Demystifying Sound Transmission Class (STC Ratings)
When you’re designing a control room, studio, operator cabin, or acoustic enclosure, one question always comes up: How much sound isolation do you need? In North America, that answer often starts with Sound Transmission Class (STC)—but what does the number actually mean for your project? STC 35, STC 40, STC 45, STC 50, and STC 55 are common benchmarks. Here’s what STC measures, how it’s determined, what these ratings mean in real-world use, and how to choose the right level for your space.
What you’ll learn: what Sound Transmission Class (STC) is, how it’s measured, what each rating typically blocks, and how to select the right STC for your application—so you can make confident, effective decisions about sound isolation and soundproofing.
What is Sound Transmission Class (STC)?
STC stands for Sound Transmission Class. It is a rating system that classifies the airborne sound isolation provided by a building element/partition. Simply put, STC is a single-number rating classifying soundproofing.
- It allows you to qualify a single number rating for how much sound a wall, door, window, etc., will block from getting out of a room and being transmitted to another (or vice versa).
- STC is the most common rating system for airborne sound isolation in North America.
What influences the STC rating?
- The STC rating depends on the composition of the building element/partition that separates the two rooms.
- The building element could be composed of a single material or several.
- In general, a partition with more mass will provide greater STC ratings (mass law).
- Multiple-layer assembly (double leaf, triple leaf, etc.) with a separating insulation-filled gap may increase the performance in some frequency bands.
- The incorporation of damping material and/or resilient material will also improve STC rating (decoupling).
In simpler words, Sound Transmission Class is a single number rating metric for a building partition based on its ability to reduce sound passing through it. STC measures how well the partition blocks sound transmission. But what do these numbers actually tell us about a material’s real-world performance? Does a higher STC always mean better soundproofing?
Does a Higher or Lower STC Rating Provide Better Soundproofing?
The higher the STC rating number is, the better the soundproofing performance. Think of it like a barrier’s ability to block noise: as the STC number increases, more noise is prevented from passing through the material.
For example, a wall with an STC rating of 60 offers better sound isolation than one rated at 30. To put this in familiar terms, imagine STC ratings like sunscreens’ SPF number:
- Higher STC = higher SPF, more protection against the sun’s rays (better at blocking sound).
- Lower STC = lower SPF, more sunburn risk (lets more sound through).
Before we dive into how STC ratings are calculated, let’s start with a simple visual explanation of what STC means for soundproofing. The Sound Transmission Class table below, inspired by the one from Mecanum, a manufacturer of noise and vibration control instruments, shows how different STC ratings perform.
Sound Transmission Class (STC) Chart vs Perception
These STC ratings are subjectively described and put in perspective with one another.
STC Rating Chart – STC Ratings Chart |
||
|---|---|---|
STC Rating |
Subjective Description of Insulation Performance |
Example of STC Rating Recommendations |
| STC 50 | Superior Insulation | STC 50 is recommended by IBC between two dwellings, and by ANSI/ASA S12.60 for school classrooms |
| STC 45 | Very Good Insulation | STC 45 is recommended by FGI for between patient rooms in hospitals |
| STC 40 | Recommended for Offices | STC 40 is recommended by the GSA for office partitions where normal speech privacy is desired |
| STC 35 | Loud Speech Muffled | STC 35 is recommended by FGI for walls separating treatment rooms from corridors in hospitals |
| STC 30 | Loud Speech Audible | STC 30 is the minimum recommended for dwelling unit entrance doors according to the NYC Building Code |
| STC 25 | Normal Speech Audible | N/A |
| <25 | Poor Insulation | N/A |
Perceived loudness varies with many factors: the noise source’s spectrum (broadband, tonal, or impulsive), its rhythmic/temporal character, sound directionality (angle of incidence), background noise in the receiving room, listeners’ hearing, and room/environmental conditions.
Because of this, values in Sound Transmission Class (STC) charts are guides—not guarantees. Always interpret them in the context of your specific space and use case. For example, an STC that’s acceptable for an office may be inadequate for a bedroom or a recording studio. Also note that building codes and authorities having jurisdiction (AHJs) may set minimum STC requirements you must meet.
!It’s important to approach generic STC charts online with caution
Many offer oversimplified or inaccurate information that can mislead users about what constitutes effective soundproofing and what levels of noise are actually perceptible.
✔ Always consult acoustic experts when planning for sound isolation, as real-world performance depends on many variables.
✔ There’s a big difference between the STC rating of a single component (such as a window) and the overall sound isolation of a constructed building.
✔ STC ratings alone do not define total sound isolation of a finished space. Other measurements like ASTC may provide a clearer picture.
But before comparing sound isolation metrics like ASTC and NIC, it’s critical to understand how STC ratings are tested and what they actually measure. Field performance can differ from lab-based results—knowing this prevents misinterpretation and ensures your project meets both regulatory standards and real-world acoustic needs.
How is the Sound Transmission Class (STC) Rating Determined?
First, it’s important to know that Sound Transmission Class is measured inside a laboratory. Typically, if a building element is rated with an STC number, the element has been laboratory tested, and a report should be available to support the STC result.
Per ASTM E90, airborne Sound Transmission Loss (STL) is measured in one-third-octave bands (typically 125–4,000 Hz) under controlled conditions (flanking control, calibrated instrumentation, defined source/receive rooms).
Those data are then fit to the STC reference contour per ASTM E413, producing the single-number STC.
- ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements
- ASTM E413: Classification for Rating Sound Insulation
In short: E90 defines how the test is run; E413 defines how the results are classified into an STC number.
Note: STC is a lab rating; field results (e.g., ASTC per ASTM E336) can differ due to installation and flanking.
Testing, Measuring & Analyzing: Collecting the Sound Transmission Class Data
Now that we’ve covered the theory behind STC and the standards that define it, let’s look at what the testing process actually involves. Determining an STC rating requires a carefully controlled setup, precise measurements across frequencies, and a standardized analysis method. The process can be broken down into three main stages: testing setup, measurement, and data analysis. Here is a high-level description of the process to determine the STC rating of building elements such as walls, windows, doors, etc.
1. STC Testing Setup:
- Two adjacent rooms are separated by the building element being tested.
- One room is designated as the source room, where sound is generated.
- The receiving room measures the sound transmitted through the element.
2. STC Measurement Process:
- Sound pressure levels are measured in both rooms using specialized instrumentation (sound level meter, microphone, etc.).
- The sound pressure levels are measured at various frequencies (at a minimum from 125 Hz to 4 kHz).
- Sound Transmission Loss (STL) is calculated by comparing sound pressure levels between source and receiving rooms, factoring in the tested element’s surface area and the receiving room’s absorption.
3. Analyzing & Transforming the Data Into a Sound Transmission Class Rating:
- Sound Transmission Loss values are plotted on a graph.
- These STLs are compared to a classification curve following the algorithm defined by ASTM E413.
- The STC rating is the output number of the classification algorithm.
Now that we’ve explored how STC ratings* are measured and determined, we will focus on analyzing and explaining the results of a comparative STC analysis for different types of assembly.
*One caveat: As powerful as STC is to classify soundproofing a single number rating, STC does not convey information on the specific performance per frequency.
A soundproofing system with a high STC rating may be effective at blocking high-frequency noise, but not low-frequency noise, and vice versa, as illustrated on the sound transmission loss graph.
Sound Transmission Loss and STC Rating Example: Understanding the Results
The graph below compares the sound isolation performance of the MECART standard panel with two common construction assemblies: drywall partitions and concrete blocks. As a reminder, a higher STC number indicates better sound isolation.
In this case, the MECART panel performs about the same as drywall in the mid and upper-mid frequencies (630–2,000 Hz) and outperforms it at both low and high frequencies. Against this concrete block assembly, it shows nearly identical results from 125–1,600 Hz—but at a fraction of the weight: 7 lbs/ft² compared with 48 lbs/ft². This lighter surface mass makes installation easier while maintaining comparable performance.
STC Chart: Comparison Between MECART Standard Panels VS Common Construction Assemblies
Figure 1: Transmission Loss (dB) of MECART panels compared with drywall and concrete block assemblies, measured per ASTM E90.
It’s important to note, however, that STC is only part of the story. While it remains the most widely used benchmark, laboratory-tested STC values don’t always reflect real-world performance, where factors like installation quality, flanking paths, and actual room conditions come into play.
That’s where Apparent STC (ASTC) and Noise Isolation Class (NIC) come in. Closely related to STC, these metrics extend the picture by measuring how well sound is contained under actual building conditions, providing a more complete view of room-to-room isolation.
In the next section, we’ll look at how STC, ASTC, and NIC connect, where they differ, and why these distinctions matter when designing spaces that must reliably achieve their acoustic goals.
STC vs. ASTC vs. NIC: How Are They Different, and Why Does It Matter?
When it comes to soundproofing, not all ratings are made for the same purpose/application. You’ll often see these three terms: STC (Sound Transmission Class), ASTC (Apparent Sound Transmission Class), and NIC (Noise Isolation Class). They might seem similar, but each one tells a slightly different story about soundproofing.
What These Ratings Have in Common
- All three ratings are single numbers designed to make it easy to compare the sound-blocking ability of different walls or assemblies.
- In all these cases, the higher the number, the better the sound isolation—which means less noise travels from one room to another.
- They’re all used in building design to set standards and expectations for acoustical comfort.
| STC, ASTC & NIC — Where and How They Differ | |||
| STC | ASTC | NIC | |
| What It Measures | Sound isolation of an assembly in a laboratory |
Sound isolation of an assembly in the field (e.g., real building) |
Sound reduction between two specific enclosed spaces/rooms in the field |
| Includes Flanking Path? | No | Yes | Yes |
| Influenced by Environmental Factors | No | Yes, by flanking paths if any | Yes, result is unique to the specific room-to-room tested |
| Influenced by Reverberation | No | No | Yes |
| How It Is Measured | Transmission Loss (TL) as per ASTM E90 |
Apparent Transmission Loss (ATL) as per ASTM E336 |
Noise Reduction (NR) as per ASTM E336 |
| Tip: STC values come from laboratory tests, while ASTC and NIC reflect real-world conditions and may be lower. | |||
Note that, depending on the territory, some Code requirements or Authority Having Jurisdiction may refer to one or several of these metrics. However, STC and ASTC are the more frequent ones.
STC vs. ASTC: Lab vs. Reality
STC
STC is a lab rating. It measures how much sound gets through a wall or floor under perfectly controlled conditions. It does not account for sound sneaking around the wall (like through gaps, ceilings, or adjacent structures).
ASTC
ASTC is measured in real buildings. It measures not only the wall itself, but also those “flanking paths” where sound can slip through other routes. As a result, ASTC ratings are typically lower than STC for the same wall composition, but they’re a better predictor of what you’ll actually hear in a finished space.
In short:
STC is what the wall “could” do at its best; ASTC is what it actually does after construction, with all real-world imperfections.
STC vs. NIC: Assembly vs. Specific rooms
NIC measures the total sound transmitted between two rooms, capturing every possible path—walls, doors, windows, even air leaks. It’s measured after everything is finished and in use. It is influenced by the quantity of sound absorbing material in each room, the common area between the rooms. It is specific and unique to the adjoining rooms measured. Results are therefore harder to extrapolate for different conditions (room size, etc.).
STC, by contrast, relates to the performance of the assembly. It is influenced mainly by the assembly composition and his independent of the acoustic environment of either side nor its area. Therefore, it can be easily use as its value remains the same for the same composition and doesn’t vary in different conditions (room size, etc.)
In short:
STC is analogous to the R-value for thermal isolation. It is specific to the composition independently to its size, location, etc. In the same spirit, NIC would relate to the temperature difference for a specific building envelope considering all environmental factors.
Sound Transmission Loss and STC Rating Example: Understanding the Results
The graph below compares the sound isolation performance of the MECART standard panel with two common construction assemblies: drywall partitions and concrete blocks. As a reminder, a higher STC number indicates better sound isolation.
In this case, the MECART panel performs about the same as drywall in the mid and upper-mid frequencies (630–2,000 Hz) and outperforms it at both low and high frequencies. Against this concrete block assembly, it shows nearly identical results from 125–1,600 Hz—but at a fraction of the weight: 7 lbs/ft² compared with 48 lbs/ft². This lighter surface mass makes installation easier while maintaining comparable performance.
STC Chart: Comparison Between MECART Standard Panels VS Common Construction Assemblies
Figure 1: Transmission Loss (dB) of MECART panels compared with drywall and concrete block assemblies, measured per ASTM E90.
It’s important to note, however, that STC is only part of the story. While it remains the most widely used benchmark, laboratory-tested STC values don’t always reflect real-world performance, where factors like installation quality, flanking paths, and actual room conditions come into play.
That’s where Apparent STC (ASTC) and Noise Isolation Class (NIC) come in. Closely related to STC, these metrics extend the picture by measuring how well sound is contained under actual building conditions, providing a more complete view of room-to-room isolation.
In the next section, we’ll look at how STC, ASTC, and NIC connect, where they differ, and why these distinctions matter when designing spaces that must reliably achieve their acoustic goals.
STC vs. ASTC vs. NIC: How Are They Different, and Why Does It Matter?
When it comes to soundproofing, not all ratings are made for the same purpose/application. You’ll often see these three terms: STC (Sound Transmission Class), ASTC (Apparent Sound Transmission Class), and NIC (Noise Isolation Class). They might seem similar, but each one tells a slightly different story about soundproofing.
What These Ratings Have in Common
- All three ratings are single numbers designed to make it easy to compare the sound-blocking ability of different walls or assemblies.
- In all these cases, the higher the number, the better the sound isolation—which means less noise travels from one room to another.
- They’re all used in building design to set standards and expectations for acoustical comfort.
| STC, ASTC & NIC — Where and How They Differ | |||
| STC | ASTC | NIC | |
| What It Measures | Sound isolation of an assembly in a laboratory |
Sound isolation of an assembly in the field (e.g., real building) |
Sound reduction between two specific enclosed spaces/rooms in the field |
| Includes Flanking Path? | No | Yes | Yes |
| Influenced by Environmental Factors | No | Yes, by flanking paths if any | Yes, result is unique to the specific room-to-room tested |
| Influenced by Reverberation | No | No | Yes |
| How It Is Measured | Transmission Loss (TL) as per ASTM E90 |
Apparent Transmission Loss (ATL) as per ASTM E336 |
Noise Reduction (NR) as per ASTM E336 |
| Tip: STC values come from laboratory tests, while ASTC and NIC reflect real-world conditions and may be lower. | |||
Note that, depending on the territory, some Code requirements or Authority Having Jurisdiction may refer to one or several of these metrics. However, STC and ASTC are the more frequent ones.
Understanding Transmission Loss Results
For example, if you look at the transmission loss result graph, you can see that at a frequency of 125 HZ, the number of decibels blocked is only about 12 decibels for the STC-29 gypsum board and 18 decibels for the STC-26 steel sheet (26 GA). However, at high frequencies above 2000 HZ, the steel sheet performs better than the gypsum and blocks over 30 decibels.
In addition, frequencies outside or close to the lowest and highest part of the spectrum are not well-represented. As mentioned before, STC ratings only cover 125 Hz to 4000 Hz frequencies. Yet, many noises are below 125 Hz. For example, power transformers’ noise levels hover on the 125 Hz to 250 Hz octave band. These low-frequency noises are harder to stop. Aircraft engines, heavy equipment, machinery, industrial processes and tapping machines are other low-frequency noise sources. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.
What we mean when we say STC-35, STC-45 and STC-52
Mecart’s standard acoustic products are labelled STC-35, STC-45 and STC-52. As previously explained, this means they have been tested in a laboratory for their sound transmission loss. The test results are available upon request.
A sound reduction of 35 decibels is very high. Most similar soundproof enclosures on the market reduce noise by only 15 to 25 decibels. However, Mecart’s lowest sound attenuation products are 35 decibels and are used in the noisiest environments.
Why aren’t products as soundproof as labelled once installed?
For most material available on the market, the noise reduction once installed is less than that achieved by the material during product testing.
The problem isn’t with the product itself, it’s with everything around it. Airborne sound travels through the air and can also pass under doorways, under or around a wall, through ventilation, through common ductwork, plumbing or corridors, and over, under, around, and through any obstructions. Therefore, even with a high STC rating, any penetration, air-gap, or “flanking” path seriously degrades the isolation quality of a wall. For example, even if you buy a STC-45 acoustic door, if noise passes through the walls, floor and ceiling, the actual sound reduction will be far less than 45 decibels.
However, this is not the case with Mecart enclosures.
Mecart doesn’t only test the acoustic components (walls, doors, etc.) individually but also complete assemblies to ensure that they are as acoustically effective—both alone or assembled. We also guarantee the acoustic performance of our products; therefore, if they are rated STC-45, they will achieve a reduction of 45 decibels.
See a demonstration of an acoustic test here.
Noise Reduction Coefficient (NRC)
NRC is a rating used to measure sound absorption. Like a sponge absorbs water, an acoustic product absorbs sound. It is used to improve the acoustics within a room or the sound in a studio. The NRC rating is between 0 and 1; 0 means that the solution absorbs no sound, while 1 means that it absorbs all the sound.
In an anechoic room, also called a quiet room, the NRC is at the highest level; this is why when you shout, you can’t hear yourself.
In other words, NRC measures how well a product reduces sound bouncing around or echoing within a room, whereas STC represents how well a material will block sound from entering or leaving the room.
NRC is not often discussed in our industrial noise control projects, where soundproofing rather than sound absorption is needed.
Want to protect your operators from noise?
Contact us and a member of our team will gladly discuss your project with you.