What’s That You Said? (Noise Pollution Lab)

Theory

Sound is trapped or “ducted” within a layer in which the speed of sound increases with increasing altitude. Thus, sound produced at ground level will tend have a greater impact on the surrounding area when the sound speed increases from the ground through at least the first few tens of meters in the atmosphere. It is critical to understand that it is the profile of sound speed, not the absolute value at any given height, that determines whether or not sound will be ducted!

The Speed of Sound

The calculation is based upon the temperature, wind speed and wind direction,

20.05ÖT + U (downwind), 20.05ÖT – U (upwind) [Upwind à Downwind]

where T is the temperature (Kelvin) and U is the component of the wind from the sound source toward (downwind) or away from (upwind) a particular direction. Notice how the wind direction is critical for determining areas where noise pollution impact will be the most severe. Those downwind of the noise source will generally suffer the most annoyance.

Data

Graphical and Tabular Sample Temperature and Wind Profiles

Sample Temperature (deg C) and Wind Speed Profiles, Morning and Afternoon

z(m, agl) T (early a.m.) T (mid p.m.) Speed (early a.m.) Speed (mid p.m.)

10 / 5.0 / 27.0 / 0 / 8
20 / 10.0 / 23.5 / 0 / 9
30 / 15.0 / 23.0 / 0 / 10
40 / 14.9 / 22.9 / 3 / 11
50 / 14.8 / 22.8 / 6 / 12
60 / 14.7 / 22.7 / 12 / 14
70 / 14.6 / 22.6 / 14 / 15
80 / 14.5 / 22.5 / 15 / 15
90 / 14.4 / 22.4 / 16 / 15
100 / 14.3 / 22.3 / 17 / 15
110 / 14.2 / 22.2 / 18 / 15

Actual Wind Direction Statistics- Happy Valley (% of time)

Calm NE SE SW NW

41 / 9 / 12 / 28 / 10

Procedure

A proposed limestone mine is slated to open at the “X” on the map. The mine wants operate 12 hours a day, seven days per week. Citizens from Park Forest have hired you as a consultant to determine if their rural and quiet neighborhood will be threatened by the mine.

You have gathered the data listed in the tables above and will be paid to do an impact assessment. The first step is to calculate the sound speed vertical profiles for the given morning and afternoon conditions. To do this, construct a table with six columns added to those above and list the following, then plot (construct graphs of) the resultant sound speed profiles for each case (Excel will work well). Remember, temperature must be in Kelvins for proper sound speed calculation.

Col.1-5 As above

6,7. Morning and afternoon sound speed profiles using only temperature- ignore winds

8,9. Morning and afternoon sound speed profiles for a location downwind of noise source

10,11. Morning and afternoon sound speed profiles for a location upwind of noise source

1. Hand in the completed table and graphs. (3 points)

Remember that your primary concern is areas to the northeast of the proposed mine- where most people live. The assessment should attempt to answer the following questions.

2. How do diurnal temperature (profile) variations affect the impact of noise from the mine? In other words, ignoring wind, would you expect more noise in the morning or during the afternoon? Why? (2 points)

3. What would be the effect (if any) of wind on the sound speed profile for a location 90 degrees right (or left) of the downwind wind direction (i.e. a house to the northeast of the mine with a wind blowing from the northwest or southeast)? (1 point)

4.  What would be the effect of wind on the sound speed profile for a location upwind of the mine? It will help to assume an isothermal profile and wind speed increasing with height- then sketch the upwind sound speed profile. What does this mean with respect to how noise pollution produced by the mine will affect that location, as compared to a downwind location (sketch a downwind sound speed profile if you like)? (2 points)

5.  How will your answer to question 4 change if the wind speed maximum were at and below 10 meters with speed decreasing with height? Explain your answer. (2 points)

6.  What range of wind directions will most directly affect the Park Forest neighborhood, given the wind speed profile diagrams above (speed increasing with height)? (1 point)

7.  Would a Bermuda high or a Nor’easter cause more noise pollution from the mine to affect Park Forest? Why? (1 point)

8.  What about other pollution (suggest some) that the mine might produce? Based upon the wind direction statistics above, where will it be carried most often? (1 point)

9.  Does it really make sense to treat this project as a point source for sound? In other words, might this project generate sounds or other pollution some distance from the mine? If so, name at least two sources. (2 points)

10.  How do you think the elevation between the mine and the neighborhood will affect potential noise impact? Draw a cross section of elevation between the mine and the center of Park Forest (y axis = elevation, x axis = distance). (3 points)

11.  Will the quality of life be adversely affected in Park Forest? Explain. (1 point)

12.  When should the mine be closed each (24-hour) day to reduce impact? Why? (1 point)

13.  Finally, if you had been hired by the mine- would the report have been different? Would you slant the data differently? What would you have suggested to the mine owners to help smooth over relations with their neighbors? (5 points)