How To Determine STC Rating Of A Wall in Easy Steps

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You are probably considering a building project if you are researching STC rating, and it’s perhaps not a tool shed. Sound transmission classification ratings are often relevant in locations where we wish to keep sound in or out, such as a recording studio, a road-facing outside wall, or a bedroom over a garage.

When assessing building materials and acoustical items for noise reduction, most people rely on the product’s sound transmission class or STC rating. By understanding this rating of materials, we can determine how much sound these materials will muffle.

In the same manner that NRC is the standard value for how effectively a material absorbs sound, sound transmission class (STC) is a single-number rating approximating how much sound a particular type of material could absorb.

What is Sound Transmission Class (STC)?

Sound Transmission Class (or STC) is a numeric value that indicates how effectively a building partition dampens airborne sound. It is commonly used in the United States to rate interior partitions such as floors, ceilings, windows, doors and external wall layouts.

The ISO Sound Reduction Index (SRI) is applied outside the United States. The sound transmission class rating approximates the decibels of noise reduction that a partition may give. The STC is adequate for assessing discomfort caused by speech sounds, not music or equipment noise since these sources include more low-frequency energy than speech.

The STC provides a standardized method for comparing items from rival manufacturers, such as doors and windows. The greater the number, the more efficient the sound insulation.

The STC is a standard grade issued by ASTM E413 based on lab measurements conducted according to ASRM E90.  On the other hand, ASTM E413 can also be used to get comparable ratings from field measurements conducted in line with ASTM E336.

STC What can be heard
25 Everyday speech can be understood
30 Loud speech can be understood
35 Loud speech, audible but not intelligible
40 Loud speech audible as a murmur
45 Loud speech can be heard but not audible
50 Loud sounds are faintly heard
60+ Good soundproofing; most sounds do not disturb neighboring residents

Calculating STC Ratings for Masonry Walls

Sound transmission class ratings for masonry or CMU walls are determined by the weight of the block, whether or not the cells are filled, and the type of material used to fill the cells. On the other hand, the STC rating of CMU walls can be calculated based on their weight using the formula below:

STC = 0.18W + 40

W = pounds per square foot (PSF)

When Do STC Products Work?

Remember that the STC only tests on frequency ranges between 125 to 4000 Hz. A major difficulty with this categorization is that many common noise problems can be ascribed to sounds that frequently fall below or above this range.

Examples include deep male voices, bass drums, and trucks, all of which can theoretically begin at 50 Hz; sounds that exceed 4000 Hz include high-pitched female voices, the shrill ringing of phones, and automobile horns.

This will explain why, after spending tens of thousands of dollars on skilled acoustical engineers and obtaining hundreds of pages of tested soundproofing materials with laboratory testing of STC 50 and higher, many large projects continue to get complaints about low-quality acoustic performance.

How Is a Wall’s STC Rating Calculated?

STC rating can be determined by employing the following STC calculation formula:

  1. Use an audiometer to record the decibel levels in the room containing the noise source.
  2. Measure the sound in the neighboring room: Next, proceed to the adjacent room and note the number of transferred decibels.
  3. Determine the transmission loss: Subtract the second measurement from the first to determine the total transmission loss.
  4. Test multiple frequencies: Repeat steps one through three with about 16 distinct sound frequencies between 125 and 4000 Hz. Thus, you will obtain the most precise findings.
  5. Determine the median transmission loss: The total number of frequencies recorded is multiplied by the total number of frequencies evaluated. For instance, if you recorded 16 different frequencies, divide your total by 16. This value represents your STC rating.

Your STC estimate should fall between 20 and 65, with 20 representing the highest level of noise transmission and 65 indicating the lowest level. In general, it is believed that a higher STC rating is preferable. However, it is crucial to note that many causes of noise complaints are below the 125 Hz threshold.

 The sound generated by home movie theaters, drums, aircraft, and industrial equipment does not exceed 125 Hz.

an image showing STC of a wall with mass loaded vinyl attached

Lab Comparisons and Field Tests

Lab Comparisons

First, suppose the initial architectural plans are accessible. In that case, locating the design assembly and estimating the intended STC rating may be feasible by comparing this data to available laboratory test results. While this is a useful starting point for evaluating the STC rating, not all constructions adhere to the specifications. Therefore, our subsequent test is considerably more accurate for a particular attribute.

Field Tests

As you may have guessed from the name, field tests are done on-site or “in the field.” The ASTC, or “Apparent Sound Transmission Class,” is derived from the outcomes of field tests. To ensure testing is accurate, dependable, and repeatable, testing in line with established standards is often the best methodology.

This testing method utilizes an amplified speaker that generates broadband noise in a single room (referred to as the “source”). The volume produced by the “source” is then measured in both the “source” and “receiver” rooms (adjacent room).

From that point on, the reverberation time in the “receiver” permits the calculation of the respective sound absorption alongside the source and receiver values. All of these factors contribute to the ASTC grade of the wall.

The Factors That Contribute to Calculating the STC Rating of A Surface Are:

  • Decibels: Its acoustic measurement determines the loudness of a sound; the greater the decibel level, the louder the sound. For example, a whisper is 30 decibels, whereas a rock concert may reach up to 140 dB, which is dangerously loud.
  • Transmission loss: Transmission loss measures how many decibels are lost while transmitting across a surface from one room to another. Transmission loss measures, for instance, the amount of sound lost before it reaches the recipient’s ears, like when shouting from one room to another. The transmission loss is 40 dB if your voice begins at 70 decibels and reaches the room at 30 decibels.
  • Sound frequency: This metric describes the sound’s pitch. The frequency of a high-pitched sound, such as whistling, is greater than that of a low-pitched sound, such as bass drumming. It is determined by the total sound waves produced per second; the greater the frequency, the more waves are produced.

How To Improve the STC Rating

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;

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.

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.

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

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.


1 thought on “How To Determine STC Rating Of A Wall in Easy Steps”

  1. What would the STC rating be for a furring wall of 3 5/8″ steel stud, Roxul insulation and 1/2″ Silent FX drywall 1 side to u/s deck be?


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