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Presenters: Peter Wasko, Mn/DOT Metro District

Basics of Acoustics. Presenters: Peter Wasko, Mn/DOT Metro District Mel Roseen, Mn/DOT Environmental Services Anne Claflin, Minnesota Pollution Control Agency. Mn/DOT. FHWA. MPCA. What is Noise?. Noise is any unwanted sound that a person hears.

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Presenters: Peter Wasko, Mn/DOT Metro District

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  1. Basics of Acoustics Presenters: Peter Wasko, Mn/DOT Metro District Mel Roseen, Mn/DOT Environmental Services Anne Claflin, Minnesota Pollution Control Agency Mn/DOT FHWA MPCA

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

  3. 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.

  4. Common noise sources

  5. Source-Path-Receiver Concept Source Receiver Path

  6. Frequency

  7. 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

  8. 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

  9. 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

  10. 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.

  11. 200,000,000 mPa= 2 X108mPa 20,000,000 mPa= 2 X107mPa 2,000,000 mPa= 2 X106mPa 200,000 mPa= 2 X105mPa 20,000 mPa= 2 X104mPa 2,000 mPa= 2 X103mPa 200 mPa= 2 X102mPa 20 mPa= 2 X101mPa Sound Pressure

  12. 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.”

  13. Sound Pressure Levels and Decibels

  14. 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

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

  16. Addition and Subtraction of Sound Pressure Levels (SPL) Doubling sound energy increases sound levels by 3 decibels

  17. Addition and Subtraction of 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

  18. It’s movie time!!

  19. 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

  20. A, B,& C Weighting Network Filters

  21. 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)

  22. Refraction and Wind Gradients

  23. Refraction and Temperature Gradients

  24. Noise Path without a Barrier Direct Source Receiver

  25. Noise Path with a Barrier Barrier Diffracted Direct Transmitted Reflected Source Receiver

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

  27. Geometric Relationship Between Traffic and Receiver D 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 Soft absorptive ground surface 24

  28. Importance of Breaking Line of Sight Source Receiver

  29. Importance of Breaking Line of Sight

  30. L (h)= 72 dBA L (h)= 63.5 dBA EQ EQ Effect of Barrier on Attenuation Over Distance - 4.5 dBA/DD Line Source 100’ Wall Attenuation 10 dBA -3 dBA/DD Line Source Field Insertion Loss = “Before” – “After” = 8.5 dBA 27 Wall Attenuation = 10 dBA

  31. Parallel Barrier Reflections

  32. Questions?

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