Sound Transmission Classification Assessment Tool (STraCAT)


The Sound Transmission Classification Assessment Tool (STraCAT) is an electronic version of Figures 17 and 19 in The HUD Noise Guidebook. The purpose of this tool is to document sound attenuation performance of wall systems. Based on wall, window, and door Sound Transmission Classification (STC) values, the STraCAT generates a composite STC value for the wall assembly as a whole. Users can enter the calculated noise level related to a specific Noise Assessment Location in front of a building façade and STraCAT will generate a target required attenuation value for the wall assembly in STC. Based on wall materials, the tool will state whether the composite wall assembly STC meets the required attenuation value.

How to Use This Tool

Location, Noise Level and Wall Configuration to Be Analyzed
STraCAT is designed to calculate the attenuation provided by the wall assembly for one wall of one unit. If unit exterior square footage and window/door configuration is identical around the structure, a single STraCAT may be sufficient. If units vary, at least one STraCAT should be completed for each different exterior unit wall configuration to document that all will achieve the required attenuation. Additionally, if attenuation is not based on a single worst-case NAL, but there are multiple NALs which require different levels of attenuation around the structure, a STraCAT should be completed for each differing exterior wall configuration associated with each NAL.

Exterior wall configurations associated with an NAL include those with parallel (facing) or near-parallel exposure as well as those with perpendicular exposure. When a façade has parallel or perpendicular exposure to two or more NALs, you should base the required attenuation on the NAL with the highest calculated noise level. For corner units where the unit interior receives exterior noise through two facades, the STraCAT calculation should incorporate the area of wall, window and door materials pertaining to the corner unit’s total exterior wall area (i.e., from both walls).

Information to Be Entered
Users first enter basic project information and the NAL noise level that will be used as the basis for required attenuation. This noise level must be entered in whole numbers. STraCAT users then enter information on wall, window and door component type and area. Again, as noted above, the wall, window and door entries are based on one unit, and one wall (except for corner units as discussed above). The tool sums total wall square footage based on the combined area of walls, doors and windows for the façade being evaluated.

Users may input STC values for materials in one of two ways. The tool includes a dropdown menu of common construction materials with STC values prefilled. If selected construction materials are not included in this dropdown menu, the user may also enter the STC for a given component manually. Verification of the component STC must be included in the ERR. Documentation includes the architect or construction manager’s project plans showing wall material specifications. For new construction or for components that will be newly installed in an existing wall, documentation also includes the manufacturer’s product specification sheet (cut sheet) documenting the STC rating of selected doors and windows.

Required STC Rating and Determination of Compliance
Finally, based on project information entered the tool will indicate the required STC rating for the wall assembly being evaluated and whether or not the materials specified will produce a combined rating that meets this requirement. Note that for noise levels above 75 dB DNL, either HUD (for 24 CFR Part 50 reviews) or the Responsible Entity (for 24 CFR Part 58 reviews) must approve the level and type of attenuation, among other processing requirements. Required attenuation values generated by STraCAT for NALs above 75 dB DNL should therefore be considered tentative pending approval by HUD or the RE.


Part I - Description

Part II - Wall Components

Wall Construction Detail Area STC
Browse wall materials by their sketches.
{{getWallSTC(| number}}
  {{stracatCtrl.getWallAreaTotal()| number}} Sq. Feet {{getWallSTCTotal() | number:2|number}}
Window Construction Detail Quantity Sq Ft/Unit STC
Browse window materials by their sketches.
{{stracatCtrl.getApertureArea(window.material.area)|number}} {{|number}}
Door Construction Detail Quantity Sq Ft/Unit STC
Browse door materials by their sketches.
{{stracatCtrl.getApertureArea(door.material.area)|number}} {{|number}}

Part III - Results

Wall Statistics
Stat Value
Area: {{stracatCtrl.totalWallArea || "0"}} ft²
Wall STC: {{stracatCtrl.totalWallSTC|number:2|number}}

Aperture Statistics
Aperture Count Area % of wall
Windows: {{stracatCtrl.getApertureQuantityTotal('window')|number}} {{stracatCtrl.getApertureAreaTotal('window')|number:3|number}} ft² {{stracatCtrl.getApertureWallPercentage('window')|number:2|number}}%
Doors: {{stracatCtrl.getApertureQuantityTotal('door')|number}} {{stracatCtrl.getApertureAreaTotal('door')|number:3|number}} ft² {{stracatCtrl.getApertureWallPercentage('door')|number:2|number}}%

Evaluation Criteria
Criteria Value
Noise source sound level (dB): {{stracatCtrl.intro.noiseLevel}}
Combined STC for wall assembly: {{stracatCtrl.getCompositeAttenuation()|number:2|number}}
Required STC rating: {{stracatCtrl.calculateRequiredAttenuation()}}
Does wall assembly meet requirements? {{stracatCtrl.getMeetsRequirements().value}}

Part 4 - Tips

What do you do if the preferred wall design is not sufficient to achieve the required attenuation? Another wall design with more substantial materials will work, but may not be the most cost-effective solution. Try adding some other elements for just a little more attenuation.

For example:

  • Staggering the studs in a wall offers approximately 4dB of additional protection.
  • Increasing the stud spacing from 16” on center to 24” can increase the STC from 2-5dB.
  • Adding a 2” air space can provide 3dB more attenuation.
  • Increasing a wall’s air space from 3” to 6”can reduce noise levels by an additional 5dB.
  • Adding a layer of ½” gypsum board on “Z” furring channels adds 2dB of attenuation.
  • Using resilient channels and clips between wall panels and studs can improve the STC from 2-5dB.
  • Adding a layer of ½” gypsum board on resilient channels adds 5dB of attenuation.
  • Adding acoustical or isolation blankets to a wall’s airspace can add 4-10dB of attenuation.
  • A 1” rockwool acoustical blanket adds 3dB to the wall’s STC.
  • Filling the cells of lightweight concrete masonry units with expanded mineral loose-fill insulation adds 2dB to the STC.