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BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL)

BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL) DEPARTMENT OF ARCHITECTURE. FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE ONDO STATE. Outline Introduction The Nature of Sounds Properties of Sound Propagation of Sound

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BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL)

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  1. BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL) DEPARTMENT OF ARCHITECTURE. FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE ONDO STATE.

  2. Outline Introduction The Nature of Sounds Properties of Sound Propagation of Sound Sound Power and Intensity Effects of Barrier on Sound Conclusion

  3. Introduction Acoustics is the science of sound and it covers two areas, those of room acoustics and control of noise. Noise is unwanted or damaging sound which interferes with what people are trying to do, or sound which has an adverse effect on health and safety.

  4. Introduction This lecture covers basic architectural acoustics including the properties and nature of sound, the terms used to describe sound waves, and the relationship between sound pressure, sound intensity and sound power.

  5. The Nature of Sound Sound is a disturbance, or wave which moves through a physical medium (such as air, water, or metal) from a source to cause the sensation of hearing in animals.

  6. Sound waves Sound waves are longitudinal waves originating from a source and conveyed by a medium. They are characterized by velocity (v), frequency (f), wavelength (٨), and amplitude (a).

  7. Sound waves Compression in sound waves is a region of raised pressure. Rarefaction in sound waves is a region of lowered pressure.

  8. Sound waves Figure 1: Compression And Rarefaction Of Sound By A Vibrating Tuning Fork.

  9. Sound waves Figure 2: Visualization of sound rarefaction and compression in a coiled spring.

  10. Sound waves Figure 3: Sound wave illustration.

  11. Frequency range of sound Sounds produced by various sources can range from frequencies below 20Hz to 20,000Hz and above. Infrasound are sounds with frequencies below 20Hz.Ultrasound are sounds with frequencies above 20,000Hz.

  12. The Audible range of sound Audible sounds range from the threshold of audibility to the threshold of pain. The threshold of audibility is the lower Limit of hearing and it has a standard value of 1 picowatt per metre square (1pW/m²).

  13. The Audible range of sound The threshold of pain is the upper Limit of hearing and it has a standard value of 1 watt per metre square (1W/m²). Sounds below the lower limit of hearing are inaudible while the those above the upper limit may cause pain or even damage the human ear.

  14. The Audible range of sound Figure 4: Audible range of sound.

  15. The Audible range of sound Figure 5: Audible range of sound.

  16. The Audible range of soundThe sound level or decibel scale is the logarithm of the ratio of measured sound intensity to the intensity at the thresholdof audibility.

  17. The Audible range of soundThe loudness of a sound is determined by referring to the loudness or phon scale which shows sounds of various levels and frequencies which are perceived as of the same sound loudness.

  18. The Audible range of sound Figure 6: Equal loudness contours.

  19. The Audible range of sound Figure 7: Psychological and physiological effects of sounds

  20. Properties of Sound •Wave lengthThis is the distance between two successive pressure peaks. Its symbol is ٨ and it is measured in units of metres (m).

  21. Properties of Sound •PeriodThis is the time taken for one vibration cycle. Its symbol is T and it is measured in units of seconds (s).

  22. Properties of Sound •FrequencyThis is the number of vibration cycles per seconds. Its symbol is F and it is measured in units of Hertz (Hz). For the relationship between frequency and period, F=1/T

  23. Properties of Sound •Speed or wave velocityThis is the speed with which sound travels through a medium. Its symbol is C and it is measured in units of metres per seconds (m/s). For the relationship between the speed (C), frequency (F) and wave length (٨), C=F٨

  24. Properties of Sound Figure 8: Variation of speed of sound with the medium of transmission. .

  25. Properties of SoundFactors that affect the speed of sound through a medium •Elasticity of the medium•Density of the medium•Temperature of the medium

  26. Properties of Sound•Amplitude This indicates the intensity of sound. Its symbol is I and it is measured in units of watts per metres square (W/m²).

  27. Properties of Sound Figure 9: Amplitude illustration.

  28. Properties of SoundThe inverse square law of sound states that the intensity of sound in a free field is indirectly proportional to the square of the distance from the source. This infers that there is a decrease in the intensity of sound the farther the observer is from the source.

  29. Properties of Sound Figure 10: The inverse square law.

  30. Properties of Sound Figure 11: Amplitude illustration.

  31. Properties of Sound•Pitch This is the property of sound that is perceived as highness and lowness depending on the rapidity of the vibrations producing it . It is measured in cycles per second (cps).

  32. Properties of Sound Figure 12: Pitch illustration.

  33. Properties of Sound•Sound pressure This is the force per unit area and it gives the magnitude of the sound wave. Its symbol is p and it is measured in units of Pascal (Pa).

  34. Properties of SoundThe pressure changes produced by a sound wave are also known as sound pressure. Compared with atmospheric pressure on which they are superimposed (about 100,000 pascals), they are very small (between 20 micropascals and 200 pascals).

  35. Properties of Sound Figure 13:Changes in sound pressure over time.

  36. Properties of Sound Figure 14:Sound pressure superimposed on atmospheric pressure.

  37. Properties of Sound Figure 15:Relationship between sound pressure and sound frequency in a pure tone.

  38. Properties of Sound Figure 16:The characteristics of Machine noise.

  39. Propagation of SoundInside a room, close to a source like a machine, the direct sound dominates, and the sound pressure may vary significantly with just small changes in position.

  40. Propagation of SoundThis area is called the near field and its extent is about twice the dimension of the machine or one wavelength of the sound.

  41. Propagation of SoundThe area beyond the near field is called the far field made up of two sections, •The free field•The reverberant field

  42. Propagation of SoundIn the free field the direct sound still dominates and the sound pressure level decreases by 6 dB for each doubling of distance.

  43. Propagation of SoundIn the reverberant field the reflected sound adds to the direct sound and the decrease per doubling of distance of the sound pressure level will be less than 6 dB.

  44. Propagation of Sound Figure 17:The near field and far field of sound. Source: National Institute for Occupational Health and Safety (1988).

  45. Propagation of Sound Figure 18: Decrease in sound intensity for a point source with doubling of distance.

  46. Properties of Sound•Spherical wave frontsThese are produced when sound spreads out from a point source in a free space. The wave fronts are spherical and the sound pressure level decreases 6 dB for each doubling of distance.

  47. Propagation of Sound Figure 19:Decrease in sound intensity for an omnidirectional point source. source.

  48. Properties of Sound •Cylindrical wave frontsThese are produced when sound spreads out from a line source (such as a road with constant traffic or a pipe carrying fluid). The waves are cylindrical and the sound pressure level decreases 3 dB for each doubling of distance.

  49. Propagation of Sound Figure 20: Decrease in sound intensity for a line source with doubling of distance.

  50. Propagation of Sound Figure 21:Decrease in sound pressure level for a line source.

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