When evaluating a site for compliance with the HUD (U.S. Department of Housing and Urban Development) guidelines, there are generally two steps.
- Predict the noise levels outside the proposed building
- If necessary, predict the interior noise level and insure the components (windows, walls, roof, doors) provide interior noise levels below the required DNL 45 dBA.
The DNL descriptor is a 24-hour descriptor computed by averaging (on an energy basis) the hourly equivalent sound level (Leq) measured in each hour during a 24-hour period after 10 dB is added to the levels measured between 10 pm and 7 am.
As stated in Title 24, Code of Federal Regulations 51.103(c), the degree of acceptability of the noise environment is determined by the sound levels at a location two meters (6.5 feet) from the building housing noise sensitive activities in the direction of the predominant noise source. The site acceptability standards are shown in Table 1.
HUD Site Acceptable Standards
|Acceptability||DNL (dBA)||Special approvals and requirements|
|Acceptable||Not exceeding 651||None|
|Normally Unacceptable||Above 65 but not exceeding 75||Special approvals, environmental review, attenuation2|
|Unacceptable||Above 75||Special approvals, environmental review, attenuation3|
|Note 1 Acceptable threshold may be shifted to 70 dBA in special circumstances.|
Note 2 5 dB additional attenuation required for sites above 65 dBA but not exceeding 70 dBA and 10 dBA additional attenuation required for sites above 70 dBA but not exceeding 75 dBA.
Note 3 Attenuation measures to be submitted to the Assistant Secretary for Community Planning and Development for approval on a case-by-case basis.
Source: 24 CFR 51.103
The exterior noise level is predicted using HUD formulas (or online calculator) to prediction the noise from roads, aircraft, and rail. The details for each noise source are entered to generate the total DNL. For road noise, the traffic speed, mix, volume, and barriers are all considered. For aircraft noise, the noise contours from the airports are used. For rail noise, the number of engines, number of rail cars, speed, horns, and number of trains per day are entered.
To predict the interior noise level, the HUD Sound Transmission Classification Assessment Tool (STraCAT) is used. This model uses standard Sound Transmission Class (STC) ratings for partition materials and applies adjustments for the size of each partition and the exterior noise level. If necessary, components may need to be improved to reach to required interior noise level.
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