Specifying facade sound insulation using STC ratings is one of the most common errors in building acoustic design — not because STC is a bad metric, but because it was designed for the wrong application. STC was developed in the 1960s to evaluate interior partition performance against speech and common indoor noises. Its frequency contour emphasises 500–2000 Hz — exactly the range where human speech is concentrated. Traffic noise, aircraft noise, and rail noise concentrate at much lower frequencies: 63–500 Hz. Applying STC to facade elements systematically overestimates their performance against real outdoor noise sources.
OITC (Outdoor-Indoor Transmission Class) was developed specifically to address this gap.
The Problem with STC for Outdoor Noise
To understand why STC overestimates facade performance, you need to understand how both metrics are calculated.
How STC is Calculated (ASTM E413)
STC is calculated by comparing a partition's measured 1/3-octave transmission loss (TL) values from 125 Hz to 4000 Hz against a standard contour. The contour is shifted vertically until the sum of deficiencies (measured TL below the contour) does not exceed 32 dB, with no single deficiency exceeding 8 dB. The STC number is the value of the contour at 500 Hz when this condition is met.
The ASTM E413 STC contour has these key characteristics:
- Falls steeply from 1250 Hz downward (allows significant deficiency in low-frequency performance)
- Peaks at 2000–4000 Hz (emphasises speech clarity frequencies)
- The 32 dB total deficiency allowance means a partition can perform poorly at 125 Hz and 250 Hz without reducing its STC significantly
The Outdoor Noise Spectrum
Road traffic noise is dominated by:
- Engine combustion (50–250 Hz, especially from diesel trucks): 65–80 dBA octave band levels
- Tyre/road interaction (500–2000 Hz, from tyre pattern noise): 60–75 dBA
- Aerodynamic (1000–4000 Hz, at high speed): 55–65 dBA
| Octave Band (Hz) | Typical Level (dBA re reference) |
|---|---|
| 63 | 65 |
| 125 | 72 |
| 250 | 74 |
| 500 | 73 |
| 1000 | 71 |
| 2000 | 67 |
| 4000 | 60 |
The 125–500 Hz bands dominate the outdoor noise spectrum. STC's contour treats these bands permissively. OITC's contour weights them proportionally to their contribution to the outdoor source.
OITC: ASTM E1332
OITC was standardised in ASTM E1332 in 1990 and updated in subsequent editions. The calculation methodology differs from STC in both the reference spectrum and the calculation method.
The OITC Calculation
OITC uses a fixed reference noise spectrum that represents the composite of transportation noise sources (road traffic at different speeds, aircraft, and rail). The reference spectrum is defined across 1/3-octave bands from 80 Hz to 4000 Hz.
For each 1/3-octave band, the calculation computes the sound level that transmits through the partition:
- Start with the reference spectrum level at each band
- Subtract the measured transmission loss at that band
- Convert the resulting transmitted levels to sound power
- Sum all transmitted sound powers
- Compare to the reference spectrum overall level
- OITC is the difference, rounded to the nearest integer
Typical OITC–STC Differences
The gap between STC and OITC depends on the partition's low-frequency performance:
| Construction | STC | OITC | Difference |
|---|---|---|---|
| 6 mm monolithic glass | 29 | 23 | 6 |
| 8–16–8 air-filled IGU | 34 | 28 | 6 |
| 10–12–10 argon-filled IGU | 37 | 31 | 6 |
| 6.38–15–6.38 laminated/air/laminated | 40 | 34 | 6 |
| 6.38–15–8.38 asymmetric laminated | 42 | 38 | 4 |
| Timber stud wall (90 mm, 2× 13 mm plasterboard) | 52 | 46 | 6 |
| Concrete masonry (200 mm, plastered) | 54 | 50 | 4 |
| Curtain wall facade (double skin) | 48 | 41 | 7 |
The 4–8 point difference is remarkably consistent: it reflects the systematic low-frequency deficiency of most building elements, which STC masks and OITC reveals.
Where the Difference Matters Most
Glazing in Traffic Noise Corridors
This is the highest-stakes application. A glazed residential facade on a road carrying 25,000 vehicles per day generates an external noise level of approximately 70–75 dBA Leq (15 m from road edge). To achieve an interior level of 38–40 dBA (criterion for bedroom in a mixed-use urban zone under most residential noise standards), the facade requires approximately 32–35 dBA of insulation.
If STC is specified: a window rated STC 38 appears to meet the target. If OITC is applied: the same window has OITC 32, and actual field attenuation against the traffic spectrum may be only 28–30 dB — falling 2–4 dB short of the interior target.
This 2–4 dB shortfall, after installation, means measured interior levels of 40–44 dBA — exceeding WHO Guidelines for Nighttime Noise (40 dBA outside, implying 30 dBA inside with standard facade insulation). The occupants experience noise that disturbs sleep and is legally problematic for the developer.
Aircraft Noise
Aircraft noise is dominated by frequencies below 500 Hz (engine fan tones and turbine broadband noise). For buildings within flight corridors (> 63 dB Leq Day-Night Level), STC systematically overestimates facade performance by 6–10 dB against aircraft spectra. FAA Technical Guidance for Quieter Homes (HUD Guidelines) explicitly specifies that facade insulation must be evaluated against the actual aircraft noise spectrum — effectively a site-specific OITC calculation — rather than standard STC.
Rail Noise
Rail noise has two distinct components: wheel-rail interaction noise (500–2000 Hz, similar to speech spectrum, well-captured by STC) and structure-borne "rumble" transmission (50–250 Hz, especially from tunnel operations and heavy freight). For surface rail, STC is adequate. For underground rail or sites adjacent to heavy freight lines, OITC or a site-specific spectrum analysis is essential.
When to Use Each Rating
Use STC when:
- Specifying interior partitions between rooms (STC is appropriate for the interior speech noise spectrum)
- Comparing products for residential party wall compliance (most building codes cite STC for interior separating elements)
- Evaluating performance against HVAC or mechanical noise (which concentrates in 63–500 Hz range — and OITC is actually better here too, but STC is typically the code metric)
- Specifying facade elements (windows, curtain walls, external walls, roofs) where the noise source is outdoors
- Any site where transportation noise (road, rail, air) is the design criterion
- LEED v4.1 BD+C Eq Credit 9 (Acoustic Performance) for facades — LEED recommends OITC for exterior envelope evaluation
- WELL v2 Feature 74, Section L02 for noise intrusion from exterior sources
- An interior element (e.g., a hotel room window wall) must simultaneously meet interior speech privacy requirements (STC) and outdoor noise requirements (OITC)
- Providing a complete facade specification that must satisfy both the architect's interior privacy goals and the acoustic consultant's outdoor noise intrusion analysis
Worked Example: Residential Facade on Arterial Road
Site conditions: Mixed-use residential block, ground floor setback 8 m from the kerb of a major arterial road carrying 35,000 vehicles per day (AADT). Measured external facade noise level: 72 dBA LAeq (daytime), 65 dBA LAeq (night).
Interior target: WHO Night Noise Guidelines 2009 recommend a maximum of 40 dBA LAeq inside the bedroom. Bedrooms face the road.
Required attenuation: 65 – 40 = 25 dBA net. With 3 dB combined attenuation from the surrounding wall and floor plate (not the window), the window needs to provide approximately 22 dBA attenuation against the traffic spectrum.
STC specification vs. OITC reality:
The initial specification from the architect: "glazing to achieve STC 28 minimum" (based on a mistaken application of an old code baseline). This appears to exceed the 22 dBA need.
Checking the traffic spectrum against the standard double-glazed unit at STC 28:
- At 125 Hz, traffic contributes 70 dBA; the window's TL at 125 Hz is approximately 14 dB (below the STC 28 contour); transmitted level = 56 dBA
- At 250 Hz, traffic contributes 72 dBA; TL at 250 Hz ≈ 18 dB; transmitted = 54 dBA
- At 500 Hz, traffic contributes 70 dBA; TL ≈ 27 dB; transmitted = 43 dBA
- At 1000 Hz, traffic contributes 67 dBA; TL ≈ 31 dB; transmitted = 36 dBA
Against 65 dBA external, this provides only 8 dBA net attenuation — far below the 22 dBA needed. The OITC for this unit is 22 (not STC 28), and even OITC 22 is insufficient for the target.
Correct specification: OITC ≥ 33 for this application. This requires a 10.4–16–10.4 mm asymmetric laminated glass unit with argon fill — OITC 34 laboratory rating, OITC ≈ 28 field-installed (with 5–6 dB field derating). Still marginal, which suggests the design requires secondary mitigation: lobby buffer zones, facade setback, or acoustic treatment within the bedroom to manage residual intrusion.
This worked example is representative of the systematic errors that arise when STC is applied to outdoor noise design. The consequence is discovered during post-occupancy measurement — after the glazing is installed and the occupants are trying to sleep.
The Field Performance Caveat
Both STC and OITC are laboratory ratings. Field performance is consistently lower than laboratory due to:
- Flanking transmission through wall frames, window frames, and perimeter seals
- Incomplete sealing at window perimeters (even small gaps of 1 mm can reduce effective OITC by 5–10 points)
- Mechanical connections between inner and outer elements
- Workmanship variation in installation
Use AcousPlan's sound insulation calculator to assess facade performance against site-specific outdoor noise spectra and determine whether STC or OITC targets are appropriate for your project.