Understanding STC

When discussing the reduction of sound vibration as it travels from one side of a wall to the other, we need to quantify and measure this loss. Do all construction techniques or soundproofing materials work equally well? Exactly how much sound is being reduced, and perhaps more importantly, what frequencies of sound are being affected?

In short, STC gives you a rough idea how much sound a wall, for example, might stop. STC, Sound Transmission Class, is the most common sound reduction measurement in use. As common as this measurement is, it is quite limited and should not be totally relied upon for real world soundproofing expectations.

Lets first cover a few concepts.

Decibels: Written as dB in literature, dB is simply a measurement of how loud a noise is. 50dB is quiet, while 140 dB is so loud that is can immediately injure your ears. Think of dB as the volume knob on your receiver.

Transmission Loss is a measurement of the dB (volume) difference on either side of a wall. Let’s say we have a 100dB tone on one side of a wall. Pretty loud. We measure this same tone on the other side of the wall and find we have 75 dB. So we would say that at this tone or pitch, we have 25 dB Transmission Loss. 25 dB less sound energy made it through the wall to the other side.

Interestingly, a test tone with a different pitch sent through that same wall might only see a 4 dB Transmission loss. The performance of a wall will vary a great deal depending on the tone (frequency) of the sound.

 

Frequency: Written as Hertz or Hz., this is the measurement of the tone or musical note of the sound. Is it a really high pitch like a Flute might make (2000 Hz) or a low pitch from a Tuba (as low as 29 Hz)? Most humans are born with the ability to hear frequencies from about 20 Hz (low) up to 20,000 Hz (high) but that range shrinks as we get older.

What is STC?

Now that we have an understanding of a few basic terms we can describe what STC is. Way back in 1961, STC was introduced as the method for comparing various wall, ceiling, floor, door, and window assemblies. STC is calculated by taking the Transmission Loss (TL) values tested at 16 standard frequencies over the range of 125 Hz to 4000 Hz and plotted on a graph. Your curve (what you actually measured) is compared to standard STC reference curves (see appendix). If your wall graph is closest to a standard STC 35 curve, your wall is said to have an STC of 35.

 

The MAGENTA line in that graph is the standard reference STC contour. The BLUE line is the performance of the wall. To calculate TL and STC, the performance data should be obtained from a certified laboratory.

Higher STC is generally better, though not always, as we will see below.

Caution: The largest problem with relying on an STC number alone is that STC only considers frequencies down to 125 Hz. This can be very misleading due to the fact that most sound isolation complaints are from noise sources that are below 125 Hz.

Here are a few examples of noise sources that are below 125 Hz

  • Most of the sound energy generated by the average home theater
  • Traffic noise from airplanes, trucks and heavy equipment operation
  • Guitar, bass, drums
  • Industrial Equipment, especially pump systems.

 

The example above demonstrates the problem with not considering data below 125 Hz. Frankly, neither of these walls stop much sound. Both are mediocre, with a low frequency problem near 125 Hz. However one wall is STC 32, the other is STC 42!

This is because with the blue wall, the big problem occurs at 125 Hz, and is therefore measured by STC. The black wall has essentially the same problem; however it occurs just below 125 Hz, and is therefore not calculated.

Here’s another example of how relying on STC alone is a bad idea when low frequency noise sources are present.

 

We see two walls, one has a STC 47, the other a STC 48. Note that in the low frequency range – important for music, theaters, traffic, aircraft, and most other real-world noise sources – the lower STC wall is literally 30 decibels better, yet lower STC

The actual 1961 standard which explains how to calculate STC (ASTM E413) describes the limitations of its use: “These single-number (STC) ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling allies, power transformers, musical instruments, many music systems and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.”

Caution: STC is not a measure of how many decibels of sound a wall can stop. If you have an STC 45 wall, this does not mean the wall stops 45 dB of sound.

Caution: You cannot add STC ratings. They are logarithmic values and cannot simply be added. If you have an STC 33 wall and decide to add another sheet of drywall with an STC of 20 you do not get a finished wall with an STC of 53. You might get something around STC 35.

STC Examples

STC Track Application
25 Normal speech can be easily heard and understood
30 Loud speech can be easily heard and understood
35 Loud speech heard, but not understood
40 Loud speech now only a murmur
45 Loud speech not heard, music systems / heavy traffic noise still a potential problem
50 Very loud sounds such as musical instruments or a stereo can be faintly heard
60+ Excellent soundproofing

 

STC Examples

STC Partition types: Wall, Floor or Ceiling Low Frequency Performance
33 Single layer of 1/2″ drywall on each side, wood studs, no insulation (typical interior wall) Poor
44 Double layer of 1/2″ drywall on each side, wood studs, batt insulation in wall Poor
52 Single 5/8″ drywall on one side, double 5/8″ drywall and Green Glue on other side wood studs Good
55 Double layer of 1/2″ drywall on each side, on staggered wood stud wall, batt insulation in wall Fair
60 Double 5/8″ drywall on either side of a steel stud wall with insulation and 1 load of Green Glue both sides. Good
62 Double 5/8″ drywall on either side of a steel stud wall with insulation and 2 loads of Green Glue both sides. Excellent
63 Double layer of 1/2″ drywall on each side, on double wood/metal stud walls (spaced 1″ apart), double batt insulation Good

STC ratings courtesy of the NRC and Green Glue Company, reprinted with permission.

 

For new construction, the International Building Code (IBC) requires lab-tested STC 50 for walls, floors and ceilings. Note that this is a minimal amount of isolation and you will likely be disturbed by neighbors. Better to look at STC 55 to STC 60 for best satisfaction.

Changes in STC/Changes in Apparent Loudness

Changes in STC Rating Changes in Apparent Loudness
+/- 1 STC points Almost imperceptible
+/- 3 STC points Just perceptible
+/- 5 STC points Clearly noticeable
+/- 10 STC points Twice (or half) as loud

 

Appendix – calculating STC.

 

Noise is generated in one room and the sound pressure levels in decibels (dB) are measured in both rooms at 16 distinct frequencies between 125 Hz and 4,000 Hz. The difference in levels is corrected to account for the acoustical properties of the receiving room.

We are looking for the Transmission Loss at these 16 frequencies (see table below). This is ASTM E90 Standard Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions. The Sound Transmission Class (STC) is determined in accordance with ASTM E413, Classification for Rating Sound Insulation.

An excellent description for the STC calculation process is provided by Brian Ravnaas at Green Glue Company.

Calculating STC involves adding something called “deficiencies” and utilizing the STC contour In the table you supply the Transmission Loss data. Then you add the STC contour adjustment to attain the adjusted transmission loss. Then you basically play with numbers in the “STC you wish to test column” until one of the 2 limiting conditions listed below are met. The highest number that satisfies both of those conditions is the STC.

The two STC Conditions:

  1. No single frequency band may have more than 8 deficiencies
  2. The total deficiencies may not exceed 32

In the case above, the highest # of deficiencies in any frequency band is 4, and the total deficiencies are 21. So both condition 1 and condition 2 are passed. The appropriate thing to do in this situation would then be to raise the STC you are testing to 56. In this case, the wall would pass again at 56, as shown in this table here:

 

Now the total deficiencies are 30, and it is clear that if we raised the number we wish to test to 57 that the number of deficiencies would exceed 32, and thus the wall could not pass at 57.

Therefore, the STC of this wall is 56.

Lastly, it is important to note that to accommodate the fact that the human hearing system is more sensitive to some frequencies than others, an equal-loudness contour is applied to the frequencies tested. This accommodation is known as A-weighting. In general, low frequency and high frequency sounds appear to be less loud than mid-frequency sounds, and the effect is more pronounced at low levels, with a flattening of response at high levels. A-weighted frequencies are measured in units called phons.