Normal speech is audible but not necessarily understood. Loud speech is clearly audible and easily understood.
Loud speech is clearly audible but probably unintelligible noise
Loud speech is still audible but only as a murmur. Music and TV’s are easily heard
Shouting barely audible
“An STC rating roughly equals the decibel (dB) reduction in noise volume a wall or partition can provide. For example, if an 80 dB sound on one side of a wall/floor/ceiling is reduced to 50 dB on the other side, that partition is said to have an STC of 30”
Change in Sound LevelChange in Loudness
+/- 1 dB Requires close attention to notice
+/- 3 dB Barely perceptible
+/- 5 dB Quite Noticeable
+/- 10 dB Dramatic; nearly twice or half as loud
+/- 20 dBStriking, fourfold change
Source: “Quieting: A Practical Guide to Noise Control, National Bureau of Standards Handbook 119” as published by the “U.S. Department of Commerce, National Bureau of Standards”
Difference between STC and dB
Then, what is NIC?
NIC (Noise Isolation Class) is a single number rating derived from a standard ASTM (E-336) field (not in the lab) conducted test procedure.
The FIELD test is conducted on an installed moveable wall with the NIC value determined by measuring the sound pressure level difference through the installed product.
AND also taking into account sound that is transmitted through the surrounding construction including floors, permanent walls, ceiling, plenums, HVAX, electrical and plumbing systems: all building elements. The results are then calculated as per ASTM E-412 and reported as the moveable wall NIC. NIC is indicative of what the users of the room will experience. An NIC of 40 or more on a moveable wall is considered to be excellent. NIC is more indicative as to what the ear will hear. Typically an NIC value will be minimum 10 points less than the STC value obtained in the laboratory. If you can obtain an NIC of 40 or a bit more this is a great result.
Marginal. Aware of presence of others in adjacent areas. Conversations occasionally understood.
Always aware of activities in adjacent areas. Activity and speech always heard and understood.
Conversations always heard and understood. All adjacent activities clearly audible.