Why STC Doesn’t Tell You Everything

The sound isolating properties of wall and floor/ceiling assemblies are often presented using a single number rating system, the Sound Transmission Class (STC). The STC rating of a partition is determined by following an ASTM Standard which compares the sound transmission loss from 125 Hz to 4000 Hz at the 1/3-octave band center frequencies to a set of reference curves. The range from 125-4000 Hz constitutes the majority of the energy in speech sounds. Therefore, the single number STC rating system does a good job at estimating the reduction of a partition in regards to speech noise. An STC 50 wall will reduce speech sounds by approximately 50 dB.

However, audio systems, mechanical equipment, traffic noise, etc., generate sounds well below the 125 Hz limit of the STC rating system. For sounds below 125Hz, the STC rating system tells us nothing about how much sound the partition can block. For example, a standard acoustically-rated partition consisting of two layers of 5/8-inch gypsum board on both sides of studs, with batt insulation placed in the stud cavity, has a laboratory rating of STC 55-57. An 8-inch thick reinforced concrete wall has a laboratory rating of STC 58. In the speech range, the walls will attenuate approximately the same amount of sound. However, in the 125 Hz octave band, the stud wall will block approximately 32 dB, while the concrete wall will block 42 dB. That is a 10 dB difference, which subjectively is perceived as half as loud! In the 63 Hz octave band and below, where musical instruments such as bass drums have their fundamentals, and where mechanical equipment is often very loud, the stud wall will only block approximately 18 dB, while the concrete wall will block over 30 dB.

While the STC rating system provides a good estimate for the amount of speech sound reduction, it should not be used in situations where the goal is to reduce low frequency sounds. This is why we suggest architects not put STC ratings on details whenever possible. When low frequency sounds are the issue, the partition must be selected based on 1/3-octave band transmission loss data.