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1.3.1 Sound Level Measurements > Conclusion - Pg. 10

10 SecTion | 1 Test and Measurement Noise criteria ratings provide a one-number specification for allowable levels of ambient noise. Sound level measurements are performed in octave bands, and the results are plotted on the chart shown in Fig. 1.4. The NC rating is read on the right vertical axis. Note that the NC curve is frequency-weighted. It permits an increased level of low-frequency noise, but becomes more stringent at higher frequencies. A sound system specification should include an NC rating for the space, since excessive ambient noise will reduce system clarity and require additional acoustic gain. This must be considered when designing the sound system. Instrumentation is available to automate noise criteria measurements. Conclusion Stated sound level measurements are often so ambiguous as to become meaningless. When stating a sound level, it is important to indicate: 1. The sound pressure level 2. Any weighting scale used 3. Meter response time (fast, slow or other) 90 Octave band sound pressure level ­ dB 80 70 60 50 40 30 20 10 0 125 250 500 1000 Frequency­Hz 2000 TEF 20 NC - 40 70 65 60 55 50 45 40 35 30 25 20 15 8000 4000 FigURe 1.4 A noise criteria specification should accompany a sound system specification.