You may well be hearing the need for STC50 all the time. This episode is all about why you need STC50 and at the same time do not need STC50.

STC full form is Sound Transmission Class and is an acoustical rating that determines how much airborne noise is blocked by a product, system or building element. See graphic illustration below.


Hence STC is used as a rating to classify externally – building envelopes, windows, ventilators (for which OITC or Outdoor-Indoor Transmission Class also exists) and internally – building elements like partitions, ceilings, floorings, doors, windows, duct openings and conduit penetrations at room boundaries. See infographic of this below.


An acoustical rating of STC30 would roughly translate to 30dB noise reduction. A higher number signifies a better noise rating. See graphic illustration below.


Noise isolation, noise mitigation, acoustic insulation, sound attenuation, soundproofing – all these terms mean the same thing and have to do with STC ratings to achieve a low background noise. See picture and graphic illustration below.

Hence STC is the acoustical characteristic of the product or system or building element and is the means to the goal of achieving low background noise in an enclosed space.

A low background noise means if all openings like doors, windows to an enclosed but unoccupied room are shut and then HVAC switched on then the measured ambient noise must be low as in say 35 dBA. A low background noise, ensures signal audibility and speech intelligibility.


Let’s say am conversing at 65dBA right now. This is called signal and background noise is 35dBA so there is a delta of 30dB which is called signal noise ratio or SNR in short. SNR of 30dB it is a very good number for an audience in a hall to hear me. See infographic below.


Achieving low background noise in an enclosed space of say 35dBA depends on what is the peak external noise which if it is 85dBA and hence the IL or STC needed is for 85 minus 35 that is 50dBA mitigation. See graphic illustration below.

Now we are ready to get back to STC and STC50. Here are 6 myth busters.


1 STC rating is all that you need

STC considers 1/3rd octave-band frequencies from 125Hz to 4kHz. And I can tell you it is not the most accurate at the LF which along with those below 125Hz would tend to induce structure-borne vibrations in buildings. So do not depend on STC ratings where LF is dominant. See infographic below.


2 You can depend on lab reports of STC ratings

STC of a product is tested in a dual-reverb chamber with a small test opening under ideal lab conditions with proper caulking of joints and edges to arrest leakages. In real life, in field conditions the field STC known as FTSC is always 5~10dBA lower than lab STC or LSTC. So discount lab reports. See graphic representation below.



3 STC ratings can be easily executed in the field

If every element is rated STC50 yet the installation lacks the sensitivity to achieve the result then the chain will be as strong as its weakest link. For example a soundproof partition is bridged on site during installation, the gasketing of a window pane to its frame or of the window assembly to the wall opening is carelessly executed then overall ratings will be severely degraded. Execution is as important and given the current sensitivity of project teams and available skillsets in India it is better to aim for a lower STC rating. See picture below.


4 STC50 would be economical to a project

STC50 means everything if every building element for an enclosed space is equally rated. If you have STC50 rated partitions, glazing, doors, windows, HVAC openings, pipe penetrations then you can achieve upto 45dBA of insertion loss. But this is rarely the case as it shoots up the budget to impractical levels and would make sense for a high-end studio, e.g. In normal projects, openings are likely to be much lower rated, say STC38 or STC41. Hence an expensive STC50 rated partition doesn’t make sense. What makes sense is a less expensive STC41 rated partition. So choose wisely. See graphic illustration below.


5 STC50 will make us achieve the goal of low background noise

STC as I said earlier is just a means to the goal of achieving low background noise that is called NC Noise Criteria  or RC Room Criterion. Then again low background noise is a means to the end goal of audio audibility or signal clarity. So you need to ask what is the audio levels going to be in the room and hence what is the background noise required as also what is the external noise levels going to be and so what is the insertion loss necessary. Ask the right questions instead of just harping on STC50 which is just a means and not the goal. See infographic of RC/NC curves below.


6 STC50 is required in every project

Finally with so many uncontrollable sounds that are likely to exist in an enclosed space including mobile phones would you really need unrealistically low background noise and hence high ratings of STC50 for noise isolation. It might mean double rebated doors, double glazed windows, noise rated gaskets and fire seals etc. Think about it and in the process save some money on projects. An illustrative picture is below.

Like NRC0.9 can be an industry driven myth so also STC50. Trust this video helps bust it. Thanks for reading through and gaining knowledge.


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