Quote:

Originally Posted by

**Gdupproductions**
I whas searching about this 2 a while back without any results...

How can one determine the Q he wants to absorb or how far the absorbing low pass will go ?.

I don't have any software to model this exact design, there are some on this forum who may be able to offer a solution... PrairieDog? Jens?

However, here is my take on it. In general the whole point of this design is that it does not exhibit a specific Q as such. It is

__not__ meant to be a tuned system in any sense since each slot/gap pair has a variability, deliberately so. This is different to the slotted Helmholtz panelling where

**all** the slats are designed to be the same width, with the

**same** gap, to produce a tuned type of absorption where the Q would be determined to a large extent by the flow resistivity of the mineral wool/fibreglass behind the slats.

So, I will take a practical approach. As a starting point attached is a random incidence plot for 100mm of a generic medium density mineral wool. To all intents and purposes above 4kHz it is pretty much a total absorber. So this is the part of the spectrum that we are trying to

__reduce__ the absorption in the room to get back some of the highs? The effect of the absorbent material on the room is also dependent on the surface area treated. If we cover

__half __of the fibre surface area up by wood slats then we are reducing the effective area treated at high frequencies, ie reducing the absorption. Or another way of thinking about this could be to say we are reducing the absorption coefficient to half it's starting value for the

__same__ area.

At the other end of the spectrum the low frequencies will not "see" the slats since they (the slats) are small with to respect to wavelength. For example a 30mm slat is quarter wavelength for 2800Hz, a 50mm slat is quarter wavelength for 1716Hz. So, below 1700Hz the incident sound will just be absorbed as per the pure porous performance of the mineral wool.

There will be some slotted helmholtz effect, however, since there are a range of slat/gaps pairs it will not be tuned to a specific frequency. For example lets suppose we used 10mm slat thickness and a

__uniform__ wall of 50mm slat and 30 mm gap(or slot), then peak absorption would be around 680Hz, or for 30mm slat and 50mm gap the peak would be around 885Hz. (note these are just the sort of points where the pure porous absorption is starting to drop off). For 5mm slat thickness the equivalent peaks would be 970Hz and 1250Hz.

I realise that this is a simplification and I am not putting forward measured data here, however, it is a practical suggestion. The 5:4:3 design by Newell effectively reduces the open area of the fibre by approx half. At high frequency, therefore, the area treated gradually becomes more reflective with increasing frequency to "some" limit. At low frequency the depth and flow resistivity of the mineral wool will mainly determine the absorption.