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Old 13th February 2011 | Show parent
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Originally Posted by Hannes_F ➡️
OK. You don't want to read the linkes threads yourself, so here comes

Absorption for dummies

Everything depends on the linear gas flow resistivity. This is how much the air is slowed down in the absorber when it flows through it.

To be more precise it is the resistance that the absorber gives when the air goes through it. Think you glide either on ice (low resistance) or on asphalt (high resistance). That would be a helpful picture (very rough).

Heavier stuff is more compressed and therefore the pores are smaller, so the gas flow resistivity is higher ... usually. Think gliding on asphalt. But this really depends on the details of the material: what is it made from, how big are the particles, which glue is used and so on. Therefore it is rather flakey to deal with density and the like ... but if you know the gas flow resistivity you don't need that any more. Two materials with same gas flow resistivity will perform nearly identical, no matter how different they look, weigh, touch etc.

There are four typical ranges of that:

  1. Glass wool from the building center typically has 5000 Pa*s/m² which is equal to 5000 rayls/m. That is the same as the lighter Knauf products.
  2. Then we have the 10000 rayls/m class. That would be the heavier Knauf products.
  3. Then we have 16800 rayls/m which is the famous OC703 which is quite rigid and used for studios on the american market.
  4. And finaly the quite compressed and heavy OC705 which is made for low bass absorption and has 45000 rayls/m.

Fine you say, file closed. We take the heavy OC705 stuff because it absorbs best in the low end.


Imagine you are an engineer constructing a ship. You know the motor will make much noise, so you want to block that. You need a material that will be the best sound blocker for a given depth. You make tests and find that the OC 705 (the heavy stuff) blocks best for the bass range. You market the OC705 as 'the best low end absorber'.

However what you don't care about is, how much of this low end is reflected towards the motor. It simply does not matter because the motor does not care.

Look at this picture. You see the incoming sound from left to right, and at the border to the absorber it splits into three parts (schematically):
  1. The part that goes through (the transmitted part)
  2. The part that is absorbed
  3. The part that is reflected
The same thing happens when the sound comes out at the back of the absorber (I am making this very simple and rough, reality is much more complex).

What the engineer wants is actually to minimize the transmission. If transmission is low he says 'that is a fine absorber'. Because he does not care about the part that is reflected. Who cares if the motor hears a few decibels more of its own noise.

Now we come and hang this absorber into our studio. Suddenly the reflected part does matter! And we have a double problem:
1. We want the sound wave to be attenuated as much as possible when it travels through the absorber, because the rest of it will bounce at the wall, go back through the absorber a second time and then cause trouble in our studio.
2. But the other problem that we have is that a part of the wave is already reflected when it meets the absorber. And we want that to be small, too.

In order to understand that on an extreme example, think we compress our absorber until it is as dense as stone. It will block the sound very effectively but the reflected part messes all up --- in a studio, not necessarily for a motor housing. (Again, I am making this very simple in order to explain something since reality is much more complex if you really want to block noise - experts please don't read this too thorougly ).

Back to the absorbers: People have found that for studio use

  • if your absorber is about 2" (5 cm) thick then OC705 is best for bass absorption.
  • if your absorber is around 4" (10 cm) then OC 705 or 703 are good.
  • if you can afford to make the absorber really thick (like 20" or 50 cm) then the 5000 rayls/m is the best.
You ask: Why bother with different thickness? Why not just use 2" of OC705 and call it a day?
Answer: The overall bass absorption that you can have with absorbers will be better with 50 cm of 5000 rayls/m than with 5 or 10 cm of the other stuff. But it comes with a price ... you need some space and the book shelf must go.

Now you can see that it is not possible to answer the question "which glass fibre product is the best for bass traps?". This is simply a question for which no answer exists, really.

What you need is this:
  1. You commit to a lowest frequency that has to be absorbed
  2. You commit to the maximum depth you are willing to sacrifice

and depending on these two you can find which material (which gas flow resisitivity) works best. That is what you can do with the Porous Absorption Calculator which you can find here:

Porous Absorber Calculator V1.57

Yes it is useable, and I have proven that if you use it right it absorbs down to 30 Hz. Read this:


... if you are not ready for sacrifying 30 - 50 cm on each wall for this sort of treatment then you should use the light stuff only for corner traps and search for OC703 / 705 style material for the panels. Or use tuned traps etc. etc. There are many possibilities that go way beyond your question.

You really think so?
I got it . So 5000 rayls/m would only be usefull at thickness off 20 to 50cm?. What about 10000 rayls/m at 100mm ? for bass. and 5000 rayls/m at 50mm for reflection panels ?.

Ive used the calculator thing (great thing to have , but it doesn't calculate densitys with it. And from what i know the knauf ecose tp 416 has 16kg/m3 and probally 5000 rayls/m so that would only beneft high end to mids ?.

My room is only around 1.90m in width so i can make my bass traps 100mm thick and refelction 50mm.

Also i saw you and FDW mentioning food wrap plastic ?, why would you want to use that ? doesn't it block soundwaves from being absorbed ?, would polyester wadding not be a better idea ?.

I would like to follow your advise at this point, cuz i know what your talking about.