Sound Transmission Class (or STC) is an integer-number rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used.

The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time (that is the differences in test method from the 1970’s to today are vast).

The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce. Alternatively, Outdoor-Indoor Transmission Class (OITC) is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.

STC is roughly the decibel reduction in noise a partition can provide, abbreviated ‘dB’. The dB scale is a logarithmic one and the human ear perceives a 10dB reduction in sound as roughly halving the volume - a 40 dB noise subjectively seems half as loud as a 50 dB one. (For more detail on equal-loudness curves see: Fletcher-Munson curves.) If an 80dB sound on one side of a wall/floor/ceiling is reduced to 50dB on the other side, that partition is said to have an STC of 30. This number does not apply across the range of frequencies, since the STC value is A-weighted and derived from a curve-fit of many datapoints. Any partition will have less TL at lower frequencies. For example, a wall with an STC of 30 may provide over 40dB of attenuation at 3000 Hz but only 10dB of attenuation at 125 Hz.

Typical interior walls in homes (2 sheets of 1/2″ drywall on a wood stud frame) have an STC of about 33. When asked to rate their acoustical performance, people often describe these walls as “paper thin”. They offer little in the way of privacy. Adding absorptive insulation (i.e. fiberglass batts) in the wall cavity increases the STC to 36-39, depending on stud and screw spacing. Doubling up the drywall in addition to insulation can yield STC 41-45, provided the wall gaps and pentrations are sealed properly.

Note that doubling the mass of a partition does not double the STC. Doubling the mass (going from two total sheets of drywall to four, for instance) typically adds 5-6 points to the STC. Breaking the vibration paths by decoupling the panels from each other will increase transmission loss much more effectively than simply adding more and more mass to a monolithic wall/floor/ceiling assembly.

Structurally decoupling the drywall panels from each other (by using resilient channels, steel studs, a staggered-stud wall, or a double stud wall) can yield an STC as high as 63 or more for a double stud wall (see table below), with good low-frequency transmission loss as well. Compared to the baseline wall of STC 33, an STC 63 wall will transmit only 1/1000 as much sound energy, seem 88 percent quieter and will render most frequencies inaudible.

Due to their high density, concrete and concrete block walls have good TL values (STC’s in the 40s and 50s for 4-8″ thickness) but their weight, added complexity of construction and poor thermal insulation tend to limit them as viable materials in most residential wall construction, except in temperate climates and hurricane or tornado prone areas. Various Cellulose insulation installation options can result in an STC of 50 or greater. ^[1]

Materials which can improve STC’s in walls include mass-loaded vinyl (MLV) and soundproof drywall, such as QuietRock.

It must be noted that acoustical performance values such as STC are measured in specially constructed acoustical chambers and field conditions such as lack of adequate sealing, outlet boxes, back-to-back electrical boxes, medicine cabinets, flanking paths and structure-borne sound can diminish acoustical performance. The as-built ‘field-STC’ (FSTC) is usually lower than the laboratory-measured STC.

New multifamily dwellings(apartments, condominiums, duplexes, etc.) are required by building code to meet a minimum of STC 50 between units. This is sufficient to block normal speech, but may not adequate for a home theater or loud stereo which can reach 110 dB or more.

In serious cases (for instance, a bedroom adjacent to a home theater room, and an inconsiderate nocturnal neighbor, to boot) a partition to reduce sounds from high-powered home theater or stereo should ideally be STC 70 or greater, and show good attenuation at low frequencies. An STC 70 wall can require detailed design and construction and can be easily compromised by ‘flanking noise‘, sound traveling around the partition through the contiguous frame of the structure, thus reducing the STC significantly. STC 65 to 70 walls are often designed into luxury multifamily units, dedicated home theaters, and high end hotels.

The demanding THX reference standard (a guideline for high-quality audio in movie soundtracks) requires partitions to achieve 50dB of attenuation at 63 Hz. Few walls can meet that, as that requires a wall with an STC of 80 or higher. For all practical purposes, no sound will be heard on the other side of the wall with this level of construction. However, an STC this high is not achievable in simple construction and this level of isolation is only feasible for high-end studios and theaters, where the design and construction can be carefully controlled and the additional cost is justified.

Sound Transmission Class Examples

STC partition ratings taken from: “Noise Control in Buildings: A Practical Guide for Architects and Engineers”; Cyril M. Harris, 1994

Noise health effects

Roadway noise is a major source of exposure

Noise health effects are the health consequences of elevated sound levels. Elevated workplace or other noise can cause hearing impairment, hypertension, ischemic heart disease, annoyance, sleep disturbance, and decreased school performance. Changes in the immune system and birth defects have been attributed to noise exposure, but evidence is limited.^[1] Although some presbycusis may occur naturally with age,^[2] in many developed nations the cumulative impact of noise is sufficient to impair the hearing of a large fraction of the population over the course of a lifetime.^[3][4] Noise exposure has also been known to induce tinnitus, hypertension, vasoconstriction and other cardiovascular impacts.^[5] Beyond these effects, elevated noise levels can create stress, increase workplace accident rates, and stimulate aggression and other anti-social behaviors.^[6] The most significant causes are vehicle and aircraft noise, prolonged exposure to loud music, and industrial noise.

Hearing loss

Hearing loss is somewhat inevitable with age. Though older males exposed to significant occupational noise demonstrate significantly reduced hearing sensitivity than their non-exposed peers, differences in hearing sensitivity decrease with time and the two groups are indistinguishable by age 79.^[2] Women exposed to occupational noise do not differ from their peers in hearing sensitivity, though they do hear better than their non-exposed male counterparts. Due to loud music and a generally noisy environment, young people in the United States have a rate of impaired hearing 2.5 times greater than their parents and grandparents, with an estimated 50 million individuals with impaired hearing estimated in 2050.^[3]

The mechanism of hearing loss arises from trauma to stereocilia of the cochlea, the principal fluid filled structure of the inner ear.^{[citation needed]} The pinna combined with the middle ear amplifies sound pressure levels by a factor of twenty, so that extremely high sound pressure levels arrive in the cochlea, even from moderate atmospheric sound stimuli. Underlying pathology to the cochlea are reactive oxygen species, which play a significant role in noise-induced necrosis and apoptosis of the stereocilia.^[7] Exposure to high levels of noise have differing effects within a given population, and the involvement of reactive oxygen species suggests possible avenues to treat or prevent damage to hearing and related cellular structures.^[7]

Cardiovascular effects

Noise has been associated with important cardiovascular health problems.^[8] In 1999, the World Health Organization concluded that the available evidence showed suggested a weak association between long-term noise exposure above 67-70 dB(A) and hypertension.^[9] More recent studies have suggested that noise levels of 50 dB(A) at night may also increase the risk of myocardial infarction by chronically elevating cortisol production.^[10][11][12]

Fairly typical roadway noise levels are sufficient to constrict arterial blood flow and lead to elevated blood pressure; in this case, it appears that a certain fraction of the population is more susceptible to vasoconstriction. This may result because annoyance from the sound causes elevated adrenaline levels trigger a narrowing of the blood vessels (vasoconstriction), or independently through medical stress reactions. Other effects of high noise levels are increased frequency of headaches, fatigue, stomach ulcers and vertigo.^[13]

The U.S. Environmental Protection Agency authored a pamphlet in 1978 that suggested a correlation between low-birthweight babies (using the World Health Organization definition of less than 2,500 g (~5.5 lb) and high sound levels, and also correlations in abnormally high rates of birth defects, where expectant mothers are exposed to elevated sound levels, such as typical airport environs. Specific birth abnormalities included harelip, cleft palate, and defects in the spine. According to Lester W. Sontag of The Fels Research Institute (as presented in the same EPA study): “There is ample evidence that environment has a role in shaping the physique, behavior and function of animals, including man, from conception and not merely from birth. The fetus is capable of perceiving sounds and responding to them by motor activity and cardiac rate change.” Noise exposure is deemed to be particularly pernicious when it occurs between 15 and 60 days after conception, when major internal organs and the central nervous system are formed. Later developmental effects occur as vasoconstriction in the mother reduces blood flow and hence oxygen and nutrition to the fetus. Low birth weights and noise were also associated with lower levels of certain hormones in the mother, these hormones being thought to affect fetal growth and to be a good indicator of protein production. The difference between the hormone levels of pregnant mothers in noisy versus quiet areas increased as birth approached.

Annoyance

Because some stressful effects depend on qualities of the sound other than its absolute decibel value, the annoyance associated with sound may need to be considered in regard to health effects. For example, noise from airports is typically perceived as more bothersome than noise from traffic of equal volume.^[14]Annoyance effects of noise are minimally affected by demographics, but fear of the noise source and sensitivity to noise both strongly affect the ‘annoyance’ of a noise.^[15] Even sound levels as low as 40 dB(A) (about as loud as a refrigerator or library^[16]) can generate noise complaints^[17] and the lower threshold for noise producing sleep disturbance is 45 dB(A) or lower.^[18] Other factors that affect the ‘annoyance level’ of sound include beliefs about noise prevention and the importance of the noise source, and annoyance at the cause (i.e. non-noise related factors) of the noise.^[19] Evidence regarding the impact of long-term noise versus recent changes in ongoing noise is equivocal on its impact on annoyance.^[19]

Estimates of sound annoyance typically rely on weighting filters, which consider some sound frequencies to be more important than others based on their presumed audibility to the human ear. The older dB(A) weighting filter described above is used widely in the U.S., but underestimates the impact of frequencies around 6000 Hz and at very low frequencies. The newer ITU-R 468 noise weighting filter is used more widely in Europe. The propagation of sound varies between environments; for example, low frequencies typically carry over longer distances. Therefore different filters, such as dB(B) and dB(C), may be recommended for specific situations.

When young children are exposed to speech interference levels of noise on a regular basis (the actual volume of which varies depending on distance and loudness of the speaker), there may develop speech or reading difficulties, because auditory processing functions are compromised.^{[citation needed]} In particular the writing learning impairment known as dysgraphia is commonly associated with environmental stressors in the classroom.^{[citation needed]}

Regulations

Environmental noise regulations usually specify a maximum outdoor noise level of 60 to 65 dB(A), while occupational safety organizations recommend that the maximum exposure to noise is 40 hours per week at 85 to 90 dB(A). For every additional 3 dB(A), the maximum exposure time is reduced by a factor 2, e.g. 20 hours per week at 88 dB(A). Sometimes, a factor of two per additional 5 dB(A) is used. However, these occupational regulations are acknowledged by the health literature as inadequate to protect against hearing loss and other health effects

References

1. ^ Passchier-Vermeer W, Passchier WF (2000). “Noise exposure and public health”. Environ. Health Perspect. 108 Suppl 1: 123–31. PMID 10698728. 
2. ^ ^{a b} Rosenhall U, Pedersen K, Svanborg A (1990). “Presbycusis and noise-induced hearing loss”. Ear Hear 11 (4): 257–63. PMID 2210099. 
3. ^ ^{a b} Schmid, RE. “Aging nation faces growing hearing loss“, CBS News, 2007-02-18. Retrieved on 2007-02-18. 
4. ^ Senate Public Works Committee, Noise Pollution and Abatement Act of 1972, S. Rep. No. 1160, 92nd Cong. 2nd session
5. ^ Noise: Health Effects and Controls. University of California, Berkeley. Retrieved on 2007-12-22.
6. ^ Kryter, Karl D. (1994). The handbook of hearing and the effects of noise: physiology, psychology, and public health. Boston: Academic Press. ISBN 0-12-427455-2. 
7. ^ ^{a b} Henderson D, Bielefeld EC, Harris KC, Hu BH (2006). “The role of oxidative stress in noise-induced hearing loss”. Ear Hear 27 (1): 1–19. doi:10.1097/01.aud.0000191942.36672.f3. PMID 16446561. 
8. ^ Ising H, Babisch W, Kruppa B (1999). “Noise-Induced Endocrine Effects and Cardiovascular Risk“. Noise Health 1 (4): 37–48. PMID 12689488. 
9. ^ Berglund, B; Lindvall T, Schwela D, Goh KT (1999). World Health Organization: Guidelines for Community Noise. World Health Organization.
10. ^ Maschke C (2003). “Stress Hormone Changes in Persons exposed to Simulated Night Noise“. Noise Health 5 (17): 35–45. PMID 12537833. Retrieved on 2007-12-22. 
11. ^ Franssen EA, van Wiechen CM, Nagelkerke NJ, Lebret E (2004). “Aircraft noise around a large international airport and its impact on general health and medication use”. Occup Environ Med 61 (5): 405–13. PMID 15090660. 
12. ^ Lercher P, Hörtnagl J, Kofler WW (1993). “Work noise annoyance and blood pressure: combined effects with stressful working conditions”. Int Arch Occup Environ Health 65 (1): 23–8. PMID 8354571. 
13. ^ Noise: A Health Problem United States Environmental Protection Agency, Office of Noise Abatement and Control, Washington, DC 20460, August, 1978
14. ^ Miedema and Oudshoorn 2001 cited in Hypertension and exposure to noise near airports. Medscape.
15. ^ Miedema HME, Vos H. “Demographic and attitudinal factors that modify annoyance from transportation noise”. Journal of the Acoustical Society of America 105 (6): 3336–44. doi:10.1121/1.424662. 
16. ^ [1.pdf Noise Facts and Figures!] (PDF). Chiltern District Council. Retrieved on 2007-12-13.
17. ^ Gelfand, Stanley A. Essentials of Audiology. New York: Thieme Medical Publishers. ISBN 1-58890-017-7. 
18. ^ Walker, JR; Fahy, Frank (1998). Fundamentals of noise and vibration. London: E & FN Spon. ISBN 0419227008. 
19. ^ ^{a b} Field, JM (1993). “Effect of personal and situational variables upon noise annoyance in residential areas”. Journal of the Acoustical Society of America 93 (5): 2753–63. doi:10.1121/1.405851. Retrieved on 2007-12-13.