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Sound

Sound. Chapter 15. Topics for Sound. Sound wave properties Speed of sound Echoes Beats Doppler shift Resonance Anatomy of Ear. Sound Wave Properties. Sound Waves are Longitudinal Waves.

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Sound

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  1. Sound Chapter 15

  2. Topics for Sound • Sound wave properties • Speed of sound • Echoes • Beats • Doppler shift • Resonance • Anatomy of Ear

  3. Sound Wave Properties

  4. Sound Waves are Longitudinal Waves The air molecules shown below are either compressed together, or spread apart. This creates alternating high and low pressure.

  5. Frequency • The frequency of a sound wave (or any wave) is the number of complete vibrations per second. • The frequency of sound determines its pitch.

  6. The higher the frequency, the higher the pitch

  7. Wavelength • Wavelength is the distance between two high pressures, or two low pressures. This property is dependent on the velocity of the sound and it’s frequency. • Wavelength and frequency are inversely related. • Short wavelength (high frequency) results in a high pitch.

  8. Frequency and the human ear • A young person can hear pitches with frequencies from about 20 Hz to 20000 Hz. (most sensitive to frequencies between 1000 and 5000 Hz). • As we grow older, our hearing range shrinks, especially at the high frequency end. • By age 60, most people can hear nothing above 8000 Hz. • Sound waves with frequencies below 20 Hz are called infrasonic. • Sound waves with frequencies above 20000 Hz are called ultrasonic.

  9. The Amplitude of a Sound Wave Determines its loudness or softness

  10. Velocity of Sound

  11. The velocity of sound depends on • the medium it travels through • the temperature of the medium

  12. Sound travels faster in liquids than in air (4 times faster in water than in air) • Sound travels faster in solids than in liquids (11 times faster in iron than in air) • Sound does not travel through a vacuum (there is no air in a vacuum so sound has no medium to travel through) • The speed depends on the elasticity and density of the medium.

  13. Effects of Temperature • In air at room temperature, sound travels at 343m/s (~766 mph) • v = 331 m/s + (0.6)T • v: velocity of sound in air • T: temperature of air in oC • As temperature increases, the velocity of sound increases

  14. Relationship between velocity, frequency, and wavelength • V = f • V = velocity of sound •  = wavelength of sound • f = frequency of sound

  15. Echoes: REFLECTION

  16. Echoes are the result of the reflection of sound Sound waves leave a source, travel a distance, and bounce back to the origin.

  17. Things that use echoes... • Bats • Dolphins/ Whales • Submarines • Ultra sound • Sonar

  18. REFRACTION OF WAVES

  19. Refraction of Sound • as the sound wave transmits into the warmer air at lower levels, they change direction, much like light passing through a prism

  20. DIFFRACTION: THE BENDING OF WAVES THROUGH A SMALL OPENING

  21. BENDING OF A WAVE

  22. Sound waves move out like this: • http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

  23. But when they move, the front of the wave gets bunched up (smaller wavelength) and the back of the wave starts to expand (larger wavelength): • http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

  24. Observer C hears a high pitch (high frequency) Observer B hears the correct pitch (no change in frequency)Observer A hears a low pitch (lower frequency) • http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

  25. When the source goes faster, the wave fronts in the front of the source start to bunch up closer and closer together, until...

  26. The object actually starts to go faster than the speed of sound. A sonic boom is then created. • http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

  27. Doppler Effect • The doppler effect is a change in the apparent frequency due to the motion of the source or the receiver. • Example: As an ambulance with sirens approaches, the pitch seems higher. As the object moves by the pitch drops.

  28. Police use the Doppler Shift when measuring your speed with radar • A frequency is sent out of the radar gun • The sound wave hits the speeding car • The frequency is changed by the car moving away from the radar and bouncing back • The amount the frequency changes determines how fast you are going • The faster you are going, the more the frequency is changed.

  29. Equation that describes the doppler effect. fd = fs(v + vd) (v - vs) fs is the actual frequency being emitted fd is the perceived frequency as the source approaches or recedes vd is (+) if the observer moves toward the source vd is (-) if the observer moves away from the source vs is (+) if the source moves toward the observer vs is (-) if the source moves away from the observer

  30. Example • Sitting at Six Flags one afternoon, Mark finds himself beneath the path of the airplanes leaving Hartsfield International Airport. What frequency will Mark hear as a jet, whose engines emit sounds at a frequency of 1000 Hz, flies toward him at a speed of 100 m/sec? (temp is 10oC)

  31. Solution • v = 331 + (0.6)T v = 331 + (0.6)(10) v = 337 m/s • fd = fs(v + vd) f = ? (v – vs) fs = 1000 Hz vd = 0 m/s vs = 100 m/s

  32. Solution f= 1000 (331 + 0) (331 – 100) f = 1430 m/s

  33. SOUNDINTENSITY: THE LOUDNESS OF SOUND

  34. Sound Intensity • The intensity of a sound is the amount of energy transported past a given area in a unit of time. • Intensity = power/area • The greater the amplitude, the greater the rate at which energy is transported-the more intense the sound • Intensity is inversely related to the square of the distance. As distance increases, the intensity decreases.

  35. Threshold of Hearing • The human ear is sensitive to variations in pressure waves, that is, the amplitude of sound waves. • The ear can detect wave amplitudes of 2x10-5 Pa up to 20 Pa. • The amplitudes of these waves are measured on a logarithmic scale called sound level. • Sound level is measured in decibels (dB).

  36. DECIBEL • MEASURES THE LOUDNESS OF SOUND • RELATES TO THE AMPLITUDE OF THE WAVE • EVERY INCREASE OF 10dB HAS 10x GREATER AMPLITUDE

  37. A SOUND 10 TIMES AS INTENSE IS PERCEIVED AS BEING ONLY TWICE AS LOUD

  38. NOISE POLLUTION · Prolonged exposure to noise greater than 85-90 dB may cause hearing loss   · Brief exposures to noise sources of 100-130 dB can cause hearing loss · A single exposure to a level of 140 dB or higher can cause hearing loss

  39. EXPOSURE TO LOUD NOISE Hours Per Day Noise Level (dB) 8 90 4 95 2* 100* 1 105 0.5 110

  40. Reducing Sound Intensity • Cotton earplugs reduce sound intensity by approximately 10 dB. • Special earplugs reduce intensity by 25 to 45 dB. • Sound proof materials weakens the pressure fluctuations either by absorbing or reflecting the sound waves. • When the sound waves are absorbed by soft materials, the energy is converted into thermal energy.

  41. Resonance

  42. Natural Frequency • Nearly all objects when hit or disturbed will vibrate. • Each object vibrates at a particular frequency or set of frequencies. • This frequency is called the natural frequency. • If the amplitude is large enough and if the natural frequency is within the range of 20-20000 Hz, then the object will produce an audible sound.

  43. Timbre • Timbre is the quality of the sound that is produced. • If a single frequency is produced, the tone is pure (example: a flute) • If a set of frequencies is produced, but related mathematically by whole-number ratios, it produces a richer tone (example: a tuba) • If multiple frequencies are produced that are not related mathematically, the sound produced is described as noise (example: a pencil)

  44. Factors Affecting Natural Frequency • Properties of the medium • Modification in the wavelength that is produced (length of string, column of air in instrument, etc.) • Temperature of the air

  45. Resonance • Resonance occurs when one object vibrates at the same natural frequency of a second object, forcing that second object into vibrational motion. • Example: pushing a swing • Resonance is the cause of sound production in musical instruments. • Energy is transferred thereby increasing the amplitude (volume) of the sound. • http://www.pbs.org/wgbh/nova/bridge/meetsusp.html

  46. Types of Resonance • Resonance takes place in both closed pipe resonators and open pipe resonators. • Resonance is achieved when there is a standing wave produced in the tube. • Closed pipe resonators • open end of tube is anti-node • closed end of tube is node • Open pipe resonators • both ends are open • both ends are anti-nodes

  47. Closed pipe resonator

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