I just built and placed a bunch of big membrane absorbers against my speakerwall. 2 panels of 2,44m x 1,22 x 0,15 1panel of 2,44 x 0,60 x 0,15 and 2 corners of 2,44 x0,50(speakerwall) x 0,50(sidewalls) (depth 0,45)
They are constructed out of 18mm mdf sides and 12mm mdf backs, with a hardboard membrane. Filled with rockwool not touching the membrane (low q high bandwith)The corners have a double layer of hardboard (with kit inbetween). Everything is sealed with kit to make it airtight.
My question is about the placement. Because of an old concrete windowpost I decided to place the membrane panels as a whole 10 cm away from the speaker wall. Leaving a 10 cm airgap. I figured this would be better for symetry and easier then puzzling around the concrete windowpost.
Measurements before and after show that I definately tamed the low end. But surprisingley there is a big null at 400hz that wasn’t there before. I’m hoping that all the absorption panels and cloud (all 10cm thick) will tackle this issue.
I’m just double checking if the membrane setup is a recipe for faillure. Will the 10 cm airgap cause trouble, should I be filling it with rockwool. Does the 400hz null have anything to do with the airgap. Is it an affect of mids acting different as the dimensions for these frequencies have changed?
Thanks in advance!
Room dimensions: 5,29 width x 6,10 lenght x 4,80 height (with a suspended ceiling at 4m, I’m not sure what the material is, but it’s light and porious. On top 10 cm of rockwool)
2 panels of 2,44m x 1,22m x 0,15m (diameter2,73)
2 panels of 1,90 x 1,22 x 0,15 (diameter2,26)
1 panel of 2,44x 0,58 x 0,15 (diameter 2,51)
2 corners of 2,44 x 0,50 x 0,50 (depth 0,45) (diameter 2,49). These have a double layer of hardboard.
function: Studio control room
all 2,44m high panels are lined up against the shorter wall. the two 1,90 panels are stacked on top of this wall horizontal. both moved to the corners. These two horizontal panels are right against the concrete wall contrary to the basscabinets under them. Behind the whole wall of 2,44m basstraps there’s a 10 cm airgap.
I have about 20 absorption panels with rockwool for the rest of the room. They are all 10cm thick with no backside, just a frame. A few will be put over the backcorners leaving an airgap. Some in between and next to the speakers, going around till the first reflection points to both sides are covered and some more wall area is covered. Three will form a cloud above the desk. I’ll leave a 5 cm air gap behind all panels.
The speakers will go right up against the basscabs to get the sbi a bit higher in the freq range. leaving the air gap unfortunately will cause it to be a bit lower. Putting the desk a lot further down the room isn’t possible for praticle reasons. I’ll probably do some low freq rolloff for the really low subs 20/30hz.
All this info is just to give you an idea of the situation. My main question is if the airgap is a bad idea. It’s kind of hard to changr it, but if it’s really a recipe for faillure I’d rather know now. Maybe there are workarounds to fix it.
Thanks in advance for the advice!
November 18, 2021
Dennis is this information supposed to be helpful or were you planning on following this up with a “technology” suggestion?
Redwood, you have good questions and clearly have a decent understanding of acoustics so please don’t let people like Dennis with his masterful vagueness deter you from asking.
The driver diameter is an unlikely concern at these frequencies unless you’re using 18inch drivers to produce your 400Hz where beaming may start forming, and you’re measuring off axis, but unlikely.
Although not ideal acoustically, this is actually a fairly generous room size for this application in comparison to the average that we see.
If I understand your design correctly, a noticeable dip around 400Hz could be caused by a coincidence coupling between the 2 compliant parallel surfaces, but it’s unlikely with a 10cm gap, more like 10mm. Thin panel membrane absorbers like yours are a good method for tackling low-frequency issues where space and budget is low because they work with pressure rather than velocity like fibrous materials, but their real-world behaviour is not always easy to predict. Trial and error is usually better here by adjusting the mass. I would use a generous amount of flexible adhesive rather than rigidly fixing the panels to the frame, increasing the absorption and predictability by reducing stiffness the stiffness of the system and reducing the modal behaviour as the energy travel outwards to the edges of the membrane to be partially absorbed.
The 400Hz dip could possibly be formed by the air spring – mass of the panel instead. Just give the panel a tap with the back of your knuckle and an RTA app on your phone with peak hold set. You can change this by adding masses to the centre of each panel to adjust the resonant frequency.
A few suggestions and questions,
– Do you have any small acoustic instruments in the room that may be causing this dip?
– Are you measuring the reverberation time in 1 position or in various positions throughout the room? If the former, reflected surfaces could cause this issue.
– Pre-construction reverberation measurements with the measurement method and locations and pictures of the room will really help in sorting this issue out. Happy for you to send them to me in a PM if you prefer.
– I would personally opt for a limp membrane absorber using mass loaded vinyl, giving you more predictable behaviour and a wide Q with insulation if you like, but this increases cost.
– The modes can also be controlled with a moderate budget using 4 or 2 small subwoofers placed in the centre of each wall in a cuboid room, instead of 1 large one or 2 stereo speakers placed in less than ideal locations. Have a look here, https://www.andyc.diy-audio-en…..fore_after. This site also provides links to some great studies on this topic. You can switch back to the 2 stereos speakers for mastering after mixing.