1 / 32

Sound and waves

Sound and waves. Sound waves. As the tuning fork vibrates, a succession of compressions and rarefactions of the air density are produced and propagate away from the fork A sinusoidal curve can be used to represent the longitudinal wave

yair
Télécharger la présentation

Sound and waves

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Sound and waves PHY231

  2. Sound waves • As the tuning fork vibrates, a succession of compressions and rarefactions of the air density are produced and propagate away from the fork • A sinusoidal curve can be used to represent the longitudinal wave • Crests correspond to compressions and troughs to rarefactions • The sound is a longitudinal wave because the vibrations (here compression or rarefaction of air density) are in the same direction as the direction of propagation PHY231

  3. Types of sound waves • Audible waves • Lay within the normal range of hearing of the human ear • Normally between 20 Hz to 20,000 Hz • Infrasonic waves • Frequencies are below the audible range • Earthquakes are an example • Ultrasonic waves • Frequencies are above the audible range • Dog whistles are an example PHY231

  4. wavelength • If the speed of propagation of the wave is v and its frequency f, the distance between consecutive maxima is called wavelength and usually noted l (lambda): PHY231

  5. Applications of ultrasounds • Ultrasonic waves f>20 kHz (in air l<2 cm) • Can be used to produce images of small objects • Many Applications uses the reflection of the ultrasonic wave as a locating/imaging tool • Ultrasounds to observe babies in the womb • Ultrasonic ranging unit for cameras • SONAR PHY231

  6. Sound waves • Which of the following ranges corresponds to the longest wavelengths? • A) infrasonic • B) audible • C) ultrasonic • D) all have the same wavelengths PHY231

  7. Sound waves • Which of the following ranges corresponds to the longest wavelengths? • A) infrasonic • B) audible • C) ultrasonic • D) all have the same wavelengths PHY231

  8. Wavelength • The frequency separating audible waves and ultrasonic waves is considered to be 20 kHz. What wavelength is associated with this frequency? (Assume the speed of sound to be 340 m/s.) • A) 1.7 cm • B) 5.2 cm • C) 34 cm • D) 55 cm PHY231

  9. Wavelength • The frequency separating audible waves and ultrasonic waves is considered to be 20 kHz. What wavelength is associated with this frequency? (Assume the speed of sound to be 340 m/s.) • A) 1.7 cm • B) 5.2 cm • C) 34 cm • D) 55 cm PHY231

  10. Intensity of sound wave • As a sound wave propagates, it carries energy • The rate of energy transfer by second and by unit area is the intensity I • The area A is perpendicular to the direction of the energy flow • For human ears: • Threshold of hearing I ~10-12 W/m2 • Threshold of pain I ~1 W/m2 PHY231

  11. Intensity levels in Decibels • The human ear is functional over twelve order of magnitudes of intensity • Our perception of the intensity however is not linear but logarithmic. For this reason, a logarithmic unit system, the decibels, is defined as • I0=10-12 W/m2 is the threshold of hearing • b in decibels = dB PHY231

  12. Intensity levels in Decibels Multiplying the intensity by: factor 10 means increasing by 10dB factor 100 means increasing by 20dB etc… Dividing the intensity by: factor 10 means decreasing by 10dB factor 100 means decreasing by 20dB etc… I (W/m2) b (dB) Threshold of hearing I=10-12 W/m2 Threshold Of pain I = 1W/m2 b (dB) I (W/m2)

  13. How loud? • Which of the following best describes a sound level of intensity 1 W/m2? • A) extremely loud • B) about that of a power mower • C) normal conversation • D) like a whisper PHY231

  14. How loud? • Which of the following best describes a sound level of intensity 1 W/m2? • A) extremely loud • B) about that of a power mower • C) normal conversation • D) like a whisper PHY231

  15. Intensity • Tripling the power output from a speaker emitting a single frequency will result in what increase in loudness? • A) 0.33 dB • B) 3.0 dB • C) 4.8 dB • D) 9.0 dB PHY231

  16. Intensity • Tripling the power output from a speaker emitting a single frequency will result in what increase in loudness? • A) 0.33 dB • B) 3.0 dB • C) 4.8 dB • D) 9.0 dB PHY231

  17. Spherical sound waves • A source-sphere contracting and expanding periodically will generate a spherical sound wave • The disturbance moves away from the source on a spherical wave front • The wavelength l is the distance between consecutive wave fronts PHY231

  18. Intensity for spherical waves • The spherical wave front expands in radial direction • At a radius r from the source • For example: 2m away from the source, the intensity is 4 times smaller than at 1m away • Power crossing each surface is the same but the intensity decreases with the distance • The amplitude of the wave is the square-root of the intensity

  19. Plane waves • Plane waves have Wavefronts that are parallel to each other and moving on a straight line • Such a situation can arise for example at very large distance from the source of a spherical wave source (>>l) PHY231

  20. dB for spherical waves • If the distance between a point sound source and a dB detector is increased by a factor of 4, what will be the reduction in intensity level? • A) 16 dB • B) 12 dB • C) 4 dB • D) 0.5 dB PHY231

  21. dB for spherical waves • If the distance between a point sound source and a dB detector is increased by a factor of 4, what will be the reduction in intensity level? • A) 16 dB • B) 12 dB • C) 4 dB • D) 0.5 dB PHY231

  22. Jet airliner altitude • The intensity level of sound 20 m from a jet airliner is 120 dB. At what distance from the airplane will the sound intensity level be a tolerable 100 dB? (Assume spherical spreading of sound.) • A) 90 m • B) 120 m • C) 150 m • D) 200 m PHY231

  23. A) 90 m • B) 120 m • C) 150 m • D) 200 m PHY231

  24. Solar power on earth • The sun’s surface temperature is about 5800 K, its radius 7.0x108 m and its emissivity 0.97. Assuming the distance of the earth to the sun is about 1.5x1011 m, what is the intensity received on earth from the sun? (s=5.67x10-8 W/m2/K4) • A) 1.4 W/m2 • B) 14 W/m2 • C) 0.14 kW/m2 • D) 1.4 kW/m2 PHY231

  25. Solar power on earth earth Distance sun-earth A) 1.4 W/m2 B) 14 W/m2 C) 0.14 kW/m2 D) 1.4 kW/m2 sun PHY231

  26. Doppler effect • The frequency detected by an observer varies if observer and/or source move • Higher frequency when source and observer move toward each other. Lower frequency when they move away from each other • End result (general case) fo : Freq. measured by the observer fs : Freq. emitted by the source v : speed of propagation for wave vo : observer velocity in the medium vs : source velocity in the medium !!! v0>0 when pointing toward source (<0 otherwise) !!! vs>0 when pointing toward observer (<0 otherwise)

  27. Wavelength illustration • Wavelength of the wavefront travelling with the moving source is shortened • Wavelength of the wavefront travelling opposite to the moving source is lengthened lwm Frequency doesn’t change in the direction perpendicular to the motion source doesn’t move lwm lwm source moves l- < lwm (frequency increases) l+ > lwm (frequency decreases) l same everywhere

  28. If observer moves • 1) Example showing how formula simplifies (works for all cases) Observer moves toward wave front He will have impression the wavelength is even smaller (i.e. higher frequency) PHY231

  29. Train • A train station bell gives off a fundamental tone of 500 Hz as the train approaches the station at a speed of 20 m/s. If the speed of sound in air is 335 m/s, what will be the apparent frequency of the bell to an observer riding the train? • A) 532 Hz • B) 530 Hz • C) 470 Hz • D) 472 Hz PHY231

  30. Train • A train station bell gives off a fundamental tone of 500 Hz as the train approaches the station at a speed of 20 m/s. If the speed of sound in air is 335 m/s, what will be the apparent frequency of the bell to an observer riding the train? • A) 532 Hz • B) 530 Hz • C) 470 Hz • D) 472 Hz PHY231

  31. Train • You stand by the railroad tracks as a train passes by. You hear a 1 000-Hz frequency when the train approaches, which changes to 800 Hz as it goes away. How fast is the train moving? The speed of sound in air is 340 m/s. • A) 15.7 m/s • B) 21.2 m/s • C) 28.0 m/s • D) 37.8 m/s PHY231

  32. Train • A) 15.7 m/s • B) 21.2 m/s • C) 28.0 m/s • D) 37.8 m/s PHY231

More Related