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Join Date: Jul 1999
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There are two standards, or recommended best practices, for sound isolation depending upon the objective:
1. In room ambient noise level. This is defined as NR or NC 21 or lower (STC does not count since it ignores sub 125Hz frequencies and was never intended for this application in the first place ... so don't use it). In simple terms, with all environmental elements (HVAC, fans, equipment, etc.) running the ambient noise level in the room should be NC/NR 21 or lower. Using dBSPL as an example, if the ambient noise level is 32dB in room, you must lower that to 21dBSPL. 30 to 35 is typical of a residence. Basically, you need to lower the noise floor by 11dB. [NOTE: you cannot use dBSPL for accurate measurement against the NC/NR curves; but, you will get close.] Thus, your isolation strategy is to reduce noise entering the room, or noise from stuff YOU put in the room.
2. Avoid noise contamination outside the space. The generally accepted metric here is that you will not increase ambient noise levels outside the room by more than 3dB. Let's put this in simple context: the dynamic range is from 22dB to 105dB ... 115dB for low frequency. Thus, if your ambient noise level in an adjacent room (or neighbor's apartment) is 33dB, your sound isolation strategy needs to address a requirement to reduce noise transfer by 79dB. Once again, using dBSPL will get you close; but, that is not the method used to determine NR/NC. For reference, a baby screaming or crying loudly is on the order of 130dB ... I'm sure OSHA will try to regulate that level of noise pollution at some point.
Neither of these goals is easily achieved and likely not by anyone other than very skilled and experienced individuals and contractors. One can, however, see the effort to prevent noise pollution in adjacent spaces is a much different challenge than keeping in room ambient to NR21!
Dennis Erskine CFI, CFII, MEI
Subject Matter Expert
Certified Home Theater Designer
CEDIA Board of Directors