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What does NRC actually mean?
Old 2 weeks ago
  #1
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What does NRC actually mean?

I've read that an NRC of 1.0 at a frequency means the material absorbs 100% of the sound at that frequency. I was skeptical and did a little experiment where I put a reference mic in a thick pyrex pan, covered it completely with 12 inches thick of Rockwool, aimed a speaker downward pointed straight at the rockwool and played pink noise as I measured what the mic was hearing.

12 inches of rockwool:
250Hz = 5dB down vs uncovered
1000Hz = 10dB down vs uncovered
10,000Hz = 20dB down vs uncovered

The data sheet says that only 4 inches of Rockwool should have an NRC of 1.07 at 250Hz. My test showed with 12 inches of Rockwool almost no sound was absorbed at 250Hz.

What does NRC mean? What does it actually measure?
Old 2 weeks ago
  #2
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avare's Avatar
 

Noise Reduction Coefficient (NRC) is the average of the absorption coefficient in a reverberant space (reverberation chamber) for the one-third octave bands centred on 250, 500, 1,000, and 2,000 Hz rounded to .05. If in the middle then rounded up.

From ASTM C423, section 3.2.
Attached Files
File Type: pdf ASTM C423-99.PDF (125.5 KB, 9 views)
Old 2 weeks ago
  #3
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Unclenny's Avatar
ASTM C-423 measures NRC in an acoustically balanced reverberation room.

ASTM C-384 measures NRC in an impedance tube with an anechoic plug behind the sample being tested.

You'll be hard pressed to re-create those test conditions and without doing so you are not likely to get accurate data.
Old 2 weeks ago
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Quote:
Originally Posted by Unclenny View Post
ASTM C-384 measures NRC in an impedance tube with an anechoic plug behind the sample being tested.
There is no reference to NRC in C-384.
Old 6 days ago
  #5
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Unclenny's Avatar
Quote:
Originally Posted by avare View Post
There is no reference to NRC in C-384.
ok

http://arnisco.com/astm/PDF/C423.PDF

http://www.arnisco.com/astm/PDF/C384.PDF
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