STC: Sound Transmission Class or SPEECH Transmission Class?
The venerable STC rating is used to classify and rank-order partition sound isolation performance. A high rating, especially above 50, is widely considered a baseline requirement for sound isolation. However, a radio station designed its studios around 4″ thick, modular, STC 57 steel panel partitions. What could go wrong? After moving in they discovered that bass from the rock’n’roll studios was plainly audible on open mics in adjacent newsrooms. How could that happen?
Mysteries of the STC Method
The STC rating is determined by comparing sound transmission loss (TL) test results to “reference contours” over the speech frequency range 125 – 4000 Hz. These contours presume/allow low-frequency TLs to be at least 20 dB less than high-frequency TL. Furthermore, the STC method guarantees that TLs are below the contour in at least four bands (“deficiencies”), by as much as 8 dB. Low frequencies below 125 Hz is not reflected in the STC rating – in other words, treated as irrelevant.
STC is not effective for low-frequency sound
The chart (right) illustrates why the STC rating can be misleading when the incident sound is heavy in “bass” content – e.g., music (esp. live music), industrial equipment, transportation noise, etc.
This common wallboard partition has stellar STC performance and attenuates speech very well, but the TL is 33 dB at 125 Hz, and only 13 at 63 Hz, where bass and kick drum dominate. This is not an anomaly or defective design – it’s typical for dual-leaf partitions because of the air gap resonance.
In the case of the radio station, the panel resonance was around 100 Hz, – which is the A-flat at the bottom of the bass clef – there’s a LOT of music down there. The bass player still has at least one more string left below that. So, no wonder, right? But STC ratings (and single-number ratings in general) can lull you into a false sense of security
How to not make the same mistake?
1) Always ask yourself “What could go wrong?” – recognize the assumptions and blind spots underlying your methods and data.
2) Think of STC as “Speech Transmission Class”. If you’re trying to isolate anything other than speech….
3) Select partitions based on the spectrum of the incident sound and the requirements in the receiving space. If that’s not well defined….
4) Use a low-frequency rating like OITC (“Outdoor-Indoor Transmssion Class”, ASTM E1332). (Note: the STC 56 wallboard partition has an OITC of 35.)
5) If all of this is new or surprising to you, look into Noise Control Master Class online training – Nelson Acoustics can get you up to speed quickly on noise control concepts, without going back to engineering school.
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