
Sound waves reflect from hard surfaces.
Soft materials can absorb sound waves.

31. 15.2 Fourier's theorem
Fourier’s theorem says any complex wave can be made from a sum of single frequency waves.

32. 15.2 Sound spectrum
A complex wave is really a sum of component frequencies.
A frequency spectrum is a graph that shows the amplitude of each component frequency in a complex wave.

33. Chapter 15 Sound 15.1 Properties of Sound 15.2 Sound Waves
15.3 Sound, Perception, and Music 33

34. Inv 15.3 Sound, Perception, and Music
Investigation Key Question:
How is musical sound different than other types of sound? 34

35. 15.3 Sound, Perception, and Music
A single frequency by itself does not have much meaning.
The meaning comes from patterns in many frequencies together.
A sonogram is a special kind of graph that shows how loud sound is at different frequencies.
Every person’s sonogram is different, even when saying the same word.

36. 15.3 Patterns of frequency
The brighter the sonogram, the louder the sound is at that frequency.

37. 15.3 Hearing sound
The eardrum vibrates in response to sound waves in the ear canal.
The three delicate bones of the inner ear transmit the vibration of the eardrum to the side of the cochlea.
The fluid in the spiral of the cochlea vibrates and creates waves that travel up the spiral.

38. 15.3 Sound
The nerves near the beginning see a relatively large channel and respond to longer wavelength, low frequency sound.
The nerves at the small end of the channel respond to shorter wavelength, higher-frequency sound.

39. 15.3 Music
The pitch of a sound is how high or low we hear its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected by the sounds we heard before and after.
Rhythm is a regular time pattern in a sound.
Music is a combination of sound and rhythm that we find pleasant.
Most of the music you listen to is created from a pattern of frequencies called a musical scale.

41. 15.3 Consonance, dissonance, and beats
Harmony is the study of how sounds work together to create effects desired by the composer.
When we hear more than one frequency of sound and the combination sounds good, we call it consonance.
When the combination sounds bad or unsettling, we call it dissonance.

42. 15.3 Consonance, dissonance, and beats
Consonance and dissonance are related to beats.
When frequencies are far enough apart that there are no beats, we get consonance.
When frequencies are too close together, we hear beats that are the cause of dissonance.
Beats occur when two frequencies are close, but not exactly the same.

44. 15.3 Harmonics and instruments
The same note sounds different when played on different instruments because the sound from an instrument is not a single pure frequency.
The variation comes from the harmonics, multiples of the fundamental note.

45. Sound from a Guitar
The sound of an acoustic guitar is shaped by sound waves bouncing around inside the guitar, as well as the vibration of the top.
Because the shape of the guitar is irregular, there are many resonances.
In general, large-bodied guitars have stronger long-wavelength, low-frequency sounds, and are louder.
Small-bodied acoustic guitars often lack low frequencies in their range of sounds.