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STC Calculation 101: Is Your Wall ACTUALLY Soundproof?

Silent Home Hub STC Calculation

The last thing you need is to be disturbed by others due to thin walls and poor soundproofing.

If you want to understand why you can still hear your neighbors’ noise in your home theater or apartment, you need to learn all about the STC rating of a wall.

Table of Contents

How are STC Ratings Calculated?

While you can leave the measuring to the professionals, it doesn’t hurt to understand what happens as they measure the STC of a room. You can determine STC rating in a few simple steps.

Remember to keep the doors and windows of your apartment CLOSED to control airborne noise travel when you’re measuring the STC.

1. Measure the Sound in the Room With the Noise Source

To determine the STC rating, first, you’ll need an AIRBORNE SOUND SOURCE that emits vibrations in the 16 most common sound frequencies covered by the STC (between 125 Hz and 4000 Hz).

Here are some common STC Ratings to help:

STC rating table

Measure the loudness of the airborne sound in decibels (dB). Do this as close to the wall as possible that you want to measure the STC for.

2. Measure the Sound on the Other Side

Head to the room where the other side of the wall you are measuring is. Then, measure the sound AGAIN as close to the wall as you can to help you determine STC rating.

3. Calculate the Transmission Loss

Transmission loss refers to the DIFFERENCE between the two values you’ve just measured.

Subtract the value from Step 2 from the value from Step 1 to get transmission loss.

For example:

If you measured 70 dB on the side with the sound source and 65 dB on the other, the difference or transmission loss would be 5 dB.

4. Repeat Across a Range of Different Frequencies

Next, we need to measure the same sound across various frequencies to determine STC rating.

Why? Because the airborne sound transmission loss varies DEPENDING ON THE FREQUENCIES.

And so, STC ratings are not just a single value measurement. It’s actually a set of transmission loss values arranged into a curve, measured over different frequencies.

Once you’ve constructed the curve based on the transmission loss values, you compare this to STC reference curves. The closest one becomes your STC rating of a wall.

Keep in mind there are TWO CONDITIONS that have to be met when you determine STC rating:

  • Not one sound frequency band may have more than 8 deficiencies.
  • The total deficiencies in all frequency bands may not exceed 32.

The highest number that meets both conditions is the final STC value.

STC Calculation Guide: What Is Sound Transmission Class (STC)?

Sound Transmission Class, abbreviated as STC, refers to an integer rating of how well a material reduces the transmission of sound.

In short, the STC is an integer that lets you know HOW WELL an interior wall can block noise from reaching another room.

Rule of thumb: The higher the STC rating of a wall, the better the wall can block sound.

The material in question can refer to doors, windows, the floor, and ventilation systems as well. So STC isn’t just used for walls.

Limitations of STC

Keep in mind, though, STC ONLY MEASURES the decibel reduction of sound vibrations and sound energy types that travel through the air.

What Does That Mean?

  • This means the vibrations that travel through structures (structure-borne noise) ARE NOT accounted for.
  • Moreover, STC can measure sounds only in common frequencies (between 125 Hz and 4000 Hz).

So if a sound is outside this range, it won’t be blocked as well as the STC rating indicates. In other words…STC CANNOT MEASURE structure-borne vibrations.

Typical low-frequency noise under 125 Hz include power generators, industrial processes, and repetitive mechanical tapping sounds.

High-frequencies include whistles and birds chirping.

What Is an STC Rating of a Wall For?

It’s important to have a good idea of the STC of the walls in your room. DON’T INVEST in high-end soundproofing when the STC rating of a wall is already high.

When you determine the STC rating, it also helps you find out which wall of your apartment has weaker soundproofing, so you can concentrate on building a system for that wall.

The STC Rating Scale

The reference curves you compared your results to were also discovered and compiled into an STC scale using the same procedure stated above.

The STC scale uses the ASTM E413-certified standards to establish the references in the 16 most common frequencies and the sound transmission loss for each.

For example, STC 35 would mean that the structure can reduce noise by roughly 35 dB.

Having a standardized reference means all products use the same scale. And so, they can be reliably compared with each other for their effectiveness in blocking sound.


Though the resulting STC rating translates to the average transmission loss, it is not enough to convey the sound reduction across different frequencies.

One soundproofing system with high STC may be able to block high-frequency sound, but NOT low-frequency noises.

What Is a Good STC Rating?

The STC scale ranges from 20 to 65, where the higher the rating, the more sound is blocked to the human ear.

To give you a better idea, 0 dB is the quietest you can hear, while sounds of 130 dB can cause physical pain.

A summary of the STC scale would look like this:

  • STC 25 – No soundproofing; you can hear and understand talking from the other side of the wall.
  • STC 35 – You can hear loud talking but can’t understand it through the wall.
  • STC 45 – You can’t hear any talking at all, but you can still hear music and traffic noise.
  • STC 60+  – Best soundproofing; most noise sources are blocked.

Your typical interior wall at home without extra sound insulation would be around STC 35.

If you want to soundproof your rooms, you would need walls with an STC rating between 50-80.

Improving Your Room’s STC Rating

If you’re on a budget and believe your home or office space needs a better STC rating, affordable soundproofing materials are your best soundproofing solution.

However, different spaces have different soundproofing needs. And that’s why it’s good to study up on the various soundproofing materials you can use.

It’s also beneficial to understand the basic principles behind soundproofing to help you determine the best approach to soundproofing.

Even with an STC rating, it is still not an accurate number for the exact transmission loss it can provide.

What Factors Affect STC Rating?

STC ratings can’t promise to remove the exact number of decibels because of certain factors that can influence the effectiveness:

Frequency of the sound – Noise reduction varies depending on the frequency of the sound problem. While this factor was controlled when we measured the STC rating, the frequencies may be out of the common frequency range of 125 Hz to 4000 Hz in real-life situations.

Thickness of material – Since sounds are a form of vibration, the thickness of a material can affect how much the sound can pass through it, acting as an additional barrier.

Weak points – Any weak points, such as gaps in the building partition or floor and ventilation systems in the ceilings, allow sound to travel further, no matter how high the STC value of a structure is and no matter what kind of sound (airborne, impact, or flanking noise). This also affects sound reduction.

How to Improve STC Ratings

Here are simple ways you can increase your soundproofing system to improve the STC rating of your structure:

  1. Increase mass – A common rule of thumb is that DOUBLING THE THICKNESS of a wall partition can improve the STC rating by 6 dB.
  2. Utilize soundproofing material – One of the most cost-effective ways is to install soundproofing panels in the structure. Their material can lift the drywall up to decrease the sound transmission, thereby improving the soundproofing.  One with an STC 35 rating is considered quite high.
  3. Add air space – Another way of reducing sound transmission is by adding more air space. Sound travels through the air both up and down, so making it travel farther can help decrease the loudness. This way of improving the STC rating is more difficult and is best done through a professional.

Final Thoughts

Enjoying your privacy in every room of your house is a privilege that is highly valued in this day and age as noise pollution becomes a growing issue.

More and more people are investing in soundproofing needs for a better quality of life away from noise pollution and unwanted sounds.

However, if we want to be cost-effective and smart about our soundproofing investments, we must learn to understand STC values of the walls in our house.

With the help of the STC scale, the process is made easier and more standardized, so we can make better choices in making our space our personal, silent sanctuary.

You can apply the tips we mentioned here to boost your room’s noise reduction capabilities. HOWEVER, we still strongly believe a professional can help you eliminate the problem significantly more.

You might also want to just try building your own sound diffuser to improve the ambiance of your room.


July 12, 2021 – updated article links

June 29, 2021 – fixed and updated article formatting and content, updated article links, optimized content

About the Author


Andrea has always been bombarded by the hustle and bustle outside her home. Living in the city doesn’t get any quieter. The never ending noise from construction, traffic, and dogs barking on the streets day in and day out drove Andrea to a breaking point.

For 3 years, Andrea committed herself to studying DIY hacks, performing soundproofing experiments, and installing noise-free solutions. Now, she lives a quiet life free of the stress from noisy environments.

She hopes to share this knowledge so that others don’t have to endure the noise reigning in their ears and live a peaceful, stress-free life.