I am referring to NC curves in this case (relative to the US) - and not the NR (Euro) curves for ambient noise levels. NC - Noise Criterion
The irony is that these tests are literally among the first performed in a space someone is seriously considering using. In effect, they are a variant of basic Noise Level Analysis (NLA) best practices that precede any attempts at measurement/analysis or treatment.
The larger issue
to which I was referring that hopefully might make others aware, rather than simply attempting to locate a specific source here, was simply that few here ever perform an initial evaluation regarding ambient noise - something that is fundamental
in the commercial/'pro' markets and may well determine if a space is even worth considering and if any additional steps will even help. In fact, if one ever has the opportunity to work in a large acoustical space such as an auditorium or civic center, you will quickly discover that such issues become critical
- and REAL issue to deal with!!! And I literally dare anyone to approach a facilities manager to inform them that they must turn off the HVAC systems for an event as the ambient noise levels are excessive!
And while many spaces luckily qualify, many do not. Many of the spaces mentioned here, if this fundamental analysis were performed, would be eliminated from consideration and the expenditure of a great amount of time and money (with cause) from inception.
And all of this segues into the issue of what we know and do not know - and what gets posted as if
it is indeed accurate and subsequently becomes fundamental to the larger group-think.
As many such practices allude to even larger issues that lurk in shadows and corners outside of much of the generic information tossed about here by many.
And that is the fact that MUCH IS KNOWN and understood in acoustical engineering that many are quick to deny here! Much that many dismiss requires not only the desire, but an awareness and familiarity with physics
and acoustical engineering!
I know that the mention of such is anathema to many who would prefer to imagine acoustics as a black ART and not science, but it IS science, and that is the language of the discipline.
And the field is not so ignorant of various designs as some individuals may be (be they diffusion, resonant absorbers - I mean, how many times will we read that the design of resonant absorbers is not well understood when they are a backbone of applied industrial acoustical engineering! e.g.: Almost every
mechanical engine utilizes them! - but then you are not
going to find detailed design guides that explain them logically without the use of mathematical relationships based upon determined characteristics, measurements, etc., etc. etc.!) And as such it is not valid for some to issue broad statements tot he effect that such techniques or principles are not understood. Rather it may simply indicate that one might have to pursue such information outside the confines where some are not aware - or ignore-ant - of them.
I have no problem with someone stating that the design or concept behind a particular technology is not understood by them, but it fascinates me that someone who is themselves unaware of the science feels compelled to pronounce and entire discipline of engineering as ignorant of the concept...
And rather than simply pursue brute force cures for fundamental problems, folks would benefit from understanding that there IS a bit more to "acoustics" then simply throwing up a bunch of panels! As there persists a notion that acoustics is some primitive 'black art' that few if any understand based solely on the fact that neither they nor others have explained the significant body of knowledge and best practices that DO exist in mature form.
For instance, the notion that applying facing (...we might use "FSK" as a generic term here) to the surface of an absorptive membrane is "not understood"? Please! Modifying the acoustical impedance of material is WELL understood. Perhaps not by many here
, but then such understanding involves not only a conceptual understanding, but math - the mention of which sends most scurrying!
And additionally - as absorptive panels have been mentioned, just today it was mentioned that panels should be at least 4 inches in depth to insure that they absorb a relatively flat power spectrum - and while I won't debate that general guideline - the Larger irony is that most absorptive materials, and almost ALL facings of any kind (cloth, perforated materials, etc.) render the absorptive surface exceedingly reflective
at all 'grazing' angles of incidence with perhaps a normal (perpendicular) incidence!
Yet FEW cite, post or even acknowledge the very significant fact that absorption exhibits a VERY significant reflective component especially in the normal (as in average/typical) application where the various sources are oriented at rather extreme (low) incidence angles. Yet many are under the erroneous impression that if you install absorption, that all or most of the the energy in a particular frequency range in question is effectively mitigated and 'removed' - when in fact a large part of that energy is not, and is reflected in a non-linear distribution.
And has anyone even pondered as to if or how this behavior might be identified and quantified? Such should be readily
apparent simply by the fact that an acoustic (or any other) impedance is not simply comprised of real values - or even more fundamentally - of what happens by definition in the event of impedance mis-matches! Again, while this may be qualified for acoustical issues, the basic concept is fundamental to the study and understanding of the electro-mechanical world as we know it!
The irony being that even with a lack of such test and informational data, such practical significance is EASILY determined in a practical manner via the use of ETC measurements that provide real time feedback regarding the reflected energy from any applied surface, be it absorptive, reflective or diffusive, providing an accurate comprehensive picture of the actual performance at any given time in any given location relative to any treatment (or lack thereof).
Yet this use provides significant USABLE data available essentially to anyone who takes the small amount of time to simply employ it!
Yet many still 'rely' on incomplete and unreliable NRC measurements that provide little or no practical value - in fact, with their inherent limitations and incomplete profiling of the behavior of a material - they are not much better than tire grading schemes for tire wear and temperature that, AT BEST, provide a comparative ranking among the same manufacturer's product, but are meaningless to compare product between manufacturers!
The failure to acknowledge and to employ such basic understanding as well as the practical and readily available tools - or for that matter, the active campaigns by some against such tools - simply contributes to the perception in the larger forum, as well as those whose are here to learn of the various options regarding acoustics, of acoustics being a rather 'slow' and ignorant stepchild of the engineering sciences!
The understanding is in large measure indeed available - And the tools are available as well - for those who are serious enough to take the time to simply ask and not be satisfied with the seemingly mantra-like pronouncements that little is known in acoustics! As such pronouncements reflect only the provider of such 'insight', and not the state of affairs in acoustical physics.
And at the point where our understanding reaches the limits of the known, we can indeed ponder such wonders - but for the most part, those limits are relatively distant from the boundaries of that which we are dealing here.