Originally Posted by Martycool007
Here are some of my questions that I need some help with. I have had two different people tell me two different things, so I would like to get some more opinions on it.
1. So do all of you guys believe that lower density insulation is the best?
No, you heard two opposite answers. Ethan gave you measurements he made that said higher density is better. I post reasoning from top industry people that says the answer is the opposite. The reference Arny gave you proves the same thing I said but requires material science knowledge to use and verify in this case.
The physics of this can be complicated to explain but let me say it at very high level. A "porous" absorber like we are talking about works by having the air particles enter it and then due to friction of the fibers get converted into heat and with it, give you the absorption you are after. For this to work the best, you want no resistance to particles wanting to go inside it otherwise they reflect back and you get no absorption. Once inside, you want to have higher resistance for the energy to dissipate. To optimize the former, you want the "impedance" (its resistance to flow) to be the same as air in your room. Anything higher will cause an "impedance mismatch" and cause some of the sound energy to reflect. As I noted, what is reflected will not be in the absorber so therefore by definition cannot be converted to heat/absorbed.
The other poster said we should listen to people in acoustic industry and not me (even though I gave him industry people quote, not my own). So here is one: http://www.readyacoustics.com/index.php?main_page=faq_info&fcPath=13_4&faqs_id=36
"Yes, but I have read that higher density acoustic insulation (like OC705) is better for low-end absorption.That’s simply not true.
The common misconception that higher density acoustic insulation makes for better low frequency absorbers arises from a fundamental misunderstanding of how the physics of porous acoustic absorption works in the real world [as opposed to the world of internet chat forums].
He then provides the same explanation I gave above plus these set of measurements:
The numbers you see are coefficients of absorption. The higher the numbers, the more it absorbs sound at that frequency. At 500 Hz for example with a 4 inch panel against the wall, 703 has an absorption coefficient of 1.24 vs 1.17 for 705. So higher density resulted in less performance. The difference is not huge though so if all you can get is 705, and you know the material will enhance the sound in your room not make it worse, go ahead and use it.
Note that as I said, at some point this equation reverses where you have too little resistances inside the absorber and hence effectiveness goes down. This is not an issue with respect to 703 and 705 material that are far denser than this threshold.
2. What about with fluffy insulation, is lower density still the best with it?
The problem you have with lose material is how to contain it and build a structure out of it. If you put fabric, plastic or whatever in front of it, you again create an impedance mismatch and at some frequency, the sound starts to reflect instead of going into the material. When it does, then it acts more like an equalizer, filtering/absorbing some frequencies more than others. If you have a well designed speaker that had pretty even off-axis (side-ways) response, you just screwed that up by modifying the spectrum that comes back from the absorber. So while there are products of this sort in the market, you have to be careful in using them.
3. Also with regards to the fluffy stuff, should it be compressed, or not?
4. If the fluffy stuff should be compressed, then how much should it be compressed?
Per above you want to have less "compression" than more. Also note that the relationship between density and resistivity flow is not always as I explained. If you change material from the ones we are talking about, you are on your own researching what that is
So stay with the common 703/705 material. It has a good balance between acoustic and structural aspects.
Answering the other posters comments, you do not want to use these materials as your main means of controlling low frequencies. Porous absorbers work best when the particles are moving fast by them. When the wave hits the walls, the air particles obviously are not moving anymore so there is nothing to absorb. If you move the panel further inside the room, then it starts to work better at low frequencies that have their highest velocity there. The above table does not go low enough to show this. Here is a simulation based on the (useful) spreadsheet link Arny provided to you that does demonstrate this effect:
As you see the low frequency absorption improves when you add 4 inches of air behind 4 inches of porous material. The reason for that is what I just explained: you are placing the absorber where particle velocity is the highest.
Note that even with this technique, you are still not getting a lot of absorption at low frequencies. The simulation even stops at 62 Hz. You would need to use a lot of these panels to make a dent in low frequency absorption. This is why you want to use other techniques first to reduce the impact of the problem first (see my article on Low Frequency Optimization
). Using those techniques, you may not need any absorbers for low frequencies. But if you do, then you don't need as much of it.
Above low frequencies, you *really* need to be careful to not run off with "forum wisdom" or companies selling products as that can be lead you astray, resulting in worse sound in your room, not better (see my other article on Perceptual Aspects of Room Reflections http://www.madronadigital.com/Library/RoomReflections.html
. Best use of such solution, if you have an otherwise bare room, is to control the later reflections that linger in the room which is a different animal than first/early reflections. Rooms need to be not too live, not too dead.
As you can see, there is a lot of complexity here and what seems intuitive often is not what the science says.