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Presenters: Peter Wasko, Mn/DOT Metro District

Mel Roseen, Mn/DOT Environmental Services

Anne Claflin, Minnesota Pollution Control Agency

FHWAMn/DOT MPCA

Basics of AcousticsBasics of Acoustics

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What is Noise?What is Noise?

Noise is any unwanted sound that a person hears

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What is sound then?What is sound then?

• Sound is vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being heard by the average human ear.

• Sound pressure levels are used to measure the intensity of sounds and are described in terms of decibels.

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Common noise sourcesCommon noise sources

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Source-Path-Receiver ConceptSource-Path-Receiver Concept

Source

PathReceiver

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FrequencyFrequency

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FrequencyFrequency

• Frequency is the number of pressure cycles that pass a point per second

• Frequency=cycles per second=Hertz (Hz)

• Human hearing is in the range of 20 to 20,000 Hz

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Speed of soundSpeed of sound

• Sounds travels at a rate of 1,126 feet per second in air of 58 degrees F

• Which corresponds to about 1 mile every 5 seconds

• The speed of sound is proportional to the square root of the temperature

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ExampleExample

• What is the wavelength of a sound with a frequency of 5,000 Hz? (assume speed of sound is 1,126 feet per second)

• 1,126 feet per second / 5,000 cycles per second =.23 feet or 2.7 inches

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Sound PressureSound Pressure

• Sound pressure amplitude determines the loudness of the sound.

• Sound pressure in air can be measured in units of micro Newtons per square meter (N/M2) or micro-Pascals (Pa).

• The human ear can detect a wide range of sound pressure. Usually from a range of

20 Pa to 200,000,000 Pa.

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Sound PressureSound Pressure

200,000,000 Pa = 2 X 108 Pa20,000,000 Pa = 2 X 107 Pa 2,000,000 Pa = 2 X 106 Pa 200,000 Pa = 2 X 105 Pa20,000 Pa = 2 X 104 Pa 2,000 Pa = 2 X 103 Pa 200 Pa = 2 X 102 Pa 20 Pa = 2 X 101 Pa

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Sound Pressure Levels and Sound Pressure Levels and DecibelsDecibels

• The square of sound pressure is proportional to sound power or sound energy.

• A measure of Sound Pressure Level (SPL) is the decibel; defined as

dB = 10 log10 (P1/P0)2

where: P1 = pressure value of interest P0 = a standard reference value of 20 Pa RMS

• The quantity (P1/P0)2 is called the “relative energy.”

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Sound Pressure Levels and Sound Pressure Levels and DecibelsDecibels

Relative Pressure Relative Energy Decibel, dB

(P1/P0)     (P1/P0)2     10 Log (P1/P0)

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100 = 1 100 = 1 0

100.5 = 3 101 = 10 10

101 = 10 102 = 100 20

101.5 = 32 103 = 1,000 30

102 = 100 104 = 10,000 40

102.5 = 316 105 = 100,000 50

103 = 1,000 106 = 1,000,000 60

103.5 = 3,162 107 = 10,000,000 70

104 = 10,000 108 = 100,000,000 80

104.5 = 31,623 109 = 1,000,000,000 90

105 = 100,000 1010 = 10,000,000,000 100

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Addition and Subtraction of Addition and Subtraction of Sound Pressure Levels (SPL)Sound Pressure Levels (SPL)

• dB levels may not be added or subtracted directly

• Relative energy values may be added or subtracted directly

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Addition and Subtraction of Addition and Subtraction of Sound Pressure Levels (SPL)Sound Pressure Levels (SPL)

Example: A source produces a sound pressure level of 70 dB. A second 70 dB source is added next to the first source.

What is the combined sound level of the 2 sources?• 70 dB + 70 dB does not equal 140 dB. Relative energy

values must be added.• Relative energy for each source =10(70/10)=10,000,000• Relative energy for both sources is 20,000,000• SPL for both sources=10 Log (20,000,000)=73 dB

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Addition and Subtraction of Addition and Subtraction of Sound Pressure Levels (SPL)Sound Pressure Levels (SPL)

Doubling sound energy increases sound levels by 3 decibels

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Addition and Subtraction of Addition and Subtraction of Sound Pressure Levels (SPL)Sound Pressure Levels (SPL)

When Two Values Differ By: Add to Higher Value 0 to 1 dB 3 dB 2 to 3 dB 2 dB 4 to 9 dB 1 dB 10 or more dB 0 dB

Example: 65 dB+ 70 dB = 71 dB

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It’s movie time!!It’s movie time!!

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What are A-weighted decibels (dBA)?What are A-weighted decibels (dBA)?

The sensitivity of the human ear to sound depends on the frequency or pitch of the sound. People hear some frequencies better than others. If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This occurs because people hear high frequency noise much better than low frequency noise.

A-weighting serves two important purposes:

1. gives a single number measure of noise level by integrating sound levels at all frequencies

2. gives a scale for noise level as experienced or perceived by the human ear

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A, B,A, B, & C Weighting Network Filters& C Weighting Network Filters

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Changes in noise levels in Changes in noise levels in an outdoor environmentan outdoor environment

• 3 dBA (increase or decrease) is barely perceptible

• 5 dBA (increase or decrease) is clearly noticeable

• 10 dBA (increase or decrease) is perceived as twice as loud (or half as loud)

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Refraction and Wind Refraction and Wind GradientsGradients

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Refraction and Temperature Refraction and Temperature GradientsGradients

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Noise Path without a BarrierNoise Path without a Barrier

Direct

Source

Receiver

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Noise Path with a BarrierNoise Path with a Barrier

Source

Receiver

Barrier

DiffractedTransmitted

Reflected

Direct

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Geometric Relationship Between Traffic and Receiver

Less loud by 3 dBA

dBA change = 10 log(D1/D2)

Hard reflective ground surface

Hard reflective ground surface

D

2D

70 dBA

67 dBA

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Geometric Relationship Between Traffic and Receiver

Attenuation increases by an additional 1.5 dBA for a total of 4.5 dB

dBA change = 15 log(D1/D2)

Soft absorptive ground surface

Soft absorptive ground surface

70 dBA

D

2D

65.5 dBA

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Importance of Breaking Line of SightImportance of Breaking Line of Sight

Source

Receiver

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Importance of Breaking Line of SightImportance of Breaking Line of Sight

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Effect of Barrier on Attenuation Over Distance

- 4.5 dBA/DD

-3 dBA/DD

100’

L (h)= 72 dBAEQ

L (h)= 63.5 dBAEQ

Field Insertion Loss = “Before” – “After” = 8.5 dBA

Wall Attenuation = 10 dBA

Line Source

Line Source

Wall Attenuation

10 dBA

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Parallel Barrier Reflections

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Questions?Questions?