Sound

Mechanical Vibrations in Gas, Liquid or Solid

California State Standards

Students know sound is a longitudinal wave whose speed depends on the properties of the medium in which it propagates.

Students know how to identify the characteristic properties of waves: interference (beats) , diffraction, refraction, Doppler effect, and polarization.

Anatomy of a Sound wave

Sound waves consist of alternating compressions and rarefactions in a longitudinal wave

Courtesy University of Wisconsin

Sound Waves Passes By Us

Courtesy University of Sydney

What Causes Sound

Vibrations of matter produce sounds

Sound could be a wave in air, or another medium

The original vibration makes something more massive vibrate, such as a sounding board

Frequency and Pitch

Frequency is the technical term of the number of vibrations per second in a sound wave going past us

Pitch is how we hear frequencyRange of human hearing is about 20 to

20,000 HzAs we age, we can no longer hear the

highest pitch soundsListen to Different Frequencies

Speed of Sound

About 331 m/s at 00 CIncreases by 0.6 m/s for every degree1200 km/hr1100 feet per secondDepends on properties of medium such as

Density Elasticity(ease of changing shape)

Experiencing Speed of Sound

Name two experiences you have had caused by the relatively slow speed of sound

EchosDelay between seeing lightning and

hearing thunderStarting gun delay between seeing smoke

and hearing

Thunder Delay

You hear thunder five seconds after you see lightning. How far away from you did the lightning strike?

D = s x tD = 1100 feet/sec x 5 s =5500 ft or about one mile Metric: 340 m/s x 5 s = 1700 m = 1.7 km

Sounds in Liquids and Solids

Sound travels much faster in liquids than in air

About 1500 m/s in waterMuch faster still in solidsAbout 4500 m/s in steelCan you think of a scene in a movie based

on the high speed of sound in solids?

Wave Properties Review

What are the characteristic properties of all waves?

Frequency, wavelength, speed, amplitudeIntensity is proportional to the square of

amplitudeThe sensation we experience due to the

intensity of a sound wave is called loudness

Analogy

Complete: Loudness is to intensity as pitch is to ________________frequency

Intensity in decibels

Energy in a sound wave in a certain areadB = 10 log10 (I/I0) dB is “decibels”

Logarithms Review

Log10 (10) = 1

Log10 (100) = 2

Log10 (1000) = 3

Log10 (10,000) = ?

To find the logarithm of a number to a certain base is to find the exponent to which the base needs to be raised to obtain the original number

dB(SPL) Source (with distance)

194 Theoretical limit for a sound wave at 1 atmosphere environmental pressure

180 Rocket engine at 30 m;Krakatoa explosion at 100 miles in air(160 km)[1]

150 Jet engine at 30 m

140 Rifle being fired at 1 m

130 Threshold of pain; train horn at 10 m

120 Rock concert; jet aircraft taking off at 100 m

110 Accelerating motorcycle at 5 m; chainsaw at 1 m

100 Jackhammer at 2 m; inside disco

90 Loud factory, heavy truck at 1 m

80 Vacuum cleaner at 1 m, curbside of busy street

70 Busy traffic at 5 m

60 Office or restaurant inside

50 Quiet restaurant inside

40 Residential area at night

30 Theatre, no talking

10 Human breathing at 3 m

0 Threshold of human hearing (with healthy ears)

Questions

Use dB = 10 log10 (I/I0) to answer:

(1) If the sound of a siren is 100 times more intense than that of a person speaking, how many decibels more is this?

Answer: 20 dB more

Questions

Use dB = 10 log10 (I/I0) to answer:

(2) If the sound of a rock concert is 100,000 times more intense than the sound of street traffic, how many decibels more is this?

Answer: 50 dB more

Forced Vibration, Natural Frequency and Resonance

All objects have frequencies they vibrate at naturally – natural frequencies

Any object can be forced to vibrateWhen object is forced to vibrate at its

natural frequency, the result is called resonance

Examples of Resonance

How do you need to push the person on the swing to get the maximum amplitude?

At natural frequency!

The Tacoma Narrows Bridge

What do you think happened next?

What was the cause? Wind induced

vibrations

Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks

by Yusuf Billah and Bob Scanlan, (Am. J. Phys. 59 (2), February 1991)

" . . . in many undergraduate physics texts the (1940 Tacoma Narrows bridge) disaster is presented as an example of elementary forced resonance . . . Engineers, on the other hand, have studied the phenomenon . . . and their current understanding differs fundamentally from the viewpoint expressed in most physics texts. In the present article the engineers' viewpoint is presented . . . It is then demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition of self-excitation or "negative damping" . . . This paper emphasizes the fact that. physically as well as mathematically, forced resonance and self- excitation are fundamentally different phenomena.

Wave Interference

Complete constructive

Complete Destructive

Applets http://www.

mysite.verizon.net/vzeoacw1/wave_interference.html

Describe Each of These

Answers from top down: complete destructive, partial destructive, constructive

Anti-noise Technology

Example of destructive interferenceUse microphone, amplifier and speaker to

produce opposite soundThis combines with the original sound by

destructive interference to produce…No sound!Sometimes called active noise

cancellationBuy headphones

Speakers Out of Phase

Face speakers toward each otherChange one wire so they are connected

wrong (out of phase)What do you predict will happen

Two speakers sound less loud than only one

Beats

Another example of sound wave interference

When two tuning forks close in frequency are played you hear a third sound, whose frequency is the difference between the two tuning fork frequencies

Called beat frequencyBeats simulation applet

Unequal Comb Spacing Produces Moire Pattern like Beats

Beats Questions

Tuning forks of 254 and 256 Hz are played. What is the beat frequency?

2 HzTuning forks of 514 and 518 Hz are

played. What is the beat frequency?4 Hz

Another Example of Beats

How could you use the phenomenon of beats to get these engines to run at the same speed? (synchronize them)

Doppler Effect in Sound

Source or observer moving towards – pitch increases

Source or observer moving away – pitch decreases

Fun applets http://www.ndt-ed.org/EducationResources/HighSchool/Sound/

dopplereffect.htm http://library.thinkquest.org/19537/java/Doppler.html http://www.lon-capa.org/~mmp/applist/doppler/d.htm

Wavelength decreases

Wavelength increases