The Science Behind IIC Ratings: Normal Weight vs Lightweight Concrete

Is a 5 dB difference at 3150 Hertz worth a 5 point difference in an IIC rating?

3150 Hertz.  Listen to what that sounds like here.

As the acoustic engineer at the company who manufactures the original recycled rubber acoustic underlayment, I’m pretty much positioned to become an expert in IIC ratings.  And yet, here I am, over 2 years later, not much more certain of my grasp on IIC ratings than when I first began.

I’m inspired to create this entry specifically in respect to the difference between the IIC ratings of lightweight vs. normal weight concrete.

For instance, what is the difference between the IIC rating of an 8″ normal weight concrete slab and and an 8″ lightweight concrete slab?  (Also, why is “normal weight” 2 words, and “lightweight” just one word?  Or is that just something I do?)

My brain tells me (although I am unable to cite any source) that the IIC rating of a standard, normal weight 8″ concrete slab should be around IIC 32 whereas it would drop to about IIC 28 for its lightweight counterpart.  Maybe this is something I deduced from Insul predictions over the years?  I will have to check later since my Insul key is currently out of date…

Since I can’t check Insul right now, I resort to Ecore’s pretty extensive database of IIC tests.  I find the following, both tests conducted at the same lab, 4 years apart.

Specimen: 8″ thick reinforced concrete slab (85.6 PSF)
Year: 2002
IIC 32

Specimen: 8″ thick reinforced concrete slab (85.6 PSF)
Year: 2006
IIC 27

I am familiar with the widespread range of IIC results from 6″ concrete slab Round-robin testing at North American labs (up to a 12 point discrepancy between labs!), but a 5 point difference at the same lab?  Looking into the 1/3 octave band data, I found that this 5 point difference in IIC ratings was determined by 2 dB at 2500 Hz and 5 dB at 3150 Hz.  These 2 frequency bands alone are responsible for the 5 dB difference in overall IIC rating.


IIC-Blog-Entry-1024x791I know the following logic isn’t the right way to interpret these findings, but based on the convention that 2 dB is not a noticeable difference to the human ear (a 3 dB difference is considered noticeable while a 6 dB difference is considered twice as “loud”), let’s ignore the 2 dB at 2500 Hz and focus on the 5 dB at 3150 Hz.

3150 Hz.  Do you know what that even sounds like?

Since sound is measured on a logarithmic scale (think Richter scale like with earthquakes) and a 6 dB difference results in a sound pressure level twice as “loud,” 5 dB is a relatively significant difference.

Still, should an almost-twice-as-loud sound pressure level at 3150 Hz be enough to justify a 5 point reduction in an IIC rating?  There’s a reason why single-number ratings don’t carry much weight with industry professionals…

What do you think?

'The Science Behind IIC Ratings: Normal Weight vs Lightweight Concrete' have 3 comments

  1. December 15, 2014 @ 6:07 pm Elzo Gernhart

    First Thank you for sharing your information regarding IIC in concrete slabs:
    After more than a decade of testing every type of wall and Floor-Ceiling assembly…. I have noticed 8″ thick normal weight concrete slabs have an IIC around 32 when new (wet). The IIC drifts downward at a rate of about 1 IIC per year until the slab reaches a static equivalent with the relative humidity. In most areas of the USA, the IIC of an 8″ normal weight concrete slab appear to stabilize around IIC 28. Cheers, Elzo

    • February 11, 2015 @ 10:29 am Sharon Paley

      Elzo, thank you so much for your input. If this is true (it does seem reasonable and neither do I have any reason to doubt you!) then this really calls into question the current system we have in building construction surrounding acoustical ratings and tests. E.g. scrambling and spending thousands of dollars to get a specific IIC rating for new construction only to have said ratings inevitably degrade over time…

    • January 4, 2017 @ 2:31 pm Amanda

      If I have a IIC rating of 50 with a 6″ concrete slab that includes my hardwood flooring, what is the typical difference in IIC ratings for same materials yet the concrete slab is 8″?

Would you like to share your thoughts?

Sound Advice is a division of ECORE International designed to keep you informed with the latest and greatest in architectural acoustics. Contact us at 717.598.3335 or