If you manage a flexible workspace, a conference center, or a school gymnasium, you rely on moveable walls to quickly divide large areas. But how do you ensure that the conversations in Meeting Room A stay private from the presentation in Meeting Room B? The answer lies in a confusing, three-letter acronym: STC.
Sound Transmission Class, or STC, is the single most important number when buying any moveable wall designed to block noise. For consumers and facility operators, the STC rating can appear hopelessly complicated. Yet, when reduced to its most simplistic form, STC is merely a standardized report card that tells you how effective a barrier is at stopping sound from passing through it.
What STC Represents: The Report Card of Sound
The STC rating is designed to measure the effectiveness of a building element—like a sheet of drywall, a solid door, or an entire moveable wall system—at blocking airborne sound. Think of STC as a simple measurement of difference: technicians play a sound on one side of the wall and measure how much quieter it is on the other side.
Crucially, STC is a measurement of sound blocking, also known as sound insulation. This is different from the Noise Reduction Coefficient (NRC), which measures sound absorption (how well a material soaks up sound inside a room to reduce echoes). For a moveable wall to provide true privacy, you need high STC ratings. The rule is straightforward: the higher the STC number, the better the wall is at keeping noise out.
STC ratings range from 25 to over 60. A standard, single-pane window might score a poor 25. A heavy, solid-core door might hit 35. For moveable walls, professional grade starts at 50 and goes up to 55 and 60 for superior performance.
How STC Affects the Human Ear
Understanding the STC scale is easiest when you connect the number directly to what you hear:
- STC 25-30 (Poor): Normal speech is easily understood through the wall.
- STC 35 (Little Privacy): Loud speech can be heard but is no longer fully intelligible. This is the starting point for minimal acoustic privacy.
- STC 40 (Some Privacy): Loud speech becomes a faint murmur.
- STC 48 (Privacy Threshold): This is considered the baseline for true acoustic privacy. Most conversations are blocked, and while you might hear that people are talking, you cannot understand a single word.
- STC 50-55 (Excellent Privacy): Very loud sounds, like music or shouting, can only be heard faintly.
A difference of just 5 points makes a noticeable difference to the human ear. For instance, moving from STC 40 (loud speech is a murmur) to STC 48 (loud speech is blocked) represents a significant leap in privacy most often achieved by adding mass or internal barriers to the wall structure.
How the Number is Determined (Simplified)
STC ratings are determined by independent acoustic laboratories following strict standards (ASTM E90).
- The Test Environment: A wall sample is installed between two soundproof rooms: a source room and a receiving room.
- The Measurement: Sound is played in the source room, covering the primary frequencies of human speech (125 Hz to 4000 Hz). Microphones measure the sound level in both the source room and the receiving room.
- The Result: The difference (the transmission loss) is calculated across those frequencies, and a standard curve is fitted to the data to produce the single STC number.
This standardization ensures you are comparing apples to apples when looking at products from different manufacturers.
The Hidden Weakness: Low Frequency
While STC is great for speech, it has a blind spot: low-frequency sounds. The test only measures down to 125 Hz. If your moveable wall is dividing a space used for amplified presentations or music (which contain substantial bass and low-frequency rumble below 125 Hz), the published STC rating may be misleading. A high STC wall can still allow annoying, rumbling bass sounds to pass through easily.
The Moveable Wall Reality: STC vs. NIC
The most crucial concept for users is that the lab-tested STC rating is not the performance you will achieve in the field.
The STC rating is achieved in a perfect laboratory environment. In the real world, how the wall performs is measured by the Noise Isolation Class (NIC). NIC accounts for all the messy realities: gaps, imperfect installation, and acoustic leaks (flanking paths) around the wall.
Because of these real-world issues, the installed NIC performance of a moveable wall is typically 10 points lower than its lab STC rating. If you buy a wall rated STC 50, expect it to perform closer to NIC 40 once installed. NIC 40 is considered a good result for a field-tested operable wall.
Factors to Consider When Choosing an STC Value
Selecting a moveable wall is not just about the panel’s rating; it is about the entire acoustic system.
- Define Your NIC Goal: Start by determining the minimum acceptable level of privacy (the NIC). If you need high privacy (NIC 45), you must specify a wall with an STC of at least 55.
- Address Flanking Paths: Even the best wall fails if sound leaks around it. You must ensure the surrounding construction is equally strong. Sound can leak through:
- The Ceiling Plenum: The area above the track system requires a solid, sealed acoustic barrier (bulkhead) that matches the wall's STC.
- Fixed Walls: If the moveable wall connects to a fixed wall with a low STC window or hollow door, that weak link will ruin the entire barrier.
- Unsealed Gaps: All joints where the wall meets the building structure must be sealed completely, usually with caulking.
- Ensure Proper Sealing Mechanism: Moveable walls rely on top and bottom seals to lock the system into place acoustically. Higher STC walls (50+) often require heavy panels and complex, automated seals to ensure a consistent, tight pressure lock against the floor and track. If the manufacturer achieved your desired STC rating using an "automatic" seal system, you must specify and purchase that same system to guarantee performance. Read the sound test and compare what was evaluated to the proposed moveable wall system. They must be the same. Choosing cheaper, simpler seals will negate your investment in a high-STC panel.
Demystifying STC means accepting that the published STC number is merely a starting point. True acoustic success for your moveable wall system depends on specifying a rating at least 10 points higher than your desired privacy level (NIC) and ensuring meticulous installation that seals every potential flanking path.
