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Acoustic Engineering Calculator

聲學知識 2026-02-10
Acoustic Engineering Calculator
Acoustic Tools

NIC and STC Conversion Formula

STC / NIC / EQ
\[ NIC = STC_{eq} - 10\log_{10}\left(\frac{S}{A}\right) \]
Formula Explanation
1) STCeq (Equivalent STC) is the composite rating calculated by integrating multiple components on the same partition. Example:Door STC 39dB, glass wall STC 45dB, and possibly a small portion of drywall STC 52dB.
2) S = Tested wall/partition area (m²)
3) A = Receiving room absorption (Sabins)

① Equivalent STC (Wall + Door)

Calculate the equivalent transmission loss of the entire partition using sound energy and area weighting.

STC in the NIC formula must be replaced by Equivalent STC (STCeq) because each segment of the partition has its own laboratory rating: door, glass wall, drywall, etc. all have different transmission loss performance. When a single partition contains multiple materials, their STC values must be integrated using area-weighted energy summation to produce one representative dB value. This composite value is called Equivalent STC.

In practice, glass wall projects usually consist of only two components: glass panels and doors. It is less common to have glass + drywall on the same test partition. Therefore, this tool currently provides inputs for glass wall and door only. If other materials exist, you can first compute their equivalent STC using the same area-weighted principle, then input the result here.

Enter values and click "Calculate Equivalent STC".
\[ TL_{eq}= -10\log_{10}\left( \frac{S_w10^{-TL_w/10}+S_d10^{-TL_d/10}}{S_w+S_d} \right) \]
Key point: Equivalent STC is NOT an arithmetic average; the door is typically the weakest element.

② NIC Calculator

Calculate NIC using the formula (Input STCeq, tested area S, and receiving absorption A. For quick estimation, A=2 is often acceptable).

Enter values and click "Calculate NIC".
\[ NIC = STC_{eq} - 10\log_{10}\left(\frac{S}{A}\right) \]

If the receiving room absorption comes from multiple materials, total absorption can be estimated by summation: \[ A = \alpha_1 S_1 + \alpha_2 S_2 + \alpha_3 S_3 + \dots \] The absorption coefficient \(\alpha\) ranges from 0 to 1 (1 = ideal absorption, 0 = no absorption). Typical carpet tiles (PVC backing) are often \(\alpha \approx 0.2\sim0.3\). For example, a 10 m² room with only carpet (\(\alpha=0.2\)) gives: \(A = 0.2 \times 10 = 2\) Sabins.

In typical office conditions, \(10\log_{10}(S/A)\) often results in a deduction of about 6–8dB (depending on receiving room absorption). If you want a higher NIC result, increasing absorption in the receiving room is one possible strategy.

③ Door Area Ratio vs Equivalent STC Curve

Fix wall STC and door STC, then scan door area ratio (1%–50%) and generate the equivalent STC curve.

This curve can be used to explain the nonlinear behavior: as door ratio increases, the equivalent STC drops faster.

④ Target Equivalent STC → Required Door STC

Given target STCeq, wall STC, and wall/door areas, compute the required door STC.

Enter values and click "Calculate Required Door STC".
If the result cannot be computed, it usually means: the target STCeq is too high, the door ratio is too large, or the wall STC is insufficient to support the target.
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