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