Sound transmission Class (STC) is a single number quantifier used to rate partitions, doors and windows for their effectiveness in blocking sound. The number assigned to a particular partition design as a result of STC testing represents a best fit type of approach to a set of curves which define the sound transmission class. The test is conducted in such a way to make it independent of the test environment, and yields a number for the partition or floor only. The reverberance and size of the room are factored out, unlike in a Noise Reduction (NR) test. Transmission Loss data can be measured in a laboratory or in the field by means of a standardized procedure defined in ASTM E90 for laboratories and E336 for field tests in actual buildings. The actual STC curve is defined in ASTM E413; the shape of the STC curve approximates the performance of actual walls in terms of frequency. The STC is heavily weighted in favor of the speech frequency range above 1250 Hz and correlates with human hearing acuity.

For the determination of the final STC number, the values between 400 Hz and 1250 Hz are increasingly discounted. The STC number is determined from Transmission Loss values using an algebraic formula for the sum of deficiencies (which must be less than 32) below the standard curves as well as for the maximum deficiency of 8 dB at any frequency.

The standard test method also requires minimum room volumes for the test to be correct at low frequencies. Due to the definition of the rating, STC is only applicable to air-borne sound, and is a poor guideline for construction that allows impact noise to pass through the structure. The standard also only considers frequencies above 125 Hz. Composite partitions composed of elements such as doors and windows as well as walls will tend to have an STC close to the value of the lowest STC of any of the component. Thus a wall with an STC of 50 combined with a door having an STC of 25 will result in a combined partition STC closer to 25 rather than closer to 50. In practice, the STC of the laboratory sample represents the optimum condition, and is rarely achieved in actual construction. The difference between the apparent STC (ASTC), which is the STC tested in the field, and the laboratory STC is the result of leaks and flanking.

The following chart displays STC test results and descriptions.

Table 1: Subjective interpretation of effects of STC as measured (assumes normal background level of NC 35). Note: the actual perceived effect of STC depends on the background noise levels, room volumes, surface areas, sound absorption values and spectral content of the sound source.

The actual behavior of two partitions with the same STC rating can be dramatically different, as the STC is weighted in favor of the part of the sound spectrum that represents the human voice. In practice, one of the most annoying transmitted sounds between dwelling units tends to be the bass in music, a part of the sound spectrum far removed from the voice range. For example, an eight inch concrete block wall rated at STC 50, that can block 20 dB more sound in some bass frequencies and would be a better choice than an STC 50 drywall partition for an application where music or mechanical noise will be a problem.

The 2005 National Building Code requires that partitions separating dwelling units meet an STC 50 requirement, and the building code provides sample ratings for several types of wall constructions. Unfortunately, test ratings of the same wall section vary from test to test, and in field situations, walls cannot be expected to perform as well as the test sample in laboratory conditions.

This drop in performance can leave the builder liable for additional construction to bring up the performance of the wall if the tenants obtain field test results from the dwelling units that confirm a reduced STC. For example, a wall section listed by the NRC as meeting the STC 50 requirement has staggered 38 x 89 mm wood studs that are 400 mm o.c. on a 140 mm plate with 65 mm batt insulation filling the cavity and a single layer of drywall on one side and two layers on the other side (TL-93-353).

Figure 1 - partition construction STC 50.

Due to minute differences at some frequencies from wall to wall, laboratory tests result in an STC rating of that particular wall section varies from STC 47 to STC 51. In field tests, the wall might get only about STC 46, even with caulking at the plates. Any cracks in the wall or holes for electrical or mechanical servicing will further reduce the actual field result, leaving the builder responsible for upgrades. Rigid connections between wall surfaces can also seriously degrade the wall performance. The higher the target STC, the more critical sealing and structural isolation requirements are. The builder’s best options for getting a satisfactory STC result are to specify partitions with a laboratory rating of STC 54 or better. If in doubt at an early stage in the construction, testing can be done to rate the construction and upgrades recommended before costly finishing is in place.

An STC test is conducted on partitions by placing a noise source on one side, a measuring microphone on the other side, and a spectrum analyzer connecting the two. The drop in decibels through the wall (Noise Reduction), background sound level and reverberation time RT60 are measured along with the area of the partition and the volume of the receive room resulting information compared to the standard STC definition to arrive at the STC rating of the wall or floor. In order to be applicable, the test must be carried out according to defined test procedures from ASTM E336. Professional acoustic consultants such as State of the Art Acoustik Inc can perform these tests and report on the rating, and if necessary recommend improvements in construction to achieve the code required STC.

Simply adding extra material to the wall or re-organizing the elements of the wall can be a hit and miss procedure. As an example, if you take two wall sections, both made up of five to six layers of drywall, two inches of fiberglass batt, resilient channel and a single row of 2 x 4 studs. Both walls cost about the same, but one wall gets STC 43 while the other gets STC 61, depending on the order in which the layers are arranged. More information on STC ratings can be obtained by calling us or referring to the NRC library.

It should also be noted that STC tests done prior to 1990 are generally no longer valid due to the weight and fabrication of drywall and the changes to STC test methodology.

STATE OF THE ART ACOUSTIK has designed and tested partitions with a field STC above 80, and a 63 Hz transmission loss of 56 dB, easily besting the THX standard for commercial cinemas.