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 frequency Range of human hearing is about 20 to 20,000 Hz As we age, we can no longer hear the highest pitch sounds Listen to Different Frequencies

Speed of Sound About 331 m/s at 0 0 C Increases by 0.6 m/s for every degree 1200 km/hr 1100 feet per second Depends 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 Echos Delay between seeing lightning and hearing thunder Starting 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 t D = 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 water Much faster still in solids About 4500 m/s in steel Can 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, amplitude Intensity is proportional to the square of amplitude The 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 area dB = 10 log 10 (I/I 0 ) dB is “decibels” Logarithms Review Log 10 (10) = 1 Log 10 (100) = 2 Log 10 (1000) = 3 Log 10 (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) 194Theoretical limit for a sound wave at 1 atmosphere environmental pressure 180RocketRocket engine at 30 m; Krakatoa explosion at 100 miles in air(160 km)[1] Krakatoa[1] 150JetJet engine at 30 mengine 140RifleRifle being fired at 1 m 130Threshold of painThreshold of pain; train horn at 10 mtrain 120RockRock concert; jet aircraft taking off at 100 mconcert 110Accelerating motorcycle at 5 m; chainsaw at 1 mmotorcycle 100JackhammerJackhammer at 2 m; inside discodisco 90Loud factory, heavy truck at 1 mfactorytruck 80Vacuum cleanerVacuum cleaner at 1 m, curbside of busy street 70Busy traffic at 5 mtraffic 60OfficeOffice or restaurant inside 50Quiet restaurant insiderestaurant 40Residential area at nightnight 30Theatre, no talking 10Human breathing at 3 m 0Threshold of human hearingThreshold of human hearing (with healthy ears)

Questions Use dB = 10 log 10 (I/I 0 ) 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 log 10 (I/I 0 ) 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 vibrate When 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 mysite.verizon.net/vzeoac w1/wave_interference.ht ml mysite.verizon.net/vzeoac w1/wave_interference.ht ml

Describe Each of These Answers from top down: complete destructive, partial destructive, constructive

Anti-noise Technology Example of destructive interference Use microphone, amplifier and speaker to produce opposite sound This combines with the original sound by destructive interference to produce… No sound! Sometimes called active noise cancellation Buy headphones

Speakers Out of Phase Face speakers toward each other Change 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 frequency Beats 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 Hz Tuning 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 ed.org/EducationResources/HighSchool/Sound/dopplereffect.htm ed.org/EducationResources/HighSchool/Sound/dopplereffect.htm Wavelength decreases Wavelength increases