Acoustic Panels and Bass Trapping a new lounge

[wasabijim]

5 hours ago, playdough said:

The only thing I don't understand is

 Quote from the PDF.

 

The box L x W is not critical but should be kept small compared to the wave length of the resonant frequency. The limp mass bass absorber is not effective beyond 300Hz for this reason. Typical L x W would be 60 x 40 cm for frequencies below 100Hz, and 40 x 30 cm for frequencies above 100Hz.

 

ok, a bit more reading work out why 

 

 

 

assuming this is the only maths at play, the (approximate) calculation does not account for LxD. Indeed odd, but kinda makes sense if it's the relationship of wave length (or fraction of) for a given freq and depth distance that determines the amount of potential pressure/energy reflected back that the device can offset.  justifies a multi-box approach....   

 

 

 

Edited August 8, 2023 by wasabijim

[playdough]

16 hours ago, wasabijim said:

justifies a multi-box approach....

Yes, it is an interesting situation

 

Sounds sad but I have been studying studying those walls/corner boundary with a tone generator and music, mic for off the boundary, hands on for on the walls themselves.

from about 85dB up the walls flex, which is the fundamental cause of the resonation, I see in the graphs, <150Hz The mic shows just off these walls summation at the corner  and the centre of the wall. I didn't even realise it was "that bad" until I felt it

The walls are double plaster, heavily insulated, studs are real 100x45mm hardwood, like steel beams at 70yo, to add mass I cladded the outer of the walls with a 10mm structural ply, before a foam foil board was applied, then the 1" thick pine cladding. 

 

I've this roll of 5Kgm2 @10m vinyl, 1360mm wide. Procrastinated for a moment then come up with,,,,,,,mmmm why cut the side of it, use the full width, that and why not actually further damp the wall (which flexes) and have a limp heavy membrane shorting out the width of the walls and bridging the boundary. It' wont stop the wall from flexing but it'll damp the resonation far more effective than a few small boxes hanging on the wall with ply backs (toys  IMHO)

 

The outer cases for the membranes are to be built onto the wall, 1350high and almost full wall width and bugle screwed to the studwork.

 

I can see merit in multiple toy boxes for maybe solid brick rendered walls, however there is way more going on with a timber wall. Way more than the Maths show or calculate, like applying a membrane to damp another membrane (wall) 

 

Yea, so it is a multiple box approach, they are just a bit bigger and do more. Being serious, none of this is portable or movable, it's permanent and integrated part of the dwelling and cost is no concern, a good result is more of a concern. 

 

Looking forward to the measurements, experience ive had in real world application tells me to be optimistic, so i'm grinning

Back to the real, staples are go.

 

 

[DVDHack]

I'm wondering if huge boxes have a membrane that is too large to resonate.  The recommendation in that other paper is for a size of 60x40cm, maybe that is to ensure resonance.  I know these are limp membranes but they are supposed to push against a seal box that is airtight implying some resistance to the sound pressure, does that still hold for a huge panel?  
 

As for rigid walls, they make room mode worse in the first place so they're a zero sum gain.  I build my own house and the walls are made of ICF, that's polystyrene blocks pumped with concrete so it's quite rigid, certainly very strong modes in my room.  Wish I'd planned for the listening room, I'd have made my walls a foot higher than my 3.1m and maybe made them vaulted or even porous ceiling with bass traps in the ceiling void, it's wasted space after all.

 

Im going to try some myself and I'll make my standard size boxes (1200x600) and I'll just build in internal dividers.  
 

Has anyone found any info on the Q of the traps?

[playdough]

Adding context, this track flexes the walls, when pushing 85dB+

 

[playdough]

2 minutes ago, DVDHack said:

info on the Q of the traps

Yea, the bigger the membrane, the higher the sensitivity.

The more medium density Polymax I can get in behind the membrane the wider the Q Factor and the better it's damped.

It's in the previous links and posts.

[DVDHack]

5 minutes ago, playdough said:

pushing 85dB+

There's another factor I suppose, that's about 10dB louder than I listen at, at 80dB I can listen for a short period.  10dB is a 10x sound intensity.

[playdough]

Questions will be answered as new measurements are compared with old.

 

I'm onto the inners, the membranes are cut, so are the timbers, so cracking on.

[DVDHack]

4 minutes ago, playdough said:

The more medium density Polymax I can get in behind the membrane the wider the Q Factor

I guess that's where the acousticmodelling.com calculator is better because it seems to account for the depth of the box changing with addition of absorption depth.

[playdough]

1 minute ago, DVDHack said:

80dB I can listen for a short period

10 to 50Hz at 85dB is enjoyable, more enjoyable at 90dB and eyeball distorting at 100dB, beyond that get's hectic and does all manner of weird things. All with 25w per channel.

 

[DVDHack]

3 minutes ago, playdough said:

Questions will be answered as new measurements are compared with old.

 

I'm onto the inners, the membranes are cut, so are the timbers, so cracking on.

'Which suburb are you in Glen?

[playdough]

20 minutes ago, DVDHack said:

box changing with addition of absorption depth

The model is a good guide, there are lines to read between however and actual application can be slightly different. 

I've built a few of these and fairly well know what will happen. Execution, is everything and I'm not playing with budget or time constraints, for me this time. 

I've been on this renovation full time on my own for 8 months straight, almost there.

Edited August 8, 2023 by playdough

[playdough]

This is what I think I'll achieve, nothing like a goal. 

What actually happens is another matter.

 

The actual measurement, beyond this one will be compared and the real product revealed, not long. Week maybe.

[davewantsmoore]

1 hour ago, playdough said:

It' wont stop the wall from flexing

 

Lowering the frequency of the flex is what you want (as the frequencies rises the acoustic efficiency of the "wall as a transducer" increases)

... and ensuring the are no rattles (what, if anything, is the flexing plaster touching on and vibrating)

 

Some of these "bass traps" will work quite well at ~100Hz+ ... but don't overestimate the effect at lower Hz...... but that being said, at any frequency with sparse modes, trying to fix that issue with acoustic treatment is like trying to put a genie back in a bottle... or get an egg back in the shell, or... (you get the idea, hehe).    So consider very critical the placement of sources..... > 300Hz .....  60 to 300Hz (consider multiple) ... and then a specific plan for <60Hz to counter any large peak/dip.    If you are going to go the "making everything else in the room compromise around my speaker location, etc."  (ie. it's a custom install where everything -> sound quality) .... then don't skip this bit.... and if you are going to build a bunch of stuff, build as many subs as you need, including ones which run right up into the 200Hz+ range.

 

The decay (and similar types) of charts are very hard to read if not normalised to the same (or close to) frequency response.   I would recommend you start using Dirac Live now.   Pick a reasonable target curve, and use that for all your comparisons.   This way any differences in decay, distortion, reverberation, noise floor, etc... will all be relative to the same (or at least closer) frequency response.

Later, once "everything is finished" you can then change the target in Dirac to whatever you want it to sound like.

[DVDHack]

49 minutes ago, playdough said:

This is what I think I'll achieve, nothing like a goal. 

What actually happens is another matter.

 

The actual measurement, beyond this one will be compared and the real product revealed, not long. Week maybe.

Be very, very careful there.  My main room is a bit like that except the low end needs fixing.  I'm adding a stack of diffusion because my room is just too dead.  That's a very low RT60 profile.  I'm adding BAD to the front of my diffusion to liven things up.  I've added BAD to 4 of the 12 ceiling panels and it's made a huge difference.  The low RT60 worked really well as a home theatre but for music it was just horrible.

[playdough]

46 minutes ago, davewantsmoore said:

Lowering the frequency of the flex is what you want (as the frequencies rises the acoustic efficiency of the "wall as a transducer" increases)

Your on it Dave.

Applying a shorting membrane across the resonator might go some way to fix it

Finally using the mile of Poly you supplied Cheers.

7 minutes ago, DVDHack said:

my room is just too dead

My lounge  is not, it's still quite alive with  floor and ceiling adsorb,  >2Kh z + Target

total surface area treated is roughly a third or less of the entire area and only deployed in the listening alcove.  It's just taken the lumpy hard edge off what the Giant multiway horns output, made listening "nicer"  certainly not dead. Different room, different approach, good work.

16 minutes ago, DVDHack said:

Be very, very careful there.

Yea, part of the deal. but my experience tells me more is better for below 100Hz, period., frequencies above, well that's a whole another story. and not my target.

Keep in mind, bass is provided by 4 x 21" Drivers as pairs, in 2 x 650 litre enclosures, port tuned to 27Hz + DSP Convolution. They are an acoustic coupling on top of an acoustic coupling, so somewhat efficient and a real experience to behold.

1 mm of cone excursion, flexes the walls. Vanishingly low linear distortion. Something needs to be done to fix resonation of the flexing walls, needs an industrial sized approach.

 

[playdough]

1 hour ago, davewantsmoore said:

get an egg back in the shell,

 I believe there is a poem about that 

Something to do with Humpty Dumpty

[DVDHack]

10 minutes ago, playdough said:

4 x 21" Drivers as pairs, in 2 x 650 litre enclosure

I used to do live sound, did sound engineering for a local band, we ran a double 4 way PA and that didnt use 21" bass drivers, that's just nuts.  The room my second system is in would be ideal for those, but not in winter.... There's a big window that fully opens directly behind the speaker position, can't get a better bass trap than that.  You aren't squatting in a concert hall are you?

[davewantsmoore]

1 hour ago, playdough said:

It's just taken the lumpy hard edge off what the Giant multiway horns output

Consider fixing this at the source, rather than with "room treatment".

The "target" chart you posted is very very very "dead".

 

1 hour ago, playdough said:

Keep in mind, bass is provided by 4 x 21" Drivers as pairs

 

1 hour ago, playdough said:

Yea, part of the deal. but my experience tells me more is better for below 100Hz, period

So, only 2 sources of bass... each in the L and R position?

[playdough]

Ok, this is the membrane inner + outer shell, frame material breakdown and in the order of assembly photo

Chop chop.

 

 Might be a long afternoon/evening finishing both.

 

[playdough]

3 hours ago, davewantsmoore said:

fixing this at the source,

Yea, I dunno if I'll actually use those speakers in the end, we'll work on the Unity set up after this I think.

 

3 hours ago, davewantsmoore said:

So, only 2 sources of bass... each in the L and R position

Yea, there is merit for 4, For the area though it's verging on being crowded with only the speakers/couch/coffee table. 2 more bass cabs, nah, maybe a coffee table sub.

[playdough]

5 hours ago, DVDHack said:

squatting

Nah, but had a few all night jobs, lugging boxes and patching cables, lifting HF units onto towers. Past life, learned a bit, was good. 

[DVDHack]

18 minutes ago, playdough said:

Ok, this is the membrane inner + outer shell, frame

'This is a bit confusing, it looks like you have a membrane, then an absorption layer and another membrane all touching, is that right?  Maybe a drawing?

[davewantsmoore]

6 minutes ago, playdough said:

crowded

4x 65 litre subs are better than 1x 650 lire sub.

[playdough]

7 minutes ago, davewantsmoore said:

4x 65 litre subs are better

I could scrap the ported walrus enclosures and -707 the 21's. be alright, yea.

Sort of afflicted with these sub enclosures, just now  and one thing at a time Dave 

 

11 minutes ago, DVDHack said:

confusing

 

Sorry ,yea I'll make an attempt to explain away what I've actually done there, a little later, still light out.

[playdough]

There is something, Membrane is suspended nice on the inner frame. Very limp

 

I decided to trim off the foam on the back of the MLV, around the edges so it wasn't trapped, under  the pine batten.

 

Reasoning is, so the foam then has been decoupled mechanically from and suspended by it's adhesion to the MLV, in a heavy frame that's meat to damp the whole membrane. 

Having only the vinyl at it's edges, makes for a lighter hinge (moving interface at the damped frame) .

 

No bracing for the inner frame, none, bad as it will effectively raise the stiffness and frequency it resonates at.

Sort of like tightening a bass drum skin, not a positive. This is an anti speaker, not a drum. The suspended trap diaphragm must have no tension on it.  

 

The front outer shell will be stretched over the front of the inner frame, stapled. This forms drum tight outer decorative face into the lounge. Left overs from the ceiling job.

 

 Under this outer face there just happened  a 20mm cavity to put a constrained layer of 20mm Polymax sheet, between the pine battens holding the MLV. That layer to be stapled top and bottom of the inside face of the pine battens.

 

This second constrained layer has 2 functions. It acts to further damp the suspended membrane and act as a thin surface HF absorb.

 

Decided was to face the foam back of the MLV into the room, to act as and in addition to the Polymax, so 26mm of outer absorb, for the high frequencies.

 

This trap will be right behind my head in the listening position hard on the rear wall of the room, Another one will act as a first reflection treatment on the side wall.

 

WIll get these done and look at the outer case.