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Lecture OneLecture One
Noise PollutionNoise Pollution
Introduction to noise pollutionIntroduction to noise pollution Noise is unwanted sound because it:
can cause hearing loss interferes speech communication disturbs moods, relaxation, and privacy
Sound is a form of energy produced by a vibrating object or an aerodynamic disturbance
Environmental noiseEnvironmental noise Major sources
Transportation Industry Construction Buildings and households Humans and pets
SoundSound Energy produced by
Vibrating objects Aerodynamic disturbances
Energy causes disturbances in air molecules producing variations in air pressure Causes eardrums to vibrate and transfer
vibrations to inner ear
SoundSound Must move through a medium of
molecules Results in differences in speed of sound
transmission
Sound energySound energy
Vibration of objects produces sound energy which radiates outward as acoustical energy
Energy produced by a sound described as sound power
Sound powerSound power
Sp values expressed in dimensionless units called decibels
Calculated from the following equation
Sound powerSound power Where
Wm = sound energy in watts
Wr = reference sound energy, 10 -12 watts
Logarithm of the ratio of measured and reference sound energy multiplied by 10
Sound intensitySound intensity As sound radiates outward from a
source this flow is described as sound intensity (SI) Watts/m2, expressed as dB
Sound intensitySound intensity
Where Im = measured sound intensity
Ir =reference sound intensity
Sound pressure levelsSound pressure levels
Commonly measured with instruments to record sound levels
Expressed in decibels, dB
Sound pressure levelsSound pressure levels Where
Pm = measured pressure
Pr = reference pressure, the threshold of human hearing, 2 x 10-5 N/m2 (Pa) or 2 x 10-4 μbars
Sound pressure levelsSound pressure levels
Calculation: where Pm = 2.0 μbars
Frequency Frequency
Major characteristic of sound Inversely related to wavelength Discrimination by humans constitutes
hearing Expressed as cycles/sec or hertz(Hz)
Frequency Frequency Sound sources produce a range of
frequencies Most sounds characterized by dominant
frequencies Range of human hearing 20-20,000 Hz
Range of human hearingRange of human hearing
Range of human hearingRange of human hearing
Humans here best in the middle frequencies
Low frequencies (< 16 Hz) -infrasound High frequencies (> 20,000 Hz) -
ultrasound
Sound levels, sources, and Sound levels, sources, and human responseshuman responses
Sound measurementSound measurement Use sound pressure level meters
Weighting Network or ScalesWeighting Network or Scales
Different scales have different discriminations on lower frequency waves
-A scale: < 600 Hz-B scale: < more moderate-C scale: little discrimination
20 dB(A), 20 dB(B), 20 dB(C) etc.
It is common to use A scale.
Meter responseMeter response Slow and fast options
Averaging timeFast response- 0.125 secondsSlow response - 1 second
Use of slow response recommended
Instrument accuracyInstrument accuracy
Rated type 1, type 2, and type 3 Type 1 instrument +/- 1dB Type 2 instrument +/- 2 dB Type 3 instrument +/- 3 dB
Impulse soundImpulse sound
Rapidly rising and falling sound pressure
Need specially designed instruments to measure
Spectrum analysisSpectrum analysis
Octave bands used for spectral analysis Sound pressure levels at different frequency
band Commonly 10 octave bands are used
Upper frequency twice the lower frequencyCharacterized by the center frequency
Octave bandsOctave bands
Equivalent sound levelsEquivalent sound levels
Equivalent sound level = Leq
Average of all sounds measured on the A scale
Correlates well with effects of noise in humans
Used to report environmental noise levels
Equivalent sound levelsEquivalent sound levels
Where Li = each measured value in decibels
N = number of values
Averaging sound levelsAveraging sound levels
10 values – 65, 75, 68, 70, 80, 72, 76, 78, 82, 65 dB
Addition: Similarly, when individual values are added:
Ltotal = 10 log 10Li/10
Subtraction:
L = 10 log (10Ls/10-10Lb/10)
Where: Ls is the noise source and Lb is the background
Total sound levelsTotal sound levels Two sounds of 80 dB
Doubling results in 3 dB increase
dBALa 83]1010[log10 8810
Chart for Adding Decibels:Chart for Adding Decibels:(1) Determine the difference between the two(2) use the following table to add the corresponding increment to the HIGH level