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Understanding Waves and Sound

Learn about waves, different types of waves, how a guitar creates different notes, the properties of waves, the speed of sound, power and intensity of sound waves, the decibel scale, and the Doppler effect.

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Understanding Waves and Sound

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  1. Waves… v = fλ

  2. 1. A wave is a traveling disturbance.2. A wave carries energy from place to place.

  3. Longitudinal Wave Transverse Wave

  4. How is a guitar made to create different notes/pitches/frequencies? • A wave’s speed, v, traveling through a string (via a transverse wave) is affected by… • the tension (F) in the spring, and • the mass (m) per unit length (L) of the string, also called the linear density, m/L, and follow this relationship… • v = [F/(m/L)] http://phet.colorado.edu/simulations/sims.php?sim=Wave_on_a_String

  5. The speed of a wave is affected by the properties of the material or medium through which the wave travels. • The speed of a wave also depends on the type of wave… sound wave, electromagnetic wave, water wave, seismic wave, etc.

  6. Sound Waves… • Fun with sulfur hexafluoride http://www.youtube.com/watch?v=V2FR6-gEwjU • Sound waves travel at 343 m/s through air at an air temperature of 20o C (slower if air temp is lower, faster if air temp is higher). • V sound in air =331 m/s + (0.6 m/s/o C)T • T is temperature of air through which the sound wave is traveling. T is measured in oC. • Sound waves travel at different speeds depending on the medium… Faster in solids, then liquids, then slowest through gasses.

  7. See Table 15.1 on pg. 482 in your textbook (below) for the speed of sound in different media (you’ll need to go here when doing your homework).

  8. Electromagnetic Waves (NOT sound waves)… • All electromagnetic waves (not sound waves) move at a remarkable speed of 3.0 X 108 m/s in a vacuum. • E-M waves: Radio, Infrared Radiation, X-rays, Ultraviolet Radiation, Microwave, Light, Gamma…

  9. A sound wave is a series of alternating condensations and rarefactions; each molecule executes Simple Harmonic Motion about a fixed location.

  10. Power & Sound Intensity of a Sound Wave • Power of a wave: the amount of energy transported each second. This energy can do work, such as vibrate an ear drum or cause damage to buildings, as in a sonic boom. • P = W/Δt • Units: Joules/second, or Watts!

  11. Sound Intensity, I: the sound power that passes perpendicularly through a surface divided by the area of that surface. • I = P/A • units: Watts/meter2 = W/m2 • As you move away from a sound source, the same power is spread over a greater and greater area causing a decrease in intensity (quieter). • P = IA (I & A are inversely proportional)

  12. For a sound radiated uniformly in all directions, such as in a spherical shape… • Asphere = 4r2 • I = P/A = P/(4r2) • I = P/(4r2) • Valid when no sound is reflected off walls, objects, etc. • Notice: I  1/r2 • What happens to sound intensity when the distance from a spherical source doubles? • I  1/r2 •  I  1/(2ro)2 = 1/(4ro2) • I is ¼ the original intensity!

  13. The scale used to measure and compare the loudness of sound is called the decibel scale. • The decibel is named after Alexander Graham Bell who did a lot of work in the area of sound and loudness. • He discovered that to obtain a sound that seems twice as loud as another sound, the intensity (how much sound energy per unit area per second hits the eardrum) of the sound must be multiplied by 10.

  14. Decibels=a measurement which compares two sound intensities, I/Io. I = the intensity of the sound, Io= reference level to which I is being compared. Usually, Io=the threshold of human hearing (lowest intensity perceived) = 1x10-12 W/m2 =Io • We call that apparent loudness the intensity level, ß, measured in decibels (dB) and we can find it if you know the intensity in Joules per second per square meter (or W/m2) using the following logarithmic equation: • ß = 10 log (I / Io)where Io is usually the softest sound the human ear can distinguish, at 1x10-12 W/m2 or J/s/m2. • Notice that if I = 1x10-12 W/m2, then ß = 0 dB (decibels). • Other samples of loudness are normal conversation, about 60 dB, whispering, about 15 dB, and loud music, about 120 dB. Sounds around and above this level (120 dB) can cause permanent damage to the ear.

  15. Noise levels in our-day to day lives • In our daily lives, we are rarely exposed to sound levels near either end of our huge (0 to 130 dB) audible range. Typically we encounter noise levels between about 20 and 30 dB (a faint whisper or night-time background noise in a quiet suburban bedroom) and 100 dB (un-muffled motorcycle or jackhammer operating nearby). Typical noise levels experienced include: • 40 to 50 dB in a general office situation. • 60 dB when talking normally to someone 1 to 2 m away. • 65 to 75 dB when riding in a car at highway speeds. • 85 to 95 dB while cutting the grass with power mower. • Roughly speaking, each 10 dB increase in sound level corresponds to a "doubling of subjective loudness" so that, for example, jackhammer noise at 110 dB would typically be judged to be 2 x 2 x 2 x 2 = 16 times as loud as the inside of a car at 70 dB.

  16. Doppler Effect As the Fire Truck approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the Fire Truck recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the Fire Truck is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the Fire Truck's speed.

  17. Either the source of sound could be moving, or the observer of the sound could be moving… Doppler Effect is still observed in either case.

  18. Doppler Shift • fo- fs = Doppler shift • For sound source moving toward observer: fo> fs • Observer interprets frequency, fo, at higher pitch than the frequency emitted by source, fs. • For sound source moving away from observer: fo< fs • Observer interprets frequency, fo, at lower pitch than the frequency emitted by source, fs.

  19. General Case of Doppler Effect • Source and observer both moving: • fo = fs · [(1 +/- (vo/v)] [(1 +/- (vs/v)] • fo, vo = frequency of observer, speed of observer • fs, vs = frequency of source, speed of source • v = speed of sound • Where 1 +/- (vo/v) • + if observer moves toward source • - if observer moves away from source • Where 1 +/- (vs/v) • - if source moves toward observer • + if source moves away from observer

  20. fo = fs · [(1 +/- (vo/v)] [(1 +/- (vs/v)] • If observer is stationary, and the sound source only is moving… let top part of equation = 1 • If sound source is stationary, and the observer of the sound only is moving… let bottom part of equation = 1 • http://www.youtube.com/watch?v=a3RfULw7aAY

  21. Sonic Boom! http://www.sky-flash.com/boom.htm https://www.youtube.com/watch?v=gWGLAAYdbbc http://www.youtube.com/watch?v=-d9A2oq1N38

  22. Everything About Waves • http://www.acoustics.salford.ac.uk/feschools/waves/wavetypes.htm • http://www.youtube.com/watch?v=-Zu5SGllmwc&feature=related

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