🎯 What Is NC?
NC (Noise Criteria) is a standard rating system for background noise from HVAC systems, traffic, and other steady-state sources. It considers both the level and frequency content of noise.
- Lower NC = Quieter: NC-15 is very quiet (recording studio), NC-45 is moderately noisy (open office)
- Spectrum-aware: NC curves allow higher levels at low frequencies where hearing is less sensitive
- Single-number rating: Simplifies comparison and specification
Why not just use dBA? A room can have the same dBA level but different NC ratings depending on the frequency content. Low-frequency rumble is more annoying and intrusive than high-frequency hiss at the same dBA. NC captures this difference.
📊 NC Targets by Space Type
| Space Type |
Target NC |
Notes |
| Recording studio |
NC-15 to NC-20 |
Critical listening, microphone sensitivity |
| Concert hall |
NC-15 to NC-20 |
Quiet passages require low background |
| Bedroom / hospital room |
NC-25 to NC-30 |
Sleep quality, patient recovery |
| Private office |
NC-30 to NC-35 |
Concentration, phone calls |
| Conference room |
NC-25 to NC-35 |
Speech intelligibility, video calls |
| Classroom |
NC-30 to NC-35 |
Student comprehension (ANSI S12.60) |
| Open office |
NC-35 to NC-45 |
Higher levels acceptable, helps mask speech |
| Retail / restaurant |
NC-40 to NC-50 |
Activity noise already present |
| Industrial / mechanical |
NC-50 to NC-60 |
Hearing protection may be needed above NC-60 |
📏 How to Measure NC
Equipment Needed
- Octave band analyzer or RTA (Real-Time Analyzer) with 1/1 octave resolution
- Calibrated microphone (Type 2 or better for accurate measurements)
- Apps like Studio Six Digital, AudioTools, or REW work with external mics
Measurement Procedure
- Set up: Place microphone at ear height (4-5 ft) in the listening/working area
- Conditions: HVAC running at normal operating mode, doors closed, no occupants
- Response: Use slow response averaging
- Duration: Measure for at least 30 seconds to average out fluctuations
- Frequencies: Record levels at 63, 125, 250, 500, 1k, 2k, 4k, 8k Hz
Measurement tips: Avoid measuring near walls (reflections), HVAC outlets (localized noise), or equipment. Take readings at multiple positions and average if the space is large.
🔬 How NC Is Calculated
The Tangent Method
NC rating is determined by the highest NC curve touched or exceeded by your measured spectrum at any frequency.
Example: If your measurements are:
- 63 Hz: 55 dB (below NC-30)
- 125 Hz: 48 dB (at NC-30)
- 500 Hz: 38 dB (at NC-35) ← Limiting frequency
- 4k Hz: 28 dB (at NC-30)
Your NC rating is NC-35 because that's the highest curve your spectrum touches (at 500 Hz).
Limiting Frequency
The frequency that determines your NC rating is called the "limiting frequency." Focus noise reduction efforts here for maximum improvement.
📈 Common Noise Sources
Low-Frequency Problems (63-250 Hz)
- Fan rumble: Imbalanced fans, improper mounting
- Duct-borne noise: Turbulence from undersized ducts, elbows
- Mechanical vibration: Compressors, pumps transmitting through structure
- Traffic: Low-frequency road/vehicle noise
Mid-Frequency Problems (500-2000 Hz)
- Air turbulence: High velocity at diffusers, dampers
- Duct breakout: Sound radiating through duct walls
- Equipment noise: Pumps, chillers, transformers
High-Frequency Problems (4000-8000 Hz)
- Diffuser hiss: Air velocity too high (>500 FPM)
- Damper noise: Partially closed dampers creating turbulence
- Leaks: Air escaping through gaps
🔧 Reducing Background Noise
For Low-Frequency Noise
- Vibration isolation: Spring isolators, inertia bases for equipment
- Flexible connections: Canvas or rubber connections between equipment and ductwork
- Duct silencers: Low-frequency silencers (reactive or combination type)
- Increase duct size: Lower velocity reduces turbulence
For Mid/High-Frequency Noise
- Lined ductwork: Internal acoustic lining absorbs mid/high frequencies
- Silencers: Dissipative silencers near equipment
- Reduce air velocity: Target <500 FPM in main ducts, <300 FPM near outlets
- Proper diffuser selection: Size for NC rating, not just airflow
For All Frequencies
- Locate equipment remotely: Mechanical rooms away from quiet spaces
- Acoustic enclosures: For nearby noisy equipment
- Duct routing: Longer paths provide more attenuation
Rule of thumb: Each doubling of duct length adds ~3 dB attenuation at high frequencies. Lined duct can provide 3-12 dB/ft depending on frequency and lining thickness.
📱 Using the Calculator
Entering Measurements
Enter your octave band SPL measurements (in dB) for each frequency from 63 Hz to 8000 Hz. The calculator plots your spectrum against NC curves and determines your rating.
Selecting Room Type
Choose the target room type to compare your measurement against appropriate NC criteria. The calculator shows whether you meet, exceed, or fall short of the target.
Understanding Results
- Measured NC: Your overall rating based on the limiting frequency
- dB Over Target: How much reduction is needed at the limiting frequency
- Spectrum Shape: Whether noise is LF-heavy (rumble), HF-heavy (hiss), or balanced
- Per-frequency status: Shows which frequencies exceed the target (red) or pass (green)
📚 Related Standards
NC vs. RC vs. NCB
- NC (Noise Criteria): Original 1957 curves, widely used in North America
- RC (Room Criteria): Updated curves with better low-frequency weighting, preferred for HVAC
- NCB (Balanced NC): Modified NC with rumble/hiss detection
References
- ASHRAE Handbook — HVAC Applications, Chapter 48 (Noise and Vibration Control)
- ANSI S12.60 — Classroom acoustics
- FGI Guidelines — Healthcare facility acoustics