Acoustical Jargon & Terms

The noise control business has some unique terminology, which can be confusing at first.  Here are a few key terms, and some simple descriptions of what they mean. Most people especially confuse NC, NIC and NRC. While they all sound similar, these terms really describe completely different things --- NC is background noise, NIC is the barrier performance of a wall, and NRC is the absorption properties of a material.  If you are ever in doubt, call or email and we can explain the meaning and application of nearly any acoustical quantity.

It is important to specify exactly what you mean, and to use the terminology correctly. We see project specifications all the time that talk about the “NRC” level when describing background sound from the HVAC system. They really meant the NC level. We also routinely see cases where NC is used to describe the amount of allowable traffic noise intrusion into a space. But NC is specifically intended to judge the steady state background noise coming from ventilation systems, and is ill suited to assess time varying sounds like traffic or conversations.

dBA (A-weighted sound level)
Humans hear different frequencies better than others.  For example, it takes a much higher level of very low frequency sound (thunder, bass guitar, diesel locomotive) to sound equally as loud as a high frequency sound (cymbals, whistles, turbines, squeals).  The "A" weighting filter takes the overall signal and applies a correction factor to certain frequencies that correspond to perceived loudness.  A diesel railroad locomotive and a cymbal both measuring 80 dBA will both sound comparably loud, despite the radical difference in frequency content. dBA is not perfect, but it is the quantity that is most widely used.

dBC (C-weighted sound level)
The C-weighting filter rolls off the very highest and very lowest frequencies, that tend to have little effect on the perceived loudness (but can overload test equipment). C-weighted levels are essentially “flat” with no boosting or cutting of certain frequencies. C-weighted levels are most commonly used to evaluate amplified music or machinery noise. In the example from above, a diesel locomotive might be 95 dBC, while the cymbal would be 80 dBC.

decibel, (dB)
Sound is simply fluctuating air pressure.  The human ear can detect changes in air pressure over an incredible range -- a ratio of trillions to one -- between the threshold of hearing and the onset of pain.  Put another way, if you had a scale with the same dynamic range as your ear you could weigh a single human hair (0 dB) and a skyscraper (140 dB) using the same device.  The decibel mathematically compress the range of values using logarithms, rather than describing the actual linear sound pressure levels measured for each noise.  Sound levels expressed in decibels are similar to earthquake values using the Richter scale.  An earthquake measuring 6.0 releases ten times as much energy as a 5.0 quake, and 100 times as much as a 4.0 event.  Similarly, a sound level increase of ten decibels requires ten times the sound intensity -- but 10 dB higher is perceived as being only twice as loud.

Loudness

Increases in loudness are generally judged as follows. Decreases in sound level follow the same scheme.

+ 1 dB measurable using a quality sound meter, but not perceptible

+ 3 dB noticeable if you are actively listening or expecting a change

+ 5 dB noticeable without prompting

+ 10 dB twice as loud as the original sound

Ldn (Day-Night Level)
Ldn is used to describe environmental noise.  It is a 24-hour average noise level.  The Leq for each hour is taken, then a ten decibel penalty or adjustment is added to the levels during the nighttime hours (10 pm - 7 am).  The nighttime adjustment helps account for increased sensitivity to noise during the traditional sleeping hours. Ldn is most often used to assess environmental noise, particularly for highways and airports.

Leq (Equivalent Level)
Leq is a measure typically used in environmental noise analysis.  Since noise typically varies over time, an overall descriptor is needed.  The Leq is determined by summing the total sound energy each second, then dividing the total energy by the total time. While not mathematically correct, think of Leq as the "average" sound level that occurred during the measurement.

NC (Noise Criteria)
NC is the background noise level, from 63 Hz to 8000 Hz, plotted against a series of equal loudness curves.  NC is most commonly used to express the allowable or desirable steady background noise level from HVAC systems.  Conceptually, think of NC 40 as a background level of roughly 40 decibels.  (NC 40 is actually 48 dBA).

NIC (Noise Insulation Class)
A measure of the sound barrier properties of a partition.   NIC is the field version of the STC test (see below).  It is normal and expected for the field test of a partition to be five points lower than the laboratory STC rating.  NIC should not to be confused with NC (above).

NRC (Noise Reduction Coefficient)
Sound absorption is measured in terms of the NRC, which is simply the average absorption for the material over the primary speech information frequencies (250 Hz - 2000 Hz).  An NRC 0.70 material will absorb about 70% of the incoming sound energy, leaving the remaining 30% to either transmit through the material or to be reflected back into the space. Note that 50% absorption translates to a 3 dB reduction in sound levels – barely noticeable in most situations. That is why added absorption is rarely the solution to a noise problem.

RT (Reverberation Time)

This is a measure of the “liveness” or general sound reflectivity within a room. RT specifically measures the time that it takes for a sudden sound (like a hand clap) to decay away to inaudibility. A very “live” space like a gymnasium may have a very long RT of 5.0 seconds. A fully furnished residential bedroom may have an RT of less than 0.6 seconds. A classroom, conference room or office should have an RT of 1.0 second or less, an auditorium about 1.5 seconds maximum, and a church should generally have an RT less than 2.0 seconds. When designing spaces, we tend to slightly over-treat to achieve lower RT values, to favor speech intelligibility and clarity. Music blending is best served by long RT’s.

STC (Sound Transmission Class)
STC is the measure of the sound barrier properties of a partition between 125 Hertz and 4000 Hertz.  If a wall is rated at STC 45, it will prevent the transfer of roughly 45 decibels of airborne sound between rooms.  Note that STC does not cover the very low or very high frequencies, and is therefore not directly useful for sounds like amplified music or machinery noise, or any noise with a considerable low frequency content.

IIC (Impact Insulation Class)
A cousin to STC, the IIC test measures the ability of a floor-ceiling system to resist the transmission of footfall or impact noises from above. Typical building codes require a floor-ceiling system rated at IIC 50 minimum between dwellings. Industry practice increases that code minimum value by 5 points (to IIC 55) for condos, and by 10 points (to IIC 60) for luxury condos. It is easy to meet IIC 60 with carpet, and exceedingly difficult to meet luxury condo goals using hard floor surfaces – even with high quality resilient hanger systems, careful construction, multiple layers of gypsum board, and expensive resilient underlayment materials under the flooring.