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Chapter 11--Waves And Sound


Greg Hover

on 31 January 2015

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Transcript of Chapter 11--Waves And Sound

la primavera
el verano
Chapter 11
Waves and Sound

I. Waves and energy
A. Waves defined
1. Definition--a periodic back-and-forth motion that transfers energy through a medium
2. The medium does not travel with the wave.
3. An oscillation is the back-and-forth motion of the wave.
b) Longitudinal waves
(1) The medium moves back-and-forth in the direction that the wave travels
2. Amplitude
a) Height of a wave; the maximum distance that a medium is displaced by a wave
b) Directly related to the energy of a wave
C. Wave behavior
1. Demonstrated by a ripple tank
2. Reflection
a) The way a wave changes its course upon colliding with an object or boundary
b) Incident waves strike the object and then form reflected waves that are bounced off the object.
b) Waves are bent toward the medium in which they slow down and away from the medium in which they speed up.
4. Diffraction
a) The way a wave spreads out after passing through a narrow gap
b) Most pronounced when the gap is about the size of one wavelength
5. Interference
a) When two waves meet each other and either reinforce or cancel each other
c) Destructive interference occurs when a crest and a trough cancel each other out.
b) Constructive interference occurs when two crests or two troughs meet to increase the amplitude.
2. Most often transmitted through air but can travel through other mediums
3. Travels in all directions from the source of the sound
5. The ears transmit the signals to the brain which interprets the vibrations as sounds.
6. Sound waves require a medium; Robert Boyle demonstrated that sound waves cannot be transmitted in a vacuum.
Robert Boyle
B. Characteristics of sound
1. Intensity
a) The strength or power of a sound wave
b) Measured in watts per square meter
c) Increases as amplitude and the energy of the wave increases; decreases as the energy of the wave decreases
2. Loudness
a) The human perception of the intensity of sound waves
d) Hearing can be damaged by overexposure to sound intensity over 130 decibels.
3. Pitch
a) The human perception of the frequency of sound waves
b) Directly related to frequency
f) Use of a tuning fork to produce the correct pitch for musical instruments
Low pitch
High pitch
c) The frequency range that can be heard by humans is known as audible sound (20 Hz to 20,000 Hz).
b) As the object approaches the listener, the pitch (frequency) is high; as the object moves away, the pitch becomes lower.
(1) Sound waves travel faster through warmer and denser air.
(2) The speed of sound through air at 0 °C and at sea level is 331 m/s or 740 mph.
(3) The speed of sound changes .61 m/s for every rise or drop in 1 °C.
(4) The equation to find the speed of sound at a given temperature:
331 m/s + (.61 m/s) (T / 1 °C)
A U.S. Air Force F-22 Raptor breaks the speed of sound
(2) Expressed in aviation as Mach numbers
(4) We perceive the shock wave as a sonic boom.
A condensation cloud forms as the jet breaks the sound barrier.
III. Behavior of sound waves
A. Reflection of sound
1. Sound waves follow the law of reflection.
3. Echoes
a) A reflected sound that is heard clearly from the original sound
A sonar fish finder
2. Sound waves bend toward the slower medium and away from the faster medium.
C. Diffraction of sound
1. The way that sound waves spread out after passing through a narrow gap
2. Most pronounced when the width of the gap is about one wavelength
3. Sounds of lower frequency are diffracted better.
Interference caused by a single tuning fork
e) An oscilloscope is used to study the interference of overtones.
A sound wave from a violin
An oscilloscope
4. Crest—the wave’s highest point

5. Trough—the wave’s lowest point

6. Wave train—a series of waves
7. Types of waves

a) Transverse waves

(1) The medium moves at right angles to the direction that the wave travels.

(2) Example—water waves
(2) One longitudinal wave consists of a compression pulse (particles of the medium are squeezed together) and a rarefaction pulse (particles of the medium are spread out).
(3) Examples—sound waves, springs
B. Wave characteristics

1. Wavelength

a) The distance between corresponding points on adjacent waves (such as from one crest to the next)

b) Represented by the Greek letter lambda ( )

3. Frequency

a) Number of waves that pass by per unit of time; found by the equation, f = N/t (frequency = waves / time)

b) Measured with the SI unit, the hertz (Hz); equal to one wave or cycle per second (1/s)
c) The period—time required for one wave or cycle to pass; the inverse of the frequency; found by the equation, T = 1/f (period = 1 / frequency)
4. Speed

a) Remains constant if the medium does not change; directly related to wavelength and frequency

b) The equation that relates wave speed to frequency and wavelength: v = f (speed = wavelength x frequency)

c) Examples—

d) Frequency and wavelength are inversely related.
5. Illustration of a wave—
c) Law of reflection—the angle of incidence equals the angle of reflection
3. Refraction

a) The change in a wave’s direction due to a change in the wave’s speed; occurs if the waves cross the boundary between media at an angle

c) Allows waves to get around obstacles in their path
Overhead view of waves diffracting around a ship
II. Sound waves

A. Sound defined

1. Vibrations that travel as longitudinal waves

4. A sound wave is formed when air molecules are compressed together creating a high-pressure area (the compression pulse) followed by a low-pressure area (the rarefaction pulse).
d) The inverse square law of sound intensity—intensity is inversely related to the square of the distance from the source of the sound
b) The bel—the unit of sound intensity; an increase of 1 bel represents an increase in intensity that is 10 times more intense
c) The decibel—the most commonly used unit to measure sound intensity in a way that reflects how our ears perceive sound
d) Infrasonic sound—have pitches below the audible range

e) Ultrasonic sound—have pitches above the audible range

4. Doppler effect

a) How the pitch of a sound changes because the source of the sound is moving; depends upon relative motion between the listener and the source of the sound

c) Examples—
5. Speed of sound

a) Depends on the type of medium

b) The speed of sound waves traveling through air depends mostly on the temperature of the air.
c) Supersonic speeds

(1) Speeds that are faster than the speed of sound; subsonic speeds—speeds slower than the speed of sound

(3) Can produce shock waves—the violent compression pulse produced by constructive interference between sound waves
2. Reverberations—the persistence or gradual fading of sound resulting from multiple reflections; not heard as separate sounds from the original sound
b) Our ears can detect reflected sound that is 0.1 seconds apart from the original sound; an echo cannot be heard if the source of the original sound is less than 17 m (56 ft) from the reflecting surface.
4. Uses for reflected sound waves

a) Can be used to determine the distance of objects

b) Sonar (sound navigation and ranging)

(1) Used to measure depth and distance under water in a process called echo ranging

(2) Use of scanning sonar to produce a 3-D image of the ocean floor
(3) Used by an ultrasonograph to produce a sonogram—an image of a baby in the womb
5. Reducing the reflection of sound waves

a) The reflection of sound waves can be minimized through absorption—the process of using sound-absorbing materials to dissipate the energy of sound waves.

b) Acoustics—the study of sound; relevant to engineers for designing buildings and rooms to have the desired reverberation time of sounds
B. Refraction of sound

1. The bending of sound waves caused by the waves entering a medium (at an angle) of different temperatures or densities

D. Interference of sound

1. Sound waves interact at the point of contact.

2. Colliding sound waves are subject to constructive or destructive interference.
IV. Music

A. Music relationships

1. Definition of music

a) An art form which has sound waves with specific pitches arranged and organized deliberately in a pleasing way

b) Different from noise which is random sound without an intended pattern
2. A note—a sound of a definite pitch
3. An octave

a) The difference in pitch between 2 notes in which one of the notes has twice the frequency as the other

b) Notes played an octave apart sound like the same note at different levels of pitch.

B. Interference in music

1. Harmony

a) Consonance—the pleasant result of notes interfering harmoniously

b) Dissonance—the unpleasant result of notes interfering in a disagreeable way
c) An interval—the distance between two notes
d) The consonance of a combination of notes depends upon the ratio of their frequencies; the ratio of 1:2 is the most consonant.
e) Consonance is greater between two notes when the frequency ratio is expressed in smaller whole numbers.
2. Beats

a) Bursts of pulsating sound caused by the interference of two sounds; part of what causes dissonance

b) The number of beats per second is equal to the difference in the frequencies of the two sounds.
3. Overtones

a) The fundamental—the sound with the lowest frequency produced by a musical instrument playing a single note; the dominant sound produced by an instrument

b) Overtones—the sounds produced at higher frequencies than the fundamental coming from a musical instrument playing a note
c) Harmonic series—a sequence of frequencies showing the relationships between the fundamental and the overtones
d) The distinctive sound quality (or timbre) of a musical instrument is determined by the interference between the fundamental and the overtones.
C. Resonance

1. Natural frequencies—the frequencies at which an object tends to vibrate or oscillate

2. An external force can cause an object to oscillate with a driving frequency.
3. Resonance occurs when the driving frequency approaches the natural frequency of an object, and the amplitude of oscillation increases.
Resonance caused the collapse of the Tacoma Narrows Bridge in 1940.
4. Resonator—a device, such as the hollow body of an instrument, that reverberates the sound to make the instrument louder
D. Musical instruments

1. Three classes of acoustic instruments—strings, winds, and percussion

2. The piano

a) The most versatile and widely played instrument today

b) Both upright and grand pianos have 88 keys attaches to hammers that strike strings that are free to vibrate.
c) A wooden sounding board amplifies the sound of the vibrating strings.
3. The violin family

a) Four-stringed instruments which include the violin, viola, cello, and double bass

b) Four notes possible on the open strings and other notes made possible by shortening the length of the strings
c) The motion of the bow across the strings causes the strings to vibrate.
4. Organ pipes

a) Air moves across a sharp edge (lip) at the bottom of the pipe to produce regular pulses of compression and a sound known as an edge tone.

b) The frequency of the edge tone is directly related to the speed of the air.
5. The brass

a) Includes the trumpet, tuba, and french horn

b) The vibration of the player’s lips against the mouthpiece produces sound; the player can produce more notes by adjusting the length of the air column by controlling the piston valves.
6. The woodwinds

a) Includes the clarinet, flute, and saxophone

b) The player blows into the instrument to produce vibrations; various notes are produced by adjusting the length of the air column by opening and closing holes along the instrument.
7. The percussion family

a) Instruments which are played by being struck; includes drums, cymbals, and bells

b) Produce sounds which do not follow a harmonic series
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