What Are Noise Criteria Levels?
Jul 17, 2023When designing a soundproof room, acoustic designers will use Noise Criterion Curves or NC curves for short to ensure the room meets proper sound isolation guidelines. In this article I will go over what NC curves are and how to use them as a guide for your own studio build.
1) What are NC Levels
Noise Criteria levels are benchmarks we can use to get our soundproof rooms to an industry standard. Here are some guidelines for NC Levels for various applications:
NC 15-20: Broadcasting Studios, Concert Halls, Recording Studios and Music Rooms
NC 30: Residences, Theaters, Libraries, Executive Offices
NC 35: Private Offices, Schools, Hotel Rooms, Churches, Hospitals
NC 40: Labs, Dining Rooms, General Office Use
NC 45: Retail Stores, Lobby Areas, Reception Areas
NC 50+: Computer Rooms, Print Machine Rooms
Studio designer and soundproofing expert, Rod Gervais states that his goal when designing professional recording studios is to shoot for NC 20 or below and preferably NC 15 if possible. (Gervais 158)
Each of these NC levels represents acceptable sound pressure levels at eight octave band center frequencies. Below is a diagram from Home Recording Studio: Build It Like The Pros by Rod Gervais.
Notice for a studio to fall in the NC15 range you must get 47 dB at 63Hz, 36dB or below at 125Hz and so on. Now that you understand the concepts behind NC levels you might be wondering how to calculate these levels in your own home studio.
2) How To Calculate NC Levels
NC levels are not easy to calculate without the right tools and experience. For this reason I recommend using the chart above as a guide and using your smart phone to record levels. Although, this is not a great method for professionals for the average DIY studio build getting some general ideas of how much isolation you need is helpful.
I recommend using the DecibelX app. You can download it for free and can pay for an upgrade if you want. This app allows you to measure the average dB level in your room and you can see the dB range over the frequency spectrum. This will help you see where you may have potential problems in your soundproofing design.
If you want a professional level test done then you will either need to hire someone with the right experience running NC level tests or rent the right gear and read the guidelines.
The American National Standards Institute (ANSI) provides guidelines for getting accurate measurements. You can look those up at ANSI S1.13-1971 (R 1986) American National Standard Methods for the Measurement of Sound Pressure Levels (Everest Pohlmann 313).
If you do your own measurements you will need to calibrate your SPL meter. You will also need a meter that can measure octave band frequencies and it should be mounted on a tripod for accurate results.
As you can see, there is a reason professionals do this work. It must meet stringent standards to qualify as credible data.
The choice is up to you if you want to try measuring NC level at all. I find the knowledge useful in understanding benchmarks for professional studio design, but probably overkill for the average DIY home studio build.
Works Cited
Everest, Frederick A., and Ken C. Pohlmann. “Modal Resonances.” Master Handbook of Acoustics, McGraw-Hill, New York, 2015.
Gervais, Rod. Home Recording Studio: Build It Like The Pros. 2nd Edition, Course Technology Cengage Learning, 2011.
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