1. 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

2. Noise is any unwanted sound that a person hears
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

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

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
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

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. 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

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 to 1 dB dB to 3 dB dB 4 to 9 dB dB or more dB 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
Diffracted
Direct
Transmitted
Reflected
Source
Receiver

26. 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

27. 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

28. Importance of Breaking Line of Sight
Source
Receiver

29. Importance of Breaking Line of Sight

30. 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

31. Parallel Barrier Reflections
Geometric Relationship Between Traffic and Receiver

32. Questions?