Sound Waves

Review

• Do you remember anything about _______?• Transverse waves• Longitudinal waves• Mechanical waves• Electromagnetic waves

Discussion

• Discuss with your partner about differences of transverse vs. longitudinal and mechanical vs. electromagnetic waves.

Superposition

• When two waves exist at the same time in the same space, the waves overlap. The combination of two overlapping waves is called superposition.

• The two waves interact to form an interference pattern.

Constructive Interference

Destructive Interference

After interference

Standing WavesNode

Antinode

Node and Antinode

• Node: a point in a standing wave that always undergoes complete destructive interference and therefore is stationary.

• Antinode: a point in a standing wave, halfway between two nodes, at which the largest amplitude occurs.

Wavelength of Standing Wave

L

(A)

(B)

(C)

A single loop corresponds to either a crest or a trough alone, this standing wave corresponds to one-half of a wavelength. Thus, the wavelength in this case is equal to twice the string length (2L).

The wavelength of (B) standing wave: _______

The wavelength of (C) standing wave: _______

Sound Waves

As the prong swings to the right, the air molecules in front of the movement are forced closer together.Such a region of high molecular density and high air pressure is called a compression.

As the prong swings to the left, the air molecules to the right spread apart. The region of lower density and pressure is called a rarefaction.

Sound is a longitudinal wave

• Sound travels through the air at approximately 340 m/s.• It travels through other media as well, often much faster than

that!• Sound waves are started by vibration of some other material,

which starts the air moving.

Hearing Sounds• We hear a sound as “high” or “low” depending on its frequency or

wavelength. Sounds with short wavelengths and high frequencies sound high-pitched to our ears, and sounds with long wavelengths and low frequencies sound low-pitched. The range of human hearing is from about 20 Hz to about 20,000 Hz.

• The amplitude of a sound's vibration is interpreted as its loudness. We measure the loudness (also called sound intensity) on the decibel scale, which is logarithmic.

Pure Sounds

• Sounds are longitudinal waves, but if we graph them right, we can make them look like transverse waves.

• When we graph the air motion involved in a pure sound tone versus position, we get what looks like a sine or cosine function.

• A tuning fork produces a relatively pure tone. So does a human whistle.

Graphing a Sound Wave

Complex Sounds

• Because of the phenomena of “superposition” and “interference” real world waveforms may not appear to be pure sine or cosine functions.

• That is because most real world sounds are composed of multiple frequencies.

• The human voice and most musical instruments produce complex sounds.

Beats

• As a result of interference, a fluctuation in the loudness of the combined sounds is heard. The periodic variation in the loudness of sound is called beats.

Review Questions

• At what times are two waves exactly out of phase? • At what times are the two waves exactly in phase?

t1 t2 t3 t4 t5

t1,t3, t5

t2, t4

Characteristics of Sound Waves

• Frequency determines pitch.• Pitch: how high or low sound is. Man’s vocal

sound ranges from 100 Hz to several hundred Hz. Woman’s vocal sound ranges from 1000Hz to several thousand Hz.

Let’s Practice.

• If you hear a higher pitch from a trumpet than from a saxophone, how do the frequencies of the sound waves from the trumpet compare with those from the saxophone?

• The trumpet’s waves have a higher frequency than the saxophone’s waves.

Characteristics of Sound Waves

• Speed of sound depends on the medium.• Sound waves generally travel faster through

solids than through gases.• Speed of sound in air (room temperature) 346

m/s; in water 1490 m/s; in copper 3560 m/s

The Doppler Effect

When a car is moving to a listener, the pitch of the car horn will be higher as the car approaches and will be lower as the car moves away.This frequency shift is known as the Doppler effect, named for the Austrian physicist Christian Doppler.

The Doppler Effect

• http://www.animations.physics.unsw.edu.au/jw/doppler.htm#source

• http://soundbible.com/579-Police-Siren.html

Let’s Practice

A moving bug is making disturbance on water. Which way is the bug moving? A to B or B to A

Answer: A to B

Let’s practice.

(B) (A)

1. Which wave has a stationary sound source?

Answer: A

2. Which way is the sound source in (B) moving?

Answer: to the right

Let’s practice.

• As a dolphin swims toward a fish, it sends out sound waves to determine the direction the fish is moving. If the frequency of the reflected waves is increased, is the dolphin catching up to the fish or falling behind?

• The dolphin is catching up to the fish.

Sound intensity and resonance

• Intensity is the rate of energy flow through a given area.

• Intensity = Power/area• Intensity of a spherical wave = power/4πr2

Practice

• What is the intensity of the sound waves produced by a trumpet at a distance of 3.2m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical.

• Intensity = 0.2 W /4π(3.2m)2=1.6 x 10-3 W/m2

Audibility depends on intensity and frequency of sound

Relative intensity is measured in decibels.

• Frequency of a sound wave determines its pitch.

• Intensity of a wave determines its loudness, or volume.

Ratios of Two dB Levels

Decibel(dB)

• Relative intensity: determined by relating the intensity of a sound wave to the intensity at the threshold of hearing.

• The volume doubles each time the decibel level increases by 10.

• 10 dB (1.0 x 10-11 W/m2) is twice as loud as 0 dB (1.0 x 10-12 W/m2).

• 20 dB (1.0 x 10-10 W/m2 is twice as loud as 10 dB (1.0 x 10-11 W/m2).

Resonance

• Every object vibrates at its own special set of frequencies, which together form its special sound. That is called natural frequency.

• Natural frequency: one at which minimum energy is required to produce forced vibrations.

• When the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs. This phenomenon is called resonance.

Demonstration

DescriptionDraw a glass or plastic golf tube out of a water bath while holding an excited tuning fork over one end.

DiscussionThe first resonance occurs when the length of the air column is one-fourth of the wavelength of the tuning fork, because a node occurs at the surface of the water and an antinode occurs at the top of the pipe.

• Resonance occurs at L = λ/4, and again at L = 3λ/4, and at L = 5λ/4.

• The speed of sound in the classroom can be calculated using the formula for the speed of a wave v = f λ.

Resonance

• Resonance is not restricted to wave motion. • Pushing a friend on a swing is an example of

resonance.• It occurs whenever successive impulses are

applied to a vibrating object in rhythm with its natural frequency.

• The Tacoma Narrows Bridge disaster in 1940 is attributed to wind-generated resonance.

Homework

• Conceptual Physics Page 403 #33 - #39

Homework

• Holt physics pages 507 – 509 Chapter 13 Review and Assess 1-37 (odd numbers only)