Environmental TechnologyEnvironmental Technology

Noise Assessment & Noise Assessment & AbatementAbatement

Module 7.0Module 7.0

Obj 7.1: What is sound?Obj 7.1: What is sound?

sound is sound is pressure pressure variationsvariations (high and (high and low density)low density)

that are created by that are created by mechanical vibrationsmechanical vibrations

and can be detected and can be detected by the by the tympanic tympanic membranemembrane (eardrum) (eardrum) of humans and animalsof humans and animals

How is it transmitted?How is it transmitted?

sound is sound is transmitted by any transmitted by any mediummedium that is that is capable of capable of compressioncompression and and expansionexpansion

solid, liquid, gassolid, liquid, gas concrete wall, concrete wall,

water, air, steel, water, air, steel, masonry, glassmasonry, glass

What are its characteristics?What are its characteristics? cyclecycle is distance is distance

(wavelength) between (wavelength) between peaks peaks

amplitudeamplitude (loudness) is (loudness) is a measure of pressure a measure of pressure intensityintensity

frequencyfrequency (pitch) is a (pitch) is a measure of how many measure of how many cycles per second (Hz)cycles per second (Hz)

What can humans hear?What can humans hear?

the average the average human ear can human ear can detect noise detect noise between 20 Hertz between 20 Hertz and 20,000 Hertzand 20,000 Hertz

Obj 7.2: Measuring sound....Obj 7.2: Measuring sound.... most of the sound we hear is most of the sound we hear is

transmitted by air, so we measure transmitted by air, so we measure air pressureair pressure

the human ear is capable of the human ear is capable of detecting detecting extremely smallextremely small differences in air pressuredifferences in air pressure

Obj 7.2: Measuring sound....Obj 7.2: Measuring sound.... normal atmospheric pressure is normal atmospheric pressure is

101.325 kiloPascals (kPa)101.325 kiloPascals (kPa) one one microbarmicrobar is one millionth of is one millionth of

the standard atmospheric pressure the standard atmospheric pressure the human ear is capable of the human ear is capable of

detecting pressures as small as detecting pressures as small as 0.0002 microbars and as large as 0.0002 microbars and as large as 2000 microbars without damage2000 microbars without damage

Obj 7.2: Measuring sound....Obj 7.2: Measuring sound.... A logarithmic scale called the…A logarithmic scale called the…

Decibel ScaleDecibel Scale

...is used to measure sound...is used to measure sound dBdB is the unit of Sound Pressure is the unit of Sound Pressure

Level (SPL)Level (SPL)

Obj 7.2: Measuring sound....Obj 7.2: Measuring sound....

SPL = 20 x log(P/PSPL = 20 x log(P/P00))

where Pwhere P00 = 0.0002 microbars (baseline) = 0.0002 microbars (baseline)

and P = measured sound pressureand P = measured sound pressure calculate SPL if P = 0.0002calculate SPL if P = 0.0002 calculate SPL if P = 2000 (painful)calculate SPL if P = 2000 (painful) calculate SPL if P = 1000calculate SPL if P = 1000

Obj 7.3: Weighting scaleObj 7.3: Weighting scale

sound is a combination of amplitude sound is a combination of amplitude (loudness) and frequency (Hz)(loudness) and frequency (Hz)

70 dB at 2000 Hz will seem louder 70 dB at 2000 Hz will seem louder than 70 dB at 500 Hzthan 70 dB at 500 Hz

for the urban planning field an “A” for the urban planning field an “A” weighting scale has been created to weighting scale has been created to combine the two measures of soundcombine the two measures of sound

Obj 7.3: Weighting ScaleObj 7.3: Weighting Scale

dB is adjusted to dBA to account dB is adjusted to dBA to account for frequencyfor frequency

it’s like a it’s like a wind chillwind chill factor used for factor used for temperaturetemperature

find the dBA for 70 dB at 2000 Hzfind the dBA for 70 dB at 2000 Hz find the dBA for 70 dB at 500 Hzfind the dBA for 70 dB at 500 Hz