Trane TRG-TRC007-EN manual NC-39

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period two

Sound Perception and

Rating Methods

notes

Noise Criteria (NC) Curves

 

80

 

 

 

 

 

 

 

mPa

70

 

 

 

 

 

 

 

 

NC-39

 

 

 

 

NC-65

20

60

 

 

 

 

 

 

 

 

 

 

 

 

NC-60

 

 

 

 

 

 

 

ref

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NC-55

dB

50

 

 

 

 

 

 

 

 

 

 

 

 

NC-50

 

 

 

 

 

 

 

pressure,

 

 

 

 

 

 

 

40

 

 

 

 

 

 

NC-45

 

 

 

 

 

 

NC-40

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NC-35

sound

30

 

 

 

 

 

 

 

 

 

 

 

 

NC-30

 

 

 

 

 

 

 

20

 

 

 

 

 

 

NC-25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NC-20

 

10

 

 

 

 

 

 

NC-15

 

125

250

500

1,000

2,000

4,000

8,000

 

63

octave-band frequency, Hz

Figure 27

The following steps describe how to calculate an NC rating.

1Plot the octave-band sound-pressure levels on the NC chart.

2The highest curve crossed by the plotted data determines the NC rating.

Although the NC curves are popular and easy to use, they do have a few shortcomings. Specifically, they do not account for the tonal nature and relative magnitude of each octave band. Figure 27 shows octave-band data measured in an open-plan office space and plotted on an NC chart. The resulting value, NC- 39, is generally considered to be acceptable for this type of environment. Notice that this NC value is set by the 63 Hz octave band and the sound drops off quickly in the higher octave bands.

In this particular example, sound generated by the air-handling unit travels through the ductwork, breaks out through the duct walls, and radiates into the office area. To achieve the desired NC level, two layers of sheet rock were added to the exterior surface of the duct to block the low-frequency sound. Unfortunately, because high-frequency sounds are much more easily attenuated than low-frequency sounds, the upper octave bands are now over- attenuated.

Although an objective analysis deems the resulting NC-39 sound level acceptable in this type of open-plan office space, most listeners in the space would probably perceive this unbalanced spectrum as having an annoying rumble.

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TRG-TRC007-EN

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Contents Air Conditioning Clinic Fundamentals of Hvac Acoustics Business Reply Mail One of the Fundamental Series Comment CardFundamentals of Hvac Acoustics Fundamentals of Hvac Acoustics PrefaceContents TRG-TRC007-EN Fundamentals of Sound Fundamentals of SoundWhat is Sound? What is Sound?Sound Wave and Frequency Wavelength Broadband Sound and Tones Broadband SoundOctave Bands Octave BandsLogarithmic sums One-Third Octave Bands Sound Power and Sound Pressure Sound Power and Sound PressureAn Analogy Decibel DB = 10 log10 ´ log Logarithmic ScaleEquation for Sound Pressure Equation for Sound Power50 dB + 44 dB = 51 dB Logarithmic Addition of DecibelsHuman Ear Response Sound Perception Rating MethodsHuman Ear Loudness Contours Single-Number Rating Methods Response to TonesSingle-Number Rating Methods Sound Perception and Rating Methods C WeightingWeighting Example Weighting Noise Criteria NC Curves NC-39 Room Criteria RC Curves MPa DB ref Pressure Sound Perception and Rating Methods RC-31 R Phon and Sone Octave-Band Rating Method Octave-Band Rating MethodAcoustical Analysis Acoustical AnalysisSetting a Design Goal Setting a Design GoalAcoustical Analysis Source-Path-Receiver Analysis Acoustical AnalysisSource-Path-Receiver Model Airborne Typical Sound PathsExample of Multiple Sound Paths Examples of a Single Sound PathIdentifying Sound Sources and Paths Sound-Path Modeling Modeling Sound PathsSupply Algorithms for Sound-Path Modeling Computerized Analysis Tools Terms Used in Sound-Path Modeling Attenuation and RegenerationSound Transmission Insertion loss IL Noise reduction NR Transmission loss TL Absorption Reflected Sound Receiver Sound Correction Equipment Sound Rating Equipment Sound RatingFields of Measurement Free FieldDistance Correction in a Free Field = L p1 20 logLot Near Field Reverberant Field Semireverberant Field Hvac Equipment Sound Rating Rating Hvac EquipmentReverberant-Room Method Free-Field Method Industry Standards Ducted Air-Handling Equipment Air Handler Test Configurations Former Methods of Sound Testing ARI Standard Sound Power by Octave Band Review-Period One ReviewNoise criteria NC Room criteria RC Review-Period TwoReview-Period Three Review-Period Four Review Questions for Period QuizOctave-band frequency, Hz Answers Glossary Glossary Room effect See receiver room correction Glossary Trane