Fundamentals of noise measurements

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Fundamentals of Noise Measurements

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Fundamentals of noise measurements

  1. 1. Fundamentals of Noise Measurements NIDHIN MANOHAR
  2. 2. • Sound - Defined as any pressure variation in a medium (Air, Water or other mediums) that the human ear can detect • Sounds unpleasant and unwanted are called “Noise • “Audible dynamic range of human ear 20 x 10-6 Pa to 200 Pa 20 x 10-6 Pa Threshold of Hearing of 0 dB 200 Pa - Threshold of Pain - 140 dB 160 dB - Threshold of permanent deafness • Sounds of different frequency, at a constant sound pressure level do not evoke equal loudness sensations • Loudness level expressed as Phon 0 Phon is 0 dB SPL at 1000 Kz 40 Phon is 40 dB SPL at 1000 Hz
  3. 3. The Ear’s Most Important Function : To Receive Speech Important Ranges of Human Bearing : Range of Hearing 20 Hz – 20 kHz All sounds preceived by normal subjects Range of Speech 100 Hz – 10 kHz All sound produced in human speech Range of Articulation 200 Hz – 6 Hz Range needed to hear every syllable of English speech
  4. 4. Range of Intelligibility 500 Hz – 2500 Hz Range needed to understand English speech (even though every syllable may not be properly heard) Minimum Audible Field (MAF) Level at which each tonal signal is barely received by people with normal hearing a) b) Highest Sensitivity Lowest Sensitivity 500 – 2500 Hz 20 – 100 Hz, 15,000 – 20,000 Hz
  5. 5. Why measure sound? • Provide definite quantities which describe and rate sound • Permit precise,scientific analysis of annoying sound • Engineering aspects of noise control • Ascertain the probable damage to ears • Improvement in building design • Diagnostic tool
  6. 6. Parameters 1. Sound Pressure 2. Sound Power 3. Sound Intensity Sound pressure level (SPL) P 20 LOG dB Po P = RMS Sound Pressure Po = Reference sound pressure 20 x 10-6 Pa (Threshold Hearing)
  7. 7. Equivalent sound pressure level 1 Leq = 10 log T P (t) ∫ P o 2 dt 2 T = Measurement P(t) = Sound Pressure Leq has the same energy content as the varying sound level SPL Depends on - Distance Orientation of Receiver relative to machine Environment of measurement room
  8. 8. Why Use A Logarithmic Measure Of Sound ? 1. Enables coverage of entire hearing range on single meter. 2. Corresponds roughly to ear’s behavior: 1 unit 10 units 32 units 100 units 1000 units Loud. Twice as loud. Three times as loud. Four times as loud. Eight times as loud. 3. Corresponda to ear’s perception of change: 1 db change…….. Hardly noticeable. 3 db “ ……... Noticeable, not significant. 5 db “ ……... Significant 10 db “ ……... Double loudness 0.5 db “ ……... Noticeable only in “A-B” tests.
  9. 9. Sound power level (SWL) P 20 LOG dB Po W = Acoustic power of source W0 = Reference sound power = 10-12 • SWL is a function of source only and is independent of the acoustic environment • Widely used for rating and comparing equipment and also in noise control.
  10. 10. Sound Intensity level 1 SIL = 10 LOG I0 Io = Reference Intensity = 10-12 W/M2 • Average rate of flow of energy per unit time through a unit area • Uses special intensity probes – Two microphones for measurement • Used for identification of noise sources • Provides information on direction of acoustical energy flow
  11. 11. Intensity 1. Intensity is a vector quantity as it has both magnitude and direction. 2. Intensity is a measure of the concentration of acoustic power across a unit area. 3. Intensity is dependent on the source’s properties and distance from the source. 4. Intensity is difficult to measure directly, but can be determined indirectly from sound pressure measurements.
  12. 12. Equipment A – Weighted Sound Power Levels Co Comprehensive (3.5 – 17 m3 min) Pneumatic hand tools Axial flow fans (0.05 m3 min-50 m3min); 10 mm H2O 85 - 120 105 –123 61-88 Axial flow fans (0.05 m3 min-50 m3min); 300 mm H2O 88 - 120 Centrifugal fans (0.05 m3 min-50 m3 min); 10 mm H2O 45 - 77 Centrifugal fans (0.05 m3 min-50 m3 min); 300mm H2O 75 - 108 Propeller fans (0.05 m3 min-50 m3 min); 10 mm H2O 62 - 94 Propeller fans (0.05 m3 min-50 m3 min); 300 mm H2O 94 - 125 Centrifugal pumps (>1600 rpm ) 105 - 132 Screw Pumps ( >1600 rpm ) 110 - 137
  13. 13. Equipment A – Weighted Sound Power Levels Reciprocating pumps(>1600 rpm) 115 - 138 Pile driving equipment (upto to 6 ton drop hammer 103 - 131 Electric saws 96 - 126 Generators ( 1.25 – 250 kVA) 99 - 119 Industrial vibrating screens 100 - 107 Cooling towers 95 - 120 Room air-conditioners (up to 2 hp) 55 - 85 Tractors and trucks 110 - 130
  14. 14. Sound power level data are useful • To calculate the approximate SPL at a given distance from a machine operating in a specified environment • To compare the noise radiated by machines of same type and size • To compare the noise radiated by machines of different types and sizes • To determine whether a machine complies with a specified upper limit of sound emission • To plan in order to determine the amount of transmission loss or noise control required under certain circumstances • To assist in developing quiet machinery and equipment.
  15. 15. Q 4 LP = Lw + 10 log 10 [ + ] dB 2 4πr R Sα R = Room constant = 1−α S1α1 + S 2α 2 + ........... = S1 + S 2 Q = Directivity factor depends on the shape and the complexity of source
  16. 16. Free space, spherical radiation Centre of flat surface, hemispherical radiation Centre of edge formed by junction of two adjacent flat surfaces Corner formed by junction of three adjacent surfaces For a typical air conditioned room at a distance of 1M LP = Lw – 8 dB Q=1 Q=2 Q=4 Q=8
  17. 17. Meter response control FAST – Meter responds quickly to step changes in continous sound level, approximately 0.2 second required SLOW –Meter responds slowly to step changes in continuous sound level. Approximately 1.0 second required IMPULSE – Meter responds to maximum RMS value of repetitive impulsive sounds. PEAK -Meter responds to maximum peak value of impulsive sound, even of single impulses

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