How To Determine STC Rating Of A Wall in Easy Steps

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Want to know how to determine STC rating of a wall? well, in this guide, I’m going to show you how you can achieve that in simple steps.

What is STC?

STC is Sound Transmission Class in full. It is defined as an integer rating that shows how well a wall/partition decreases airborne sound.  (Source)

In simple terms, STC is a rating of how walls block sound.

This rating often applies to inner walls, doors, and windows.

Walls with a higher STC are better at blocking sound—the higher your wall STC, the more peace you will enjoy.

Steps on How to Determine STC Rating of A Wall

1. Wall thickness and material

A drywall wall is different from a concrete one in different properties. Even in walls made using the same material, the thickness is always different. 

Concrete walls tend to have a higher STC. The same applies to thicker walls.

2. Weak Points

A wall may have weak points that may compromise the STC. Airborne sound can go through doors, windows and ventilation systems. 

Check for gaps on door and windows.

The wall STC can be improved by simply taking care of the weak points.

A thin floor can also be a weak point if it allows noise from your upper and lower floor neighbours into your home.

3. Noise Frequency

The STC of a wall is tested using 16 standard frequencies ranging between 125Hz and 4000Hz. 

The STC is different with each of these frequencies.

For example, a wall may have a higher STC at 1500 Hz, while another one may perform better in the 90 to 250 Hz range.

After examining the STC value on different frequencies, the values are compared with a standard STC to get an approximate value.

You need to note that structure-borne vibrations are not considered while calculating the STC level.

This can only mean that a wall’s ability to block these sounds has nothing to do with the STC.

4. Check Performance in Real-Life

To ensure that you do not end up disappointed with your sound reduction efforts, check your wall’s STC performance in real life.

In most instances, the performance is worse than the lab-tested results.

It helps to pay attention to the noises around, so it is easy to determine the next course of action.

If you do not have time to test performance in real life, ensure that the soundproofing system you choose is a little higher than recommended.

This will take care of any deficiencies that may arise.

5. Measuring the Wall STC

A wall STC is measured using Transmission Loss in between rooms. Transmission loss equals to the loss in noise reduction measured in decibels (dB). 

For example, if the noise is 70 dB in the room where the source is placed, and it reduces to 60 dB in the next room, the wall transmission loss is 10 dB. (Source)

Interestingly, the wall transmission loss may be completely different if another test note is used.

The variance in wall performance can be quite significant, depending on sound frequency.

Use an STC calculator to get your walls correct STC level.

Take readings at different spots and for best results, take the measurements closest to the wall.

To get the real STC value, you need to add the deficiencies using the STC contour.

Next, you need to get the adjusted transmission loss by adding the STC contour adjustments.

Play with the STC values until any of these two limiting conditions are met. 

These conditions are: 

  • No frequency band should have over eight deficiencies
  • Total deficiencies should not go above 32

Because the human ear is more sensitive to some frequencies, an equal-loudness contour is used when testing the frequencies.

This is referred to as A-weighting.

Check out this guide on how you can soundproof a wall in simple easy steps.

Improve Wall STC

Aside from knowing how to determine STC rating of a wall, it would be best if you also had the right tips to improve it.

There are many options available depending on the results you expect and your budget.

If you have problems with the noise seeping through the window, hanging noise-blocking drapes may be all you need to improve the STC rating.

However, if your apartment is right next to a gym, you may need to invest in something more comprehensive.

Some of the options available include;

6. Close leaks and gaps

Deal with the gaps in your doors and windows.

Check for any cracks, passages ways, and ducts that may allow sound to pass through and ensure they are soundproofed.

There are many solutions designed for soundproofing windows, doors, and the notorious open spaces.

These are often easy to install, and they do not cost as much.

7. Adding Mass

Adding mass  to the wall using anything from soundproofing panels to mass loaded vinyl to your walls helps to reduce the noise getting into your space.

These materials are also quite affordable and can be installed without professional help.

Adding mass works to reduce vibrations. 

Increase the air space between walls

Sound travels in the form of waves, and it is either contained or absorbed before it gets to the other point.

Air absorbs a good portion of the sound waves.

With more space between wall, sound will significantly reduce as the air absorbs some of it.

Increasing space should only be done with the help of a professional.

8. Install a soundproofing system

Get a system that considers all the weak points.

This system is the surest way to get the STC you desire.

A soundproofing system makes more economic sense if you are the homeowner.

Final Thoughts On How To Determine STC Rating Of A Wall

You have to remember that STC doesn’t consider low-frequency noise since the STC frequency cut off is set at 125Hz.

This can only mean that while this is a common measurement, it is limiting in many ways.

It would be best if you did not rely on STC alone when determining your soundproofing expectations.

Everyday noises that are below 15 Hz include:

  • The average sound generated by home theatres
  • Traffic noise from trucks, aeroplanes, and heavy equipment
  • Guitar, drums, bass
  • Industrial equipment including the pump system

Even if your wall has a high STC value, any of the above sounds can still be a problem.

Take your time to consider other factors so that you get a compressive soundproofing solution for your home.

Also Read: Resilient Channel vs. Hat Channel:

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