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| NC > About NC |
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| NC (Noise Criteria) is an index to show the calmness of a room. It was proposed by Leo L. Beranek (America) in the Noise Control on 1957. (Later, some modifications were added and the curves were introduced). NC-curves were produced from the outcome of questionnaire about steady-state noise with broadband spectrum, such as the noise of air-conditioning equipment in an office. The noise to be graded is octave-analyzed and the minimum value that does not exceed any curve in any band is designed to be a NC-curves. The less NC-curve is, the calmer the room is. For example, a room at NC 25 is quieter than one at NC40. | |
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| [The relationship of NC-curve and Conversation Intelligibility] |
| NC | Condition of Noise | Applicable Examples |
| NC 20 to 30 | Very quiet No nuisance for telephone conversation Accomodate for big conferences |
Executive Suite Large Conference Room |
| NC 30 to 35 | Quiet Can hold a comference across a 4.5m-long table Can talk over a distance of 3-9m |
Japanese-style Room Reception Room Small Conference Room |
| NC 45 to 40 | Can hold a conference across a 2-2.5m-long table No nuisance for telephone conversation. Can talk over a distance of 2-4m |
Middle-sized Office Factory Office |
| NC 40 to 50 | Can hold a conference across a 1.5m-long table Telephone communication is rather difficult. Conversation in normal voices is possible over 1~2m Can talk over a distance of 2~4m with rather loud voice |
Large Machine Room Drafting Room |
| NC 50 to 55 | Impossible to hold conversation among over 2-3 people Telephone communication is rather difficult. Conversation with normal voice is possible over 30-60cm. Can talk over a distance of 1-2m with rather loud voice |
Typing Room Computer Room Drafting Room etc |
| above NC 55 | Very noisy Inappropriate for Offices Telephone conversation is possible. |
Non-applicable |
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| NC is widely used as the reference value of noise. The recommended value according to room application is shown. |
| [Permissible NC in Rooms] |
| Recording Studios | NC 15 |
| TV Studios | NC 20 to 25 |
| Consert Hall | under NC 15 |
| Multipuropose Halls | NC 15 to 20 |
| Theaters | NC 20 to 25 |
| Opera Houses | NC 20 to 25 |
| Church, Chapels | NC 20 to 25 |
| Multipropose Event Space | NC 35 to 40 |
| Judging from the list above, studios require NC 15 to 25. Recently recording can be made with much less noise due to noise reduction in electronics and air conditioning. The calmness which is closed to M.A.F (Minumum Audiable Sound Field) is required. |
| In studios, the sound insulation against exterior noise is strongly established. So, NC-Value is determined mainly by the noise that arises in the room, such as the noise of air-conditioning equipment. There are two main types of air conditioning systems: "Duct-connected Type" and "Ceiling-mounted Cassette Type / Wall-mounted Type". |
| Duct-connected Type With the combination of the duct-type air-conditioning machine and the silencer, you can make the quiet environment even when the machine is acting. And with regard to the ventilation equipment, noise reduction type is used. Calmness at the level of about NC20 to M.A.F.can be obtained by using high-grade silencers. |
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| Ceiling-mounted Cassete Type / Wall-mounted Type Using this type of air-conditioning machine, the noise level of air-conditioning is equal to that of normal offices when the machine is acting. By using sound-reduction type for ventilation equipment, you can gain the calmness which could be obtained when the machine is not acting. NC-Value is about 40 to 30 when machine is acting. |
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