Resolution Mag : Room for voice recording by Philip Newell

The Resolution Magazine published an interesting article by Philip Newell in the last July/August 2007 issue. The task was to redo the acoustics in a small dialog replacement room that suffered from coloration in the lower vocal range. The original treatment was 4cm of 70kg/m3 mineral wool mounted flush on the walls and ceiling, covered with compressed cotton waste felt. The coloration in the lower vocal range is no surprise here. M. Newell retained the original suspended plasterboard ceiling and replaced the original acoustic treatment with :

• a 60 mm layer of 80kg/m3 open cell polyurethane foam called Arkobel glued to the walls and ceiling,

• a 35 mm layer of lightly compressed wood shavings mixed with cement called Celenit glued to the Arkobel.

According to Newell the Celenit glued to the foam acts as a highly damped panel absorber that deals well with the mid-bass frequencies and users were very happy with the results. I have a few questions :

• In the article Mr Newell exhibits a really smooth decay time over the frequency range : under 100ms down to 125Hz. According to the (correct) formula the 18kg/m2 Celenit at 60mm from the boundary would resonate at 58Hz*. To me that seems much too low as the foam absorption starts rolling off at 1.5kHz and the Celenit absorption starts rolling off at 1kHz (if mounted flush on the wall). Also, absorbing this low in a voice replacement room is not necessary. Does the damping of the Celenit by the Arkobel brings the resonant frequency up ? What kind of Newell magic is at force there ?
• Arkobel is a Spanish product, is there an equivalent in Europe ? in the US ? Would the Arkobel could be replaced with (probably cheaper) 80kg/m3 mineral wool ?
• In my country (France) it's not allowed to glue anything on the ceiling. Is it the case everywhere ?

* : Celenit density, as taken from the Celenit website for a 35mm board is 14 kg/m2 instead of 18kg/m2 as stated by M. Newell in his article, the calculated resonant frequency doesn't change much : 65Hz

msikio, There is no way you can calculate whatever resonance frequency based on that data. If you calculate a mass spring system that assumes a closed mass and knowledge of the dynamic stiffness of the spring. This wood thing is like tectum and other brands and is open cell material (thermal isolation with relative poor absorption values in itself). These wooden boards are standard stuff anywhere and are called in my language "Houtwolcementplaten". If I try to translate this litterally that should become "Wood wool cement boards", but that's most likely not the correct English translation/name for it. That material is often used in combination with mineral wool as absorption material (where it more act as a stable self-carrying acoustic transparent board, but having some absorption properties in itself). There are so many complicated unknows involved here that you can as well use a darts game trying to calculate this. To know that you need to know the dynamic stiffness of that foam and the "APPARENT" mass of that wooden stuff which relates to gasflow resistance, entrance impedance, whatever.

Eric, As usual, you're absolutely right. As the wood fiber product is not airtight the formula can't be used at all in this case... I should have though of that :? Thanks a lot, great info here again. Michel

Michel, You've the courage to ask questions. That's a very good thing. And I'm sorry if I sometimes sound impatient. If you see a formula it's good to wonder what that formula calculates in fact. A formula is just a symbolic representation of a real live story.

Eric, That's no pb. I actually prefer somebody who might sound impatient at times and know his stuff rather than the other way round :wink: