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**Basics of Acoustics Mn/DOT MPCA FHWA Presenters:**

Peter Wasko, Mn/DOT Metro District Mel Roseen, Mn/DOT Environmental Services Anne Claflin, Minnesota Pollution Control Agency

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**Noise is any unwanted sound that a person hears**

What is Noise? Noise is any unwanted sound that a person hears

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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 sources

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**Source-Path-Receiver Concept**

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Frequency

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Frequency 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 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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Example 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 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 (mN/M2) or micro-Pascals (mPa). The human ear can detect a wide range of sound pressure. Usually from a range of 20 mPa to 200,000,000 mPa.

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**Sound Pressure 200,000,000 mPa = 2 X 108 mPa**

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**Sound Pressure Levels and Decibels**

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

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**Sound Pressure Levels and Decibels**

Relative Pressure Relative Energy Decibel, dB (P1/P0) (P1/P0)2 10 Log (P1/P0)2 100 = 1 100.5 3 101 10 102 20 101.5 32 103 1,000 30 104 10,000 40 102.5 316 105 100,000 50 106 1,000,000 60 103.5 3,162 107 10,000,000 70 108 100,000,000 80 104.5 31,623 109 1,000,000,000 90 1010 10,000,000,000

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

Doubling sound energy increases sound levels by 3 decibels

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

When Two Values Differ By: Add to Higher Value to 1 dB dB to 3 dB dB 4 to 9 dB dB or more dB dB Example: 65 dB+ 70 dB = 71 dB

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It’s movie time!!

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**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, & C Weighting Network Filters**

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**Changes in noise levels in an 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 Gradients**

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**Refraction and Temperature Gradients**

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**Noise Path without a Barrier**

Direct Source Receiver

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**Noise Path with a Barrier**

Diffracted Direct Transmitted Reflected Source Receiver

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**Geometric Relationship Between Traffic and Receiver**

70 dBA Hard reflective ground surface 2D 67 dBA Geometric Relationship Between Traffic and Receiver Less loud by 3 dBA dBA change = 10 log(D1/D2) Hard reflective ground surface 23

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**Geometric Relationship Between Traffic and Receiver**

70 dBA Soft absorptive ground surface 2D Attenuation increases by an additional 1.5 dBA for a total of 4.5 dB dBA change = 15 log(D1/D2) 65.5 dBA Geometric Relationship Between Traffic and Receiver Soft absorptive ground surface 24

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**Importance of Breaking Line of Sight**

Source Receiver

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**Importance of Breaking Line of Sight**

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**Effect of Barrier on Attenuation Over Distance**

L (h)= 72 dBA EQ - 4.5 dBA/DD Line Source 100’ L (h)= 63.5 dBA EQ Geometric Relationship Between Traffic and Receiver Wall Attenuation 10 dBA -3 dBA/DD Line Source Field Insertion Loss = “Before” – “After” = 8.5 dBA Wall Attenuation = 10 dBA 27

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**Parallel Barrier Reflections**

Geometric Relationship Between Traffic and Receiver

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