Talk:Noise reduction coefficient

Not percentage?

Due to the formulas used, the coefficient is not a percentage and values larger than one are possible. (and common)/

So far as I can understand from the source cited, it is just a percentage which sometimes goes beyond 100% only due to the limitations of measurment techniques. 46.16.177.18 (talk) 06:41, 7 June 2013 (UTC)

It's impossible for a material to absorb more than 100% of sound at a given frequency. So, no, it's not a percentage. This source explains it better than I could:

The ASTM C423 standard makes it possible to arrive at absorption coefficients and, therefore, NRCs that are greater than 1.00. This may be counterintuitive since many references define the Sabine alpha and NRC as the percentage of sound absorbed by a material. This treatment of alphas and NRCs as percentages, however, is not really accurate. The formulae used in the standard to measure absorption are dependent on: the decay of sound in the test room, room volume, room temperature and the area of Chamber floor covered by the test material. Sabine absorption and Sabine alphas come from the fact that the absorption is calculated using the Sabine equation. Nowhere in the ASTM C423 standard is there a reference to Sabine alphas being equal to percentages of any kind. Therefore, numbers greater than 1.00 are possible. This means that Sabine alphas are simply a representation of the relative amount of sound absorbed by the material. (Relative to the absorption without treatment in the room.) Higher numbers mean more absorption in that frequency band.

— Preceding unsigned comment added by 50.177.226.132 (talk) 05:18, 23 October 2014 (UTC)

This page needs a rewrite

"An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption"

This is misleading and should be changed. NRC is measured at particular frequencies. So an NRC of 1 indicates 100% absorption at those frequencies on average. In other words, a material could have a lot of absorption at high frequencies and almost none at low frequencies and still have an overall NRC of 1. That's hardly "perfect absorption."

Also, NRC has been changed by many companies (it's an informal industry standard, though still largely based on ASTM C423) to include six frequencies instead of only four: 125, 250, 500, 1000, 2000, and 4000 Hz. That makes the SAA standard inferior, so I doubt it will replace NRC as this article claims.

Here are some better sources (especially considering the original link is dead): http://www.acoustics.com/nrc.asp and http://www.auralex.com/auralex_acoustics_faqs/faqs.asp?Q=10 — Preceding unsigned comment added by 50.177.226.132 (talk) 05:20, 23 October 2014 (UTC)

The Acoustic.com site no longer exists and the Auralex.com information is incorrect. If you purchase and actually read ASTM C423 you can learn that all of these are based on some average of absorption coefficients which are measured in Sab/ft². Unfortunately in section "11. Calculation" part of ASTM C423-99a, or -02 or -80 or other revisions, it also states that you can display the result without units. This is what has caused confusion for years. While Sab/ft² is technically dimensionless it really should be included for the same reason you sometimes you write kg/kg or weight based percentage. Point "4.2 Measurement of a Sound Absorption Coefficient" indicates that, when using Sound Absorption Coefficients using the inch-pound system, it should be reported in Sab/ft². When using metric system it should probably be reported in m²/m². The dividend is the sound absorption, measured in m² or Sab, while the divisor is the area of the panel surface in m² or ft². There is also the option in point "4.3 Measurement of the Sound Absorption of an Object Such as an Office Screen, a Theater Chair, or a Space Absorber" where the units for Sound Absorption are measured in m²/object or Sab/object.

Now what's really interesting is that if you look at point 4.1.1 in the ASTM C423 standard that defines equation 1 for sound absorption you actually find that sound absorption is based off the decay rate with units of dB/s in a reverberant room. So the standard really screwed up because the dB scale is logarithmic. They really should specify units as (dB*Sab)/ft² so that people are aware that the ratio is a logarithm of a quotient that divides one level by another base level. Just like you don't say that the SPL is 90, you say it's 90 dB because the unit, while technically dimensionless, is really important because you're dealing with a non-linear logarithmic scale with elements in (0,infinity) instead of [0,1] or (-infinity, infinity). Equation 8 in section 11.7 shows that you are subtracting one logarithmic based absorption coefficient from another logarithmic based value for the room which using logarithmic identities is the same as the logarithm of the ratio of the room decay rate with the absorption divided by the room decay rate without the absorption. You're still left with dB units and logarithmic scaling in the absorption coefficient result and the NRC and SAA and any other calculation that's based off the ASTM C423 definition of sound absorption.

Most manufacturers are ignorant of the math behind what they display. They're in business to sell product and make money, rather than to be accurate or even know what they're selling. Although their customers usually benefit if they do know what they're selling. The scientists that wrote the ASTM C423 could have done better in communicating results, but then again what fraction of the greater public actually understands what a logarithm is, means, or how it functions? — Preceding unsigned comment added by Aeriform (talk • contribs) 17:52, 6 June 2019 (UTC)