Question 1

What are NC (Noise Criteria) curves and how are they used?

Accepted Answer

NC curves are a family of spectral shapes used to rate steady-state background noise in occupied spaces. Developed by Beranek in 1957, they define maximum octave band levels from 63 Hz to 8 kHz for each NC rating. The NC rating of a room is the lowest NC curve that is not exceeded by any octave band measurement. For example, if the measured 500 Hz band level is 42 dB and this corresponds to NC-40 at 500 Hz, and all other bands are below NC-40, the room is rated NC-40. Common targets: NC-15 to NC-20 for recording studios and concert halls, NC-25 for churches and theaters, NC-30 for private offices and conference rooms, NC-35 for open offices, and NC-40 for lobbies. SonaVyx overlays NC curves on the octave band display and automatically reports the NC rating.

Question 2

What is the difference between NC, NR, and RC curves?

Accepted Answer

NC (Noise Criteria) is the original American system, widely used in ASHRAE standards. It uses 8 octave bands (63-8000 Hz) and a single-number rating. NR (Noise Rating) is the European/ISO equivalent (ISO 1996), with a similar structure but slightly different curve shapes and extending to 31.5 Hz. RC (Room Criteria) is a newer system that considers both level and spectral character. RC classifies noise as Neutral (N), Rumble (R), or Hiss (H) based on the spectral slope, and adds a vibration indicator (RV) for low-frequency content above 65 dB in the 16 or 31.5 Hz bands. PNC (Preferred Noise Criteria) is a modification of NC with stricter low-frequency limits. In practice, NC and NR are used most frequently. SonaVyx supports NC and NR curve overlays.

Question 3

How do I measure NC rating in a room?

Accepted Answer

Procedure: turn on the HVAC system and all normal background noise sources. Remove or silence all other noise sources (conversations, music, equipment not normally running). Place the microphone at the listening position (seated ear height, 1.2 m, at least 1 m from walls). Measure the octave band levels from 63 Hz to 8 kHz for at least 30 seconds using slow time weighting. Record the Leq per band. Repeat at 3-5 positions and average. Overlay the measured spectrum on the NC curves. The NC rating is the highest NC curve touched by any octave band. If 7 of 8 bands are below NC-30 but the 250 Hz band touches NC-35, the room is rated NC-35. SonaVyx computes the NC rating automatically and highlights which band is the limiting factor.

Question 4

What NC rating should a conference room achieve?

Accepted Answer

ASHRAE recommends NC-25 to NC-30 for private offices and small conference rooms, NC-30 to NC-35 for open offices and large conference rooms, and NC-25 for executive conference rooms and boardrooms. For video conferencing rooms, some specifications are even stricter (NC-20 to NC-25) because microphones are more sensitive to background noise than human listeners. In practice, achieving NC-25 requires: quiet HVAC diffusers with low air velocity (150 fpm or less), proper duct lining or silencers, vibration-isolated mechanical equipment, and adequate partition STC between the conference room and adjacent noisy spaces. SonaVyx SPL meter with NC curve overlay provides instant compliance assessment during HVAC commissioning.

Question 5

What causes a room to exceed the NC curve at low frequencies?

Accepted Answer

Low-frequency NC exceedance is almost always caused by mechanical systems. Common sources: air handling unit (AHU) rumble transmitted through duct walls or building structure, fan motor vibration conducted through the floor or ceiling slab, duct resonance at specific frequencies where the duct acts as a pipe organ, variable frequency drives (VFDs) on motors producing tonal noise, and transformer hum from nearby electrical rooms. Traffic noise can also contribute low-frequency energy through windows and walls. Solutions: vibration isolation of mechanical equipment (spring mounts, inertia bases), duct silencers sized for low-frequency attenuation, flexible duct connections, and structural breaks in vibration paths. SonaVyx identifies which octave band exceeds the NC curve, helping you target the investigation.

Question 6

Can background noise that meets NC criteria still be annoying?

Accepted Answer

Yes. NC curves only evaluate the spectrum shape and level, not other characteristics that affect perception. Tonal noise (a prominent hum or whine) is significantly more annoying than broadband noise at the same NC level. Fluctuating noise (cycling compressors, intermittent fans) is more noticeable than steady noise. Impulsive noise (clicking relays, popping valves) can be startling even at low levels. The RC curve system partially addresses this by classifying noise as Rumble, Hiss, or Neutral, and flagging potential vibration. For a complete assessment, measure the NC rating AND listen subjectively for tonal content, fluctuations, and impulsive sounds. SonaVyx problem detector identifies tonal components and the acoustic kurtosis metric flags impulsive character that NC curves miss.

Question 7

How do NC curves relate to speech intelligibility and privacy?

Accepted Answer

Background noise has a dual role in speech communication. Too much noise masks speech and reduces intelligibility (reduces STI). Too little noise allows private conversations to be overheard, reducing speech privacy. For conference rooms targeting NC-25, the background noise provides some masking of conversations from adjacent spaces. If the room is too quiet (NC-15), every whisper from the corridor is audible. Sound masking systems intentionally add NC-30 to NC-40 shaped broadband noise to open offices to improve speech privacy without causing distraction. The ideal NC for speech privacy depends on the partition STC and the speech effort: NC-35 to NC-40 with STC-45 partitions provides Normal Privacy per ASTM E1130. SonaVyx measures the actual background noise NC to verify the acoustic design intent.

Question 8

What is PNC and when should I use it instead of NC?

Accepted Answer

PNC (Preferred Noise Criteria) was developed by Beranek in 1971 to address limitations of the original NC curves, particularly their leniency at low frequencies. PNC curves are 3-5 dB stricter than NC below 250 Hz, better reflecting occupant sensitivity to low-frequency rumble. PNC also extends to 31.5 Hz (NC starts at 63 Hz). Use PNC when: the specification calls for it (some acoustic consultants prefer PNC), the project involves particularly sensitive spaces (recording studios, hospitals), or low-frequency noise control is a priority. In practice, NC is used more frequently because it is simpler and referenced in more standards (ASHRAE, LEED). If a specification says NC-25, achieving PNC-25 ensures you meet a slightly higher standard. SonaVyx NC curve display can be used for quick assessment; for PNC, refer to the published curves.

Noise Criteria Curves: NC, NR, RC, and PNC Ratings Explained

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