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Originally Posted by deft_bonz Scott
Do I understand this right?
If this hemp has absorption coefficient a=1 for class a measurement after ISO 11654, does it mean that it has a real coefficient of a=0.7 at 250Hz as described in figure 6 here (the link you'd posted): http://www.rockfon.co.uk/sw1165.asp?PID=116 |
DB:
I haven't studied the spec [I'm sure you can find more detail on the web] but I gather that figure 6 is a series of thresholds that a material must meet to get the rating at that line. In other words, you can get a Class A rating with .7 at 250 Hz, but that doesn't mean the material must roll off at that rate as it aproaches the low end of the spectrum.
Also, the reference curve may not be an absolute criteria - the material might still get class A rating if it exceeded the reference curve at points x y and z even if does not at a b or c... also there may be a weighting giving more credit for adherance to the class refernce line for some parts of the band versus others. So you might be able to get class A even if don't achieve .7 at 250 Hz.
I just told you more than I know about this spec, but I know of other single numbers specs that do this - so I'm always suspiscious of such things. I always look for absorption data broken down per frequency - so you can get your mind around a trend line without any fancy footwork going on where you can't see it. This is real imortant when working with musical noise - as music generally has a much greater low frequency content than something like office noise [office environments being the situation most of these sorts of specs are written around] and therefore the rule set for generating these ratings is often misleading for musical applications.
Nonetheless the customer is happy - and making a WAG from the look of the stuff, the class A rating and the overall depth of material achieved by the installation as described... I bet it really does work well - even low in the band.