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Calcimator

Noise Reduction Coefficient Calculator

Analyze sound flanking transmission paths. Input the STC of the direct path and each flanking path (walls, floor, ceiling) to find the apparent STC and weakest link.

About this calculator

This calculator finds the Apparent STC of a partition once flanking transmission is accounted for -- the real-world sound isolation a partition delivers is almost always worse than its lab-rated direct-path STC, because sound also travels around the wall through the side walls, floor, and ceiling plenum. Each path's STC is converted to a transmission coefficient (tau = 10^(-STC/10)), the coefficients from all five paths are summed as independent parallel energy paths, and the combined total is converted back to an Apparent STC in decibels -- the same energy-summation approach used in ASTM E336 field sound isolation measurement. Direct Path STC is the dominant input on Apparent STC and always moves it in the same direction: raising Direct Path STC always raises or holds Apparent STC, never lowers it, because a higher direct-path STC means a smaller tau contribution from that one path while every flanking path's contribution stays fixed.

But that relationship saturates rather than climbing without limit -- once Direct Path STC is high enough that the direct path's transmission coefficient is small compared to the combined flanking paths, Apparent STC stops improving even as Direct Path STC keeps rising, because the flanking paths become the binding constraint. That is exactly what Max Achievable STC and Weakest Path are for: Max Achievable STC shows the ceiling if every flanking path were eliminated (it always equals Direct Path STC), and Weakest Path identifies which single path -- direct or one of the four flanking routes -- is contributing the most transmitted energy right now, telling you where treatment would help most.

Inputs

Results

Apparent STC

47.4

Flanking penalty (dB)2.6
Weakest path (0-4)0
Weakest path contribution (%)54.6
Max achievable STC50
Improvement potential (dB)2.6

Figures current as of 2020. Source: ASTM E336-20, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings

How to Use This Calculator
  1. Enter Direct path STC, Flanking wall 1 STC, and Flanking wall 2 STC.
  2. Set Flanking floor STC and Flanking ceiling STC.
  3. Review the Apparent STC result.
  4. Use Flanking penalty (dB) and Weakest path (0-4) to inform your decision.

How the result changes with Direct path STC

Direct path STCApparent STC
2525
3837.8
7550.8
8050.8

What each input means

Direct path STC
STC rating of the primary separating wall or floor assembly (the direct sound path).
Flanking wall 1 STC
STC of the first flanking wall path (sound traveling through the side wall around the partition).
Flanking wall 2 STC
STC of the second flanking wall path (opposite side wall).
Flanking floor STC
STC of the floor flanking path (sound traveling through or under the floor).
Flanking ceiling STC
STC of the ceiling flanking path (sound traveling through or above the ceiling plenum).

What each result means

Apparent STC
Combined STC considering all transmission paths. Always lower than the direct path STC.
Flanking penalty (dB)
How many STC points are lost due to flanking paths (direct STC minus apparent STC).
Weakest path (0-4)
Path transmitting the most sound. 0 = Direct, 1 = Wall 1, 2 = Wall 2, 3 = Floor, 4 = Ceiling.
Weakest path contribution (%)
Percentage of total transmitted sound energy coming through the weakest path.
Max achievable STC
Maximum apparent STC if all flanking were eliminated (equals the direct path STC).
Improvement potential (dB)
Possible dB improvement if all flanking paths were perfectly treated.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Direct path STC = 50, Flanking wall 1 STC = 55, Flanking wall 2 STC = 55, Flanking floor STC = 60 = 5 input(s) provided
  2. Calculate Apparent STC
    Apparent STC = -10 * log10(tauTotal)
    47.4 = 47.4
  3. Calculate Flanking penalty
    Flanking penalty = directSTC - apparentSTC
    2.6 = 2.6
  4. Calculate Weakest path
    Weakest path
    0 = 0

Figures and sources

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why is the apparent STC always lower than the direct path STC?

Because flanking paths add their own transmitted sound energy on top of whatever passes directly through the partition. Apparent STC combines all five paths (direct plus four flanking routes) as parallel energy paths, so it can never exceed Direct Path STC -- Max Achievable STC, which equals Direct Path STC exactly, is the ceiling you would only reach if every flanking path were completely eliminated.

Why did improving the direct path stop raising the apparent STC?

Once the direct path's transmission coefficient becomes small relative to the combined flanking paths, the flanking paths -- not the direct path -- are setting how much sound energy actually gets through. Raising Direct Path STC further keeps shrinking its own contribution, but the total is now dominated by paths that haven't changed, so Apparent STC plateaus. Check Weakest Path to see which flanking route is now the binding constraint.

What does the weakest path output actually identify?

Weakest Path reports which of the five transmission paths (0 = direct, 1-2 = the two flanking walls, 3 = flanking floor, 4 = flanking ceiling) is currently contributing the most transmitted sound energy -- equivalently, the path with the lowest STC. Weakest Path Contribution shows what percentage of total transmitted energy comes through that single path, which tells you how much benefit treating just that one path would realistically provide.

If I improve the weakest flanking path, will apparent STC keep rising afterward?

It will rise, but eventually a different path -- whichever is now weakest -- becomes the new limiting factor, and further gains slow again until that one is addressed too. Because paths combine as parallel energy routes, apparent STC is always closest to whichever path currently transmits the most, not an average of all five.

Is this the same thing as the Noise Reduction Coefficient (NRC)?

No -- despite the name, this tool analyzes flanking sound transmission and apparent STC (airborne sound isolation between spaces), not the Noise Reduction Coefficient, which is a separate single-number rating of how much a material absorbs incident sound, averaged across four mid-range octave bands per ASTM C423. STC and flanking analysis address isolation between rooms; NRC addresses absorption within a single room.

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