This manual contains various procedures for identifying residential highway noise problems and selecting building modifications as solutions. The flowchart of Figure 25 shows a step-by-step procedure which applies this information directly to the problem of determining building modifications necessary to adequately insulate a building against the noise from an existing or proposed highway. The procedures of this manual can most effectively be applied by following the steps shown in the flowchart. A short explanation of each step is given below.
Step 1 | Calculate Building Noise Reduction (NRc) using the methods of Chapter 2. Use normally open or closed conditions for the windows. |
Step 2 | Calculate Interior Noise Level using the relation Lic = Lo - NRc. |
Step 3 | Check to see if the Calculated Interior Noise Level, Lic, plus 5 dB, is greater than the Interior Design Noise Level, Lc. The calculated level is increased by 5 dB to allow for uncertainty in the calculated interior level. If the calculated interior level plus 5 dB is greater than the Design Noise Level, measurements must be made. |
Step 4 | Measure the Exterior Noise Level, Lom, using the procedures of Chapter 3. If the subject highway is not in use, an artificial source must be employed. |
Step 5 | Measure the Interior Noise Level, Lim, using the procedures of Chapter 3. Use normally open or closed condition for the windows. |
Step 6 | Determine the Measured Noise Reduction using the relationship NRm = Lom - Lim - 5. |
Step 7 | If the Given Highway Noise Level, Lo, minus the quantity (NRm) is less than the Design Noise Level, then the building attenuation is sufficient. If not, building wall modifications must be planned. |
Step 8 | Plan Building Wall modifications using the procedures of Chapter 2. |
Step 9 | Calculate New Noise Reduction using the procedures of Chapter 2 for the additional building modifications selected. |
Step 10 | If the Given Noise Level, Lo, minus the Calculated Noise Reduction, NRc, is less than the Design Noise Level, then the building attenuation is sufficient. If not, additional modifications must be planned and Steps 8, 9, and 10 repeated until the condition of Step 10 is satisfied. |
Step 11 | Select Compatible Ventilation System: If ventilation has been restricted by the sealing of cracks and forced closing of windows in order to reduce interior noise levels, mechanical ventilation must be provided. This is discussed in Chapter 4. |
Step 12 | Determine Modification Costs: The costs of necessary modifications can be determined for comparative planning purposes using the methods of Chapter 5. |
Example 11: It is desired to determine whether design modifications will be necessary for a 2100 ft²(195 m²) house located adjacent to a heavily traveled highway. Because of local terrain, the only exterior building walls directly exposed to the highway are two living room walls, and the rest of the structure appears to be well shielded. The exterior unshielded noise level in the vicinity of the house is 67 dB. The living room walls and roof have the following characteristics:
Wall #1 - Constructed of standard wood studs, 16" (41 cm) on center, with an interior surface of 3/8" (1 cm) gypsum lath on ½" (1.3 cm) plaster, and an exterior surface of ¾" (1.9 cm) wood siding. There is thermal insulation in the stud space. The wall measures 16' 6" x 8'0" (4.9 x 2.4 m) with a 1-3/4" (4.4 cm) solid core wood door with drop threshold seal. The door measures 6'8" x 3'0" (2 x .9m).
Wall #2 - Constructed of furred interior surface finished with ½" (1.3 cm) gypsum board on an exterior of 6" (15.2 cm) block with ½" stucco. The wall measures 24'0" x 8' 0" (7.3 x 2.4 m) with a single glazed l/8"(.3 cm) glass window measuring 8'0" x 5'0" (2.4 x 1.5m).
Roof - This single story house has nonvented single joist ceiling construction with thermal insulation under a flat roof. Clay tiles cover the roof's exterior, with ½" (1.3 cm) fiberboard covering the interior ceiling.
The house is located in Los Angeles and has a central heating/air conditioning unit which operates on gas. Assuming the living room to be the most critical room, determine whether modifications will be necessary to achieve an interior design noise level of 32 dB.
Solution: Following the steps of tie Flow Diagram in Figure 25:
Step 1: Calculate Building Noise Reduction
Therefore: NRc = 29 dB, Calculated Noise Reduction (From end of Step 1, Figure 17)
Step 2 - Calculate Interior Noise Level Lic
Lo = Predicted outdoor noise level = 67 dB
Lic = Lo - NRc = 67 - 29 = 38 dB
Step 3 - Is Lic +5 < Lc?
Design Interior Noise Level = Lc = 32 dB
Is 38 +5 < 32? No.
Thus proceed to Step 4.
Step 4 - Measure Exterior Noise Level
Subject Highway is existing:
Exterior Leq is determined from measurements to be 71 dB = Lom
Step 5 - Measure Interior Noise Level
Interior Leq is determined from measurements to be 39 dB = Lim
Step 6 - Determine Measured Noise Reduction
Lom - Lim - 5 = 71 - 39 - 5 = 27 dB = NRm
Step 7 - ls Lo - NRm < L?
Is 67 - 27 < 32? (L is given in Step 2)
No. Thus proceed to Step 8.
Step 8 - Plan Modifications
Begin with a simple modification. Treatment of leakage paths in windows,
door, etc. will result in a structure EWNR increase of 4 dB.
Step 9 - Calculate New Noise Reduction
New Calculated Noise Reduction will be:
NRc (from Step 1) + 4 = 29 + 4 = 33 dB.
Step 10 - Is Lo - NRc < Lc?
Is 67 - 33 < 32? No.
Thus return to Step 8 and select additional modifications to give the needed Noise Reduction.
Step 8 - Plan Additional Modifications
In surveying all existing wall elements, the large window appears to be the weakest. Thus, it is specified that this window be double-glazed with
panes of 3/16 inch and ¼ inch separated by a 2-inch airspace.
Step 9 - Calculate New Noise Reduction
Worksheets No. 1 and No. 2 are again used.
The new window, from Table 7, will have an EWNR of 39 dB.
Thus the new NRc = 34 dB + 4 dB from sealing = 38 dB
Step 10 - Is Lo - NRc < Lc?
Is 67 -38 < 32? Yes.
Thus, the interior noise level criterion will be satisfied if the building sides facing the highway are well sealed and the specified double pane window is installed.
Step 11 - Select Compatible Ventilation System
Due to the high exterior noise levels, it is expected that the door and window would be kept shut at all times, relying on infiltration and mechanical ventilation throughout the year. Infiltration for the house will be greatly reduced by sealing the living room, however, placing a heavier ventilation demand on the HVAC system. This extra ventilation energy requirement is therefore a consequence of Noise Reduction modification.
The sealing and improved window will also result in heat energy savings. Proceed to Step 12 to calculate these energy changes.
Step 12 -