Why do Manufacturers Only List STC and Not Oitc?

STC numbers are higher, which makes products look better in marketing. STC is also the rating referenced in building codes (IBC, IRC), so it has regulatory importance. OITC testing requires a different reference spectrum and produces lower numbers for the same product, which is less attractive for advertising. Some manufacturers provide OITC data in technical documents but not on product pages.

Can a Wall Have a High STC but Low Oitc?

Absolutely. This is the core problem. A lightweight double-drywall wall with Green Glue might achieve STC 56 but only OITC 42. It excels at blocking speech frequencies but has a significant weakness below 125 Hz. Any assembly that relies on cavity resonance or damping without substantial mass will show a large gap between STC and OITC.

Does MLV Improve OITC Ratings?

Yes, significantly. MLV adds dense, limp mass that improves transmission loss across the entire frequency spectrum, including the low frequencies that OITC emphasizes. Adding 1 lb/sq ft of MLV to a standard wall typically improves OITC by 6-10 points — a larger proportional improvement than the STC gain, because MLV addresses the exact frequency range where lightweight assemblies are weakest.

What OITC Rating do I Need to Block Traffic Noise?

For a busy urban road, target OITC 35-40 for the overall facade (including windows and doors, which are usually the weakest links). For highway-adjacent homes, OITC 40-45 is recommended. Airport noise mitigation programs typically require OITC 40+ for habitable rooms. Remember that the overall facade OITC is limited by the weakest element — usually the windows.

Is IIC (Impact Insulation Class) Related to STC or Oitc?

IIC is a separate rating that measures impact noise isolation (footsteps, dropped objects) rather than airborne sound transmission. It is not directly related to either STC or OITC, which both measure airborne sound. However, all three ratings can be relevant for the same assembly — a floor-ceiling system has an STC for airborne noise, an OITC for exterior noise (if applicable), and an IIC for footstep noise.

STC vs. OITC Ratings: Which Soundproofing Numb…

1The Problem with a Single Number

Reducing the acoustic performance of a wall assembly to a single number is inherently reductive. Sound transmission varies dramatically across the frequency spectrum — a wall might block 50 dB at 1,000 Hz but only 20 dB at 63 Hz. Compressing that complexity into one rating requires choices about which frequencies to prioritize.

STC and OITC make fundamentally different choices about which frequencies matter. STC was designed in the 1960s for interior partitions where the primary concern was speech privacy — voices, telephones, office conversations. OITC was developed in the 1990s to address exterior noise — traffic, aircraft, construction, and other outdoor sources that contain significant low-frequency energy.

The result: two walls can have identical STC ratings but wildly different real-world performance depending on what kind of noise you are trying to block. If you pick the wrong rating system, you pick the wrong wall.

2What STC Actually Measures

STC (Sound Transmission Class) is defined by ASTM E413 and is calculated from transmission loss measurements across 16 one-third octave bands from 125 Hz to 4,000 Hz. The test procedure (ASTM E90) measures how much sound a partition blocks at each frequency, and then a standardized contour is fitted to the data. The STC number is the value of that contour at 500 Hz.

Key characteristics of STC:
• Frequency range: 125 Hz – 4,000 Hz
• Optimized for: Human speech (primary energy between 250–4,000 Hz)
• Ignores: Everything below 125 Hz — bass music, traffic rumble, HVAC, aircraft
• Contour fitting: Allows up to 8 dB deficiency in any single band, meaning a wall can have a severe weakness at one frequency and still achieve a high STC

STC works well for its intended purpose: rating interior partitions for speech privacy. If your primary concern is hearing your neighbor's conversation or telephone calls, STC is a reliable predictor. It is also the rating used in building codes (IBC requires STC 50 for party walls), so it has regulatory significance.

The problem: most real-world noise complaints are not about speech. They are about bass from music systems, traffic on nearby roads, aircraft overhead, mechanical equipment, and other sources with dominant energy below 125 Hz — the range STC essentially ignores.

3What OITC Actually Measures

OITC (Outdoor-Indoor Transmission Class) is defined by ASTM E1332 and was introduced in 1990 to address the limitations of STC for exterior noise. OITC uses transmission loss data across one-third octave bands from 80 Hz to 4,000 Hz, and critically, it applies a frequency weighting that emphasizes the low-frequency energy typical of outdoor noise sources.

Key characteristics of OITC:
• Frequency range: 80 Hz – 4,000 Hz (extends 45 Hz lower than STC)
• Optimized for: Transportation noise (traffic, aircraft, trains)
• Weighting: Uses an A-weighted spectrum based on real transportation noise data, giving more importance to low frequencies
• No contour fitting: Calculates a single weighted average — no hiding a low-frequency weakness behind a curve fit

OITC ratings are typically 5–15 points lower than STC ratings for the same assembly. This is not because the wall performs worse — it is because OITC is measuring performance in a frequency range where most assemblies actually do perform worse. OITC is telling you the truth that STC is hiding.

OITC is required or recommended for exterior facade design in many jurisdictions, airport noise mitigation programs, and highway noise barrier specifications. However, it remains largely absent from residential product marketing because the numbers are lower and less impressive on a spec sheet.

4Head-to-Head: STC vs. OITC

The following table shows how the same wall assemblies rate under both systems:

Wall Assembly	STC Rating	OITC Rating	Difference
Single 2×4 wall, 1/2" drywall each side	33	25	−8
Same wall + mineral wool cavity	39	29	−10
Same wall + 1 lb MLV + mineral wool	46	38	−8
Double drywall + Green Glue each side, mineral wool	56	42	−14
Same assembly + 1 lb MLV	58	48	−10
8" concrete block wall, painted	52	48	−4

Notice two patterns: First, the gap between STC and OITC is largest for lightweight assemblies with resonant cavities — exactly the walls most common in residential construction. Second, massive assemblies like concrete show a much smaller gap because mass provides consistent performance across all frequencies including bass.

This is why MLV is particularly valuable when OITC matters: it adds dense, limp mass that improves low-frequency performance disproportionately — exactly the range that OITC captures and STC ignores.

5When STC Misleads You

STC creates a false sense of security in several common scenarios:

Scenario 1: Highway traffic noise. You live near a highway and install an STC 50 window. Traffic noise barely changes. Why? Because highway traffic generates dominant energy at 63–125 Hz — frequencies STC does not measure. Your window might only achieve OITC 30, meaning it blocks less than half the perceived noise.

Scenario 2: Upstairs neighbor's bass system. You build an STC 55 ceiling assembly. Voices are gone. The subwoofer at 40–80 Hz shakes your light fixtures exactly as before. STC told you the ceiling was excellent; it simply was not measuring the frequencies that mattered.

Scenario 3: HVAC mechanical noise. Air handling units, compressors, and duct rumble produce significant energy from 31–125 Hz. An STC 50 partition between a mechanical room and an occupied space might only achieve OITC 35 against these sources.

Scenario 4: Aircraft noise. Airports use OITC exclusively for noise mitigation programs because STC dramatically overstates the protection provided against jet engine noise, which contains massive low-frequency energy.

💡 Rule of Thumb: If the noise you are trying to block makes your chest vibrate, your windows rattle, or your walls hum — STC is lying to you. Look at OITC or, better yet, request frequency-specific transmission loss data from the manufacturer.

6Which Rating to Use for Your Project

Choosing the right rating depends entirely on what noise you are fighting:

Use STC when:
• Your primary concern is voices, conversation, or speech privacy
• You are building interior partitions between offices, bedrooms, or apartments
• You need to meet building code requirements (codes reference STC, not OITC)
• The noise source is primarily mid-to-high frequency (TV dialogue, telephone, general office noise)

Use OITC when:
• You are blocking exterior noise (traffic, aircraft, trains, construction)
• The noise has a significant low-frequency component (bass music, mechanical equipment, HVAC)
• You are designing exterior facades or selecting windows/doors for noise reduction
• You want a more honest assessment of real-world performance across all audible frequencies

Use both when:
• You are designing a multi-family building where residents face both interior party-wall noise and exterior traffic noise
• You are building a home studio or theater where you need to block both neighbor noise and outside traffic
• You want the complete picture of how an assembly performs across the full frequency spectrum

When product manufacturers only list STC (which is most of the time), you can roughly estimate OITC by subtracting 8–12 points for lightweight framed assemblies, or 4–6 points for massive assemblies like concrete or CMU. This is not precise, but it gives you a reality check when a spec sheet makes a product look too good to be true.

The bottom line: STC is not wrong — it is incomplete. For interior speech privacy, it works well. For anything involving low-frequency noise, it overstates performance by a margin that can mean the difference between a quiet room and a failed project. OITC tells you what STC will not.

8Conclusion

STC has dominated the soundproofing industry for decades because it was first, because building codes reference it, and because the numbers are higher and easier to market. But if your noise problem includes any significant low-frequency component — and most real-world noise does — STC is giving you an incomplete and potentially misleading picture. OITC exists specifically to fill that gap. The smartest approach is to evaluate both ratings for any assembly, and when in doubt, prioritize materials that add mass across the full frequency spectrum. Mass loaded vinyl is one of the few materials that improves both STC and OITC proportionally, because dense, limp mass does not discriminate by frequency.