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my broadband absorbers take out too much mids and highs
Old 21st April 2013
  #1
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my broadband absorbers take out too much mids and highs

hey

i have a small live room. see:
can you help me with this room?

i feel the absorbers take out too much highs and mids. i now placed the absorbers across the corners and that did help regarding the low end. i also drilled holes in the frames and opened their back. i think this helped as well.

but i feel that the absorbers still take out too much highs end mids. when i talk directly into them it sounds as if id talk into a pillow. i do have some professional bass traps by gik that dont do that.

the panels are 24cm thick filled with rockwool. the frames have open backs and lots of holes on the sides. so everything is really exposed. is there anything i can do to get back some life? i heared of people placing a small layer of plastic before the rockwool are even placing slats on the frame.

please help :-)
Old 22nd April 2013
  #2
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do you think this structure would help with my problem:

my broadband absorbers take out too much mids and highs-slats.jpg
Old 22nd April 2013
  #3
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Common solutions are slats or plastic/card on the front (often under the fabric for cosmetic reasons).
Old 22nd April 2013
  #4
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I have the same problem in my room, I want to have just a little more "life"
I think with some piece of wood it's work, and it's beautiful!
May be something like this, with more space between wood :
http://blogs.onisep.fr/concours/conc...ges/bamako.jpg
Old 22nd April 2013
  #5
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Quote:
Originally Posted by salomonander View Post
do you think this structure would help with my problem:

Attachment 341297
The quick answer:- yes it would.

The Newell design that you posted will effectively reduce the open area of the broadband fibre. According to the author with "minimal tonal colouration", if you stick to his sequence, but a bit more reflective at high frequencies

Taken to extreme with a solid wooden panel in front of your mineral wool it becomes a "relatively" narrowband panel absorber at lower frequency, determined by properties/mass of the panel, and considerably more reflective at higher frequencies.
Old 22nd April 2013
  #6
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Quote:
Originally Posted by Icecube1 View Post
The quick answer:- yes it would.

The Newell design that you posted will effectively reduce the open area of the broadband fibre. According to the author with "minimal tonal colouration", if you stick to his sequence, but a bit more reflective at high frequencies

Taken to extreme with a solid wooden panel in front of your mineral wool it becomes a "relatively" narrowband panel absorber at lower frequency, determined by properties/mass of the panel, and considerably more reflective at higher frequencies.
thanks everybody

sounds good :-)
i couldn't really find out how deep the slats in this design are. but the author talks of "wood strips" so i assume not very deep at all?
also, does anyone know if the measurements are in cm or inch?

cheers
Old 22nd April 2013
  #7
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Quote:
Originally Posted by salomonander View Post
thanks everybody

sounds good :-)
i couldn't really find out how deep the slats in this design are. but the author talks of "wood strips" so i assume not very deep at all?
also, does anyone know if the measurements are in cm or inch?

cheers
The thickness of the slats here is not critical, I have used a thickness of 5mm (or1/4")before to get the desired effect of some HF reflectivity. However, for big frontal areas you may want to go a little bit thicker to prevent it flexing too much.

The important point about the sequence is the ratio. It does not matter whether it is metric or imperial, as long as you stick to one set of units for the gaps/slats in the quoted ratio of the sequence.
Old 22nd April 2013
  #8
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Glenn Kuras's Avatar
The slat design is like the following. Highly recommended when you can do it!

http://gikacoustics.com/scatter-plate-product-video/
Old 22nd April 2013
  #9
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Quote:
Originally Posted by Glenn Kuras View Post
The slat design is like the following. Highly recommended when you can do it!

GIK Acoustics Scatter Plate Product Video
thanks Glenn

i couldn't find any info on your page regarding what pattern/structure is used. i assume the drawing i posted does the same thing? cheers
Old 22nd April 2013
  #10
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Quote:
Originally Posted by Icecube1 View Post
The thickness of the slats here is not critical, I have used a thickness of 5mm (or1/4")before to get the desired effect of some HF reflectivity. However, for big frontal areas you may want to go a little bit thicker to prevent it flexing too much.

The important point about the sequence is the ratio. It does not matter whether it is metric or imperial, as long as you stick to one set of units for the gaps/slats in the quoted ratio of the sequence.
thanks icecube

i think ill go for 1cm slats then and stick to the drawing i posted regarding structure/pattern.
Old 22nd April 2013
  #11
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Gdupproductions's Avatar
 

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 ?.
Old 22nd April 2013
  #12
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avare's Avatar
 

Quote:
Originally Posted by Gdupproductions View Post
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 ?.
He has to decide what Q he wants. As far as the system Q goes, with greater than ~20% open area the slats act as a low pass filter. Below the frequency where the slat width equals the frequency's wavelength, the sound will be absorbed by the material behind as if the slats were not present

Well filtered,
Andre
Old 22nd April 2013
  #13
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Quote:
Originally Posted by Gdupproductions View Post
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.
Attached Thumbnails
my broadband absorbers take out too much mids and highs-100mmporousrandomincidence.jpg  
Old 23rd April 2013
  #14
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thanks a lot everybody

here are my rt60 measurements. everything (except low end) looks seriously low. would it be wise trying to get everything to around 0,15s - 0,2s?
is there a formula that can be applied in order to calculate correct slat dimensions/covering-ratio in order to achieve this in a controled manner?
my broadband absorbers take out too much mids and highs-rt60.jpg
Old 26th April 2013
  #15
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i decided to simply apply to formula in the attached pic. can anyone tell me if it makes a difference if i mount the slats vertically or horizontally?
it makes a rather big difference in price since id have to throw away lots of wood if i go for vertical mounting...

cheers
Old 26th April 2013
  #16
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Vertical slats will 'spread' the sound out to the sides
Horizontal will spread the sound up/down.
Usually it's better to push the sound to the sides, where it will encounter even more diffusion and have an easier path back to the ears. I guess it depends on your particular room and setup, and what you have going on floor/ceiling.
You aren't limited to one direction either. You could, for instance use vertical on the rear and horizontal on the sides that in turn push to a diffusive device on the ceiling. I'm not sure how the slats work in terms of diffusive effect, or if it's more about adding 'air', but something to consider all the same.
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