Absorber material - is expensive better?

[svenr]

I see a lot of people spending lots of money on "acoustic" absorber material.  I get PMs asking where certain expensive stuff can be bought in small quantities.

 

The message is this:  it's only the flow resistivity that matters - nothing else.  Cheap products with flow resistivity in the 5,000 Pa.s/m2 range even perform much better than expensive high density products with flow resistivity in the 12,000+ Pa.s/m2 range.

 

Below are two examples - a typical ceiling absorber and a broadband wall absorber.  I use two products - Polymax XHD and Greenstuf.  The price difference can be checked on Google.  Have a look at the performance difference and then let me know whether the higher cost for the high density material is warranted. 

 

 

[svenr]

I just saw advertisement for expensive "acoustic" putty and sealant, claiming to be "designed to perform better than traditional approaches".  What does this do?  Be more air tight than a conventional gap seal?

 

The "treat the room" mantra is certainly a thing in the market now.  We should have a forum dedicated to audiophile putty, sealant, gyprock and polyester wool.  

[Pieface]

Hi Sven,

 

I note the wall absorber is spec'd at 300mm of material and a 100mm air gap. If these are reflection point treatments...that is 800mm you have chewed out of the room for left and right walls combined...impractical for anything other than a dedicated room.

 

For anyone trying to treat a mixed use space I think the 100mm material plus 100mm airgap you modelled for the ceiling would be the maximum they could consider. At this stage is there any functional benefit to the "smoother" absorbtion curve of the denser material vs the "peaky" absorbtion of the greenstuf? Also in this instance does the better absorbtion value of the denser material lower into the frequency range offer any appreciable performance or is a bit more of not much effectively still not much?

 

Can you also clarify which greenstuf product (A ceiling batt? R rating?) has the flow resistivity number you have specc'd? I see a number of different products from them and can't seem to see that flow resistivity information in the datasheets on their website.

 

 

[Guest Peter the Greek]

Sven, most interesting.

 

What does 50m of polymax look like?

Also 25mm - plastic film (say 75 micron) - 25mm.

 

Compared to the Green Stuff

 

.....which coincidentally we've just lined our new home with....that was a fun task in 49 degree heat

[Guest Peter the Greek]

@Pieface beat me to it regarding real estate  

Edited January 18, 2017 by Peter the Greek

[Guest Peter the Greek]

1 hour ago, svenr said:

I just saw advertisement for expensive "acoustic" putty and sealant, claiming to be "designed to perform better than traditional approaches".  What does this do?  

 

Perhaps its stays flexible over its life unlike cheaper ones which harden, then crack, then dont seal? its a bit of a stretch when its say 4x the cost

 

....god knows, I use sikaflex for everything so I dont have to worry. That or the fire rated mastic. Interestingly though, the Green Glue sealant stays flexible and its really nice to work with....easy clean up etc too

[svenr]

For simple calculations use this website:  http://www.acousticmodelling.com/multi.php  

 

The point of the 300+100 example was solely to demonstrate the difference in absorption between expensive and cheap material.  You wouldn't use that at the reflection points, but I use systems up to 1m thick in control room designs (mainly ceilings).

 

@Pieface - the peakiness is due to interaction between air gap size, absorber thickness and flow resistivity.  You can improve that by adding a thin layer of denser material to the back - but the differences are of no relevance.  You are right about the low frequency difference - not significant.  But note the difference in the 300+100 system - the cheap mid density stuff strongly outperforms the expensive high density stuff (as you expect but often forget).  The flow resistivity data has to be requested from the manufacturer if you cannot find it (they will tell you a story how the heat bonding of the fibers forms a denser layer on the outside which has a bit of an effect on the absorption behaviour - but they will provide you an average value nonetheless).  But look for other datasheets of similar material and do a sensitivity check on this - you'll find that it is not that important what the actual value is.  The R rating is a function of the thickness, obviously.  The material is the same.  

 

@Peter the Greek - use the calculator to model such systems.  For plastic films, use the limp mass membrane option plus the surface mass (easy to calculate once you know the density of PE).  Try 1.8 kg/m2 (el-cheapo linoleum from your local flooring mob) and see what you can do with that.  Normal quality building materials (sealants, putty, etc) are perfectly alright.  

[davewantsmoore]

7 hours ago, svenr said:

I see a lot of people spending lots of money on "acoustic" absorber material.

 

Hah.  I am grappling with this concept right now...   and had basically pruned it down to the question of what ratio to buy of fluffy stuff, vs polymax (or similar)   [as I am already aware of the 'reality check']

 

The polymax is rigid, which will give me a couple of (practical) advantages (for construction)...... but for my big fluffy ceiling cloud  (180mm -> 270mm thick).... then it's good to have a reality check thrust upon me.

 

 

Your posts are always high quality.   Appreciated.

Edited January 18, 2017 by davewantsmoore

[Guest m_james]

I've been looking at trying some Greenstuf insulation to use as a broadband absorption. Which thickness would be best and how would you go about using it?

[Guest Peter the Greek]

Whilst I dont like it (because its nasty shit), I find Ultratel the best and easiest to work with....super easy to cut and mould....wonderful stuff in that respect