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build a VPR bass trap.uk
Old 23rd September 2012 | Show parent
  #31
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🎧 10 years
Quote:
Originally Posted by G. E. ➡️
.........Actually straddled VPRs behind the speakers have the potential to sort out dips in the FR in the ~80Hz-100Hz area -- I just don't know why.
Because non resonant interferences (usually called here S.B.I.R). Any wideband LF absorber (resonant or not), behind loudspeaker and on side walls and ceiling, near to loudspeakers may have this potential. This nulls are position dependent too. Also, area covered with this type of absorbers is important. Because large wavelengths it is not possible to decrease influence of interferences with couple small sized panels.
You can draw mirror images of speakers to calculate delay times which cause this nulls... in corners you have usually three images (two primary and one/two secondary) from side and back walls... and another three from ceiling... I assume that angle between boundaries is always pi/2.

EDIT: Example where is used Philip Newell's type of wideband LF absorbers for decreasing influence of non resonant interferences from behind of loudspeakers.



Later, it is covered with fabric, and binary diffuser slats:

Old 23rd September 2012
  #32
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🎧 10 years
Quote:
Originally Posted by Red Mastering ➡️
ok, thanks,
so what would be better shot on steel ?
VPRs depend on modal density of the plate. Mode frequencies of the plate depend mostly on stiffness and density. Neither of these values changes much for typical types of modern steel (for example check modulus of elasticity which is the fancy term for stiffness: http://www.engineersedge.com/manufac...s_strength.htm ). So really any steel plate of the correct thickness will probably work the same as any other in this application.

Different types of steel do take different force to bend permanently, have different ability to stretch, etc.... none of which matter for this application.
Old 23rd September 2012 | Show parent
  #33
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🎧 10 years
1 + 1

Quote:
Originally Posted by Red Mastering ➡️
ok, so we are getting closer
... 2x1mm ...
Please note I wanted to say that I would prefer two VPR-type absorbers with 1,0mm steel over just one with 2,5mm steel assuming its your only option.
Old 23rd September 2012 | Show parent
  #34
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🎧 10 years
Bending

Quote:
Originally Posted by aackthpt ➡️
... Different types of steel do take different force to bend permanently, have different ability to stretch, etc.... none of which matter for this application.
I do not agree -- I think it does matter if a plate is already (slightly) bent in one dimension because the stiffness in the perpendicular dimension is greatly enhanced which results in much higher (propably unwanted) eigenmodes of the plate. Apart from that I agree that this has nothing to do with the spring-mass-system the plate forms together with the absorbent.
Old 23rd September 2012
  #35
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🎧 10 years
Quote:
Originally Posted by G. E. ➡️
I think it does matter if a plate is already (slightly) bent in one dimension because the stiffness in the perpendicular dimension is greatly enhanced which results in much higher (propably unwanted) eigenmodes of the plate...
Does this mean if the steel is bent into say a Polycylindrical shape it would resonate at a higher frequency because of the inherent stiffness caused by the shape?
Old 23rd September 2012 | Show parent
  #36
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🎧 10 years
Steelwork

Quote:
Originally Posted by John White ➡️
Does this mean if the steel is bent into say a Polycylindrical shape it would resonate at a higher frequency because of the inherent stiffness caused by the shape?
... it surely does to a high extent (without going into details). Think of the use of typically hollow profiles in steelwork.
Old 23rd September 2012 | Show parent
  #37
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🎧 10 years
Quote:
Originally Posted by G. E. ➡️
Please note I wanted to say that I would prefer two VPR-type absorbers with 1,0mm steel over just one with 2,5mm steel assuming its your only option.
it's noted sir and remembered well.
I plan to get 3 or 4 panels.
3 smaller ones, let's say 120x60cm (and thickness, well still thinking about option - isobond/steel/isobond. let's say 8cm foam, 1mm steel, 4cm foam)
1 bigger 200x100 cm, same thickness - this will go on a back wall, behind sofa,
it's def. the place when 30-40Hz problem is really bad (as well as in all corners)

so far, I put 2 packs of rw3 rockwool (60kg/m3 density) behind the sofa and it is much better, almost great in sweat spot (just last octave is not perfect)
but I want to remove rockwool from there as it takes too much room space (20cm) and still does not sort out lowest octave problem

I am certainly won't break the doors already open,
so I use G.E.'s concept

now, I am not sure about steel
and mostly - how to mount it ?
I presume foam has to be at least few cm bigger then steel sheet, so it will be able to amortize the weight of steel (I plan to glue it as DanDan suggested, as it will be easier to move around (I am not sure about frame, and finish with fabric ?)
Old 23rd September 2012 | Show parent
  #38
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🎧 5 years
Quote:
Originally Posted by G. E. ➡️

..... Remember that since VPRs are "reactive" devices they'll do more to the release of the room response than the frequency response.
Very important point, Gernot! +1

Growth and decay of sound...

Cheers

Ake
Old 23rd September 2012 | Show parent
  #39
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🎧 5 years
Resonant or not?

Quote:
Originally Posted by boggy ➡️
Because non resonant interferences (usually called here S.B.I.R). Any wideband LF absorber (resonant or not), behind loudspeaker and on side walls and ceiling, near to loudspeakers may have this potential. This nulls are position dependent too. Also, area covered with this type of absorbers is important. Because large wavelengths it is not possible to decrease influence of interferences with couple small sized panels.
You can draw mirror images of speakers to calculate delay times which cause this nulls... in corners you have usually three images (two primary and one/two secondary) from side and back walls... and another three from ceiling... I assume that angle between boundaries is always pi/2.

EDIT: Example where is used Philip Newell's type of wideband LF absorbers for decreasing influence of non resonant interferences from behind of loudspeakers.
Great tutorial on the behavior of reflections and absorber positioning.
Plus... nice pictures!

Of coarse this kind o treatment will take up some volume in the room.

Cheers

Ake
Old 23rd September 2012 | Show parent
  #40
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🎧 5 years
Quote:
Originally Posted by G. E. ➡️
I do not agree -- I think it does matter if a plate is already (slightly) bent in one dimension because the stiffness in the perpendicular dimension is greatly enhanced which results in much higher (propably unwanted) eigenmodes of the plate. Apart from that I agree that this has nothing to do with the spring-mass-system the plate forms together with the absorbent.
But it IS true that the plate modes have a correlation with compliance at mode frequency?
Old 24th September 2012
  #41
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🎧 10 years
Quote:
Originally Posted by OpusOfTrolls ➡️
But it IS true that the plate modes have a correlation with compliance at mode frequency?
Assuming by "compliance" you mean young's modulus (aka modulus of elasticity) then of course. First mode (which places a lower boundary on the harmonics on which this design depends) is proportional to square root of stiffness. But steel doesn't really vary from a modulus of 30 ksi for all practical purposes.

And yes gernot different shape plate can of course have different effect in this device. I was speaking only of material properties not system ones. So more completely... assuming the plate is flat and the same thickness it should not much matter if it is annealed, cold or hot rolled, etc. I originally figured the "flat plate" part was more or less assumed, since that is one of the basic features of the design.
Old 24th September 2012 | Show parent
  #42
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🎧 10 years
Quote:
Originally Posted by aackthpt ➡️
VPRs depend on modal density of the plate. Mode frequencies of the plate depend mostly on stiffness and density. ...................
Because its mounting (steel plate is free to move front and back), functioning of VPR is mainly from steel plate pistonic type of the motion, so modal frequencies is not a main source of very low frequency absorption (they are too high). Steel plate form mass-spring damped system with IsoBond or Basotect absorbing material. This is very similar to Tim's membrane absorbers, but there, "spring" is air in the box.
Old 24th September 2012 | Show parent
  #43
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🎧 10 years
Quote:
Originally Posted by akebrake ➡️
Great tutorial on the behavior of reflections and absorber positioning.
Plus... nice pictures!

Quote:
Originally Posted by akebrake ➡️
Of coarse this kind o treatment will take up some volume in the room.
.......
Of course, absorbing below 100Hz IS always volume consuming, except if we talk about VPRs. heh

Old 24th September 2012 | Show parent
  #44
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🎧 10 years
getting back to topic,
few questions still open:
is it good idea to make 'smaller' 120x60x10 cm panels
or better to make 1-2 much bigger ?

way of mounting them (I'd like to glue steel o foam)

if anyone in UK built them - please chime in and advise on steel/factory/delivery, etc
Quote:
Originally Posted by Red Mastering ➡️
.......I plan to get 3 or 4 panels.
3 smaller ones, let's say 120x60cm (and thickness, well still thinking about option - isobond/steel/isobond. let's say 8cm foam, 1mm steel, 4cm foam)
1 bigger 200x100 cm, same thickness - this will go on a back wall, behind sofa,
it's def. the place when 30-40Hz problem is really bad (as well as in all corners)
....
now, I am not sure about steel
and mostly - how to mount it ?
I presume foam has to be at least few cm bigger then steel sheet, so it will be able to amortize the weight of steel (I plan to glue it as DanDan suggested, as it will be easier to move around (I am not sure about frame, and finish with fabric ?)
Old 24th September 2012 | Show parent
  #45
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🎧 5 years
Quote:
Originally Posted by aackthpt ➡️
Assuming by "compliance" you mean young's modulus (aka modulus of elasticity) then of course. First mode (which places a lower boundary on the harmonics on which this design depends) is proportional to square root of stiffness. But steel doesn't really vary from a modulus of 30 ksi for all practical purposes.

And yes gernot different shape plate can of course have different effect in this device. I was speaking only of material properties not system ones. So more completely... assuming the plate is flat and the same thickness it should not much matter if it is annealed, cold or hot rolled, etc. I originally figured the "flat plate" part was more or less assumed, since that is one of the basic features of the design.
I actually meant to say impedance. Assuming a free floating on foam plate, the plate modes will offer impedance closer to free air at that frequency. Otherwise, what is the point of modality? The VPR would just be a diaphragmatic absorber without the flexural properties of the steel.
Old 25th September 2012
  #46
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🎧 10 years
Red Mastering: better make them not too small. I would think nothing wrong with 1x1,5m or even bigger ones...
Old 25th September 2012 | Show parent
  #47
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🎧 10 years
Quote:
Originally Posted by JP__ ➡️
Red Mastering: better make them not too small. I would think nothing wrong with 1x1,5m or even bigger ones...
yes, but I am limited by space....
I think about getting 1 bigger, say 120cmx200cm (it really depends on steel sheet)
and 3 smaller, 120x60cm
both at least 10 cm deep

I just did quick test
and worst offending freq would be
32Hz,33, up to 44Hz

this could force me to build bigger panels
question is where to fit them?:(
Old 25th September 2012 | Show parent
  #48
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🎧 10 years
Quote:
Originally Posted by Red Mastering ➡️
.........
this could force me to build bigger panels
question is where to fit them?:(
Why you think that you need that large panels?
VPR absorbers function isn't based on steel plate modal resonances, so dimensions are (virtually) out of equation.
Commercial VPRs 1x1.5m works well below 50Hz.

Old 25th September 2012 | Show parent
  #49
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🎧 5 years
Quote:
Originally Posted by boggy ➡️
Why you think that you need that large panels?
VPR absorbers function isn't based on steel plate modal resonances, so dimensions are (virtually) out of equation.
Commercial VPRs 1x1.5m works well below 50Hz.

You are right that they will work with a smaller size. The VPR I tested according to Gernot's hypothesis showed 2dB of absorption at 70hz for a 48" x 28" 1.7mm sheet.

I am seriously wondering what effect plate modes can have. Because of the elasticity of the steel, the modes will ring given an impulse. Also, the modality is non-linear, because of flexural tension as a result of plate thickness. There is still much to be investigated.
Old 25th September 2012 | Show parent
  #50
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🎧 10 years
Quote:
Originally Posted by OpusOfTrolls ➡️
You are right that they will work with a smaller size. The VPR I tested according to Gernot's hypothesis showed 2dB of absorption at 70hz for a 48" x 28" 1.7mm sheet.

I am seriously wondering what effect plate modes can have. Because of the elasticity of the steel, the modes will ring given an impulse. Also, the modality is non-linear, because of flexural tension as a result of plate thickness. There is still much to be investigated.
at least, OP need absorption below even 50Hz, so plate modes is not his target, because he don't have a place for that big panels, as he already said.
You can look at "Tim's Limp Mass Bass Absorbers" thread, we discuss trap's dimensions. As VPR is fully based on surface density of plate and its linear movement (front-back if mounted vertically), where IsoBond act like spring and damping in the same time... there is a similarity with limp mass absorbers, where we have air (additionally damped with glasswool) as spring and damper... and pistonic motion of limp membrane...
Some similarity exist with floating floor too. This is approximately the same concept, but different realization (because different problem)

Modal activity exist for sure, but plate modes aren't dominant absorption mechanism for that low frequencies.
Old 26th September 2012 | Show parent
  #51
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🎧 10 years
Quote:
Originally Posted by boggy ➡️
at least, OP need absorption below even 50Hz, so plate modes is not his target, because he don't have a place for that big panels, as he already said.
You can look at "Tim's Limp Mass Bass Absorbers" thread, we discuss trap's dimensions. As VPR is fully based on surface density of plate and its linear movement (front-back if mounted vertically), where IsoBond act like spring and damping in the same time... there is a similarity with limp mass absorbers, where we have air (additionally damped with glasswool) as spring and damper... and pistonic motion of limp membrane...
Some similarity exist with floating floor too. This is approximately the same concept, but different realization (because different problem)

Modal activity exist for sure, but plate modes aren't dominant absorption mechanism for that low frequencies.
thanks,
I did some measurements with REW and ecm8000,
I will upload .mdat file tomorrow, I need to check 1 more setup but it's too late for re-arranging room now
the main problem is 32Hz ring, up to 40Hz (it really depends on monitor versus microphone position), but 32Hz is a king of resonance
decay time looks good (except 32Hz issue)

also I walked my room over with mic and rta analyzer
the back wall by the sofa, and especially left corner is the worst
32-39Hz literally rings there so strongly, it's kind of a torture standing there for longer then few sec

I checked Tim's thread already, just don't want to jump from 1 idea to another, vpr sound really interesting;
but on another hand, I am happy to be corrected and instructed

thanks again to all nice chaps for support
Old 26th September 2012 | Show parent
  #52
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3 Reviews written
🎧 10 years
Quote:
Originally Posted by boggy ➡️
As VPR is fully based on surface density of plate and its linear movement (front-back if mounted vertically), where IsoBond act like spring and damping in the same time... there is a similarity with limp mass absorbers, where we have air (additionally damped with glasswool) as spring and damper... and pistonic motion of limp membrane...

Modal activity exist for sure, but plate modes aren't dominant absorption mechanism for that low frequencies.
I know some people use an adhesive between the melamine foam and the steel plate. I wouldn't be surprised if different adhesives and/or application techniques had an effect on the behavior/performance of the system.
Old 26th September 2012 | Show parent
  #53
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🎧 10 years
Quote:
Originally Posted by Red Mastering ➡️
...........
the main problem is 32Hz ring, up to 40Hz (it really depends on monitor versus microphone position), but 32Hz is a king of resonance
decay time looks good (except 32Hz issue)
For best results you really need to check this thread too:
Speaker placement methods
(it is not another "second" idea)

Quote:
Originally Posted by Red Mastering ➡️
also I walked my room over with mic and rta analyzer
the back wall by the sofa, and especially left corner is the worst
32-39Hz literally rings there so strongly, it's kind of a torture standing there for longer then few sec
it is not easy to have excellent bass response near to boundaries... even with treatment...
"sofa listening position" is extremely difficult problem partly because near back wall, and partly because strong and too close diffusion which usually exists there. So it usually only looking good at pictures... but that is all...
Quote:
Originally Posted by Red Mastering ➡️
I checked Tim's thread already, just don't want to jump from 1 idea to another, vpr sound really interesting;
but on another hand, I am happy to be corrected and instructed
Both "ideas" has its good and bad things...
Be prepared not to have "instant solution" for serious acoustics problems, and try to find similarities in other "principles" and explanations for questions you definitely will have, in this similar principles, during your "adventure". I didn't want to give you a second idea, but some things are explained in this thread which is not explained in VPR thread... (dependence of size of bass trap)

Both principles has membranes, both principles don't include modal resonances of membranes in their functioning... etc.

Good thing for VPR is that you don't need to tweak damping (as in limp mass bass traps), because it is defined by IsoBond material, membrane is denser than limp mass so absorber may be thinner...
Quote:
Originally Posted by Red Mastering ➡️
thanks again to all nice chaps for support
you're welcome.

Old 26th September 2012 | Show parent
  #54
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🎧 10 years
Quote:
Originally Posted by Syncamorea ➡️
I know some people use an adhesive between the melamine foam and the steel plate. I wouldn't be surprised if different adhesives and/or application techniques had an effect on the behavior/performance of the system.
Probably, but Gernot said that this things are already self adhesive...
Old 26th September 2012
  #55
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🎧 10 years
Thanks, boggy!
Old 26th September 2012
  #56
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🎧 10 years
Boggy,
hvala ljiepa!
I am really limited by the room dimension and items within the room (desk, sofa, etc)
also i got original stands by pmc,
actually I have 2 pairs, hifi ones - much lower (14inches)
and studio stands, 33 inches
funny thing is both seems wrong,
the studio ones are too high, when i sit on chair my ears are little below midrange driver
and I am kind of tall guy (185cm)
on hifi stands is in theory bit better, as my ears are bit over the highend driver
but after initial check, higher stands, are much better, so I stick to them
I am doing few more setups with monitors right now,
I will drop .jpg with freq chart and waterfall,
plus .mdat file, so you can check it yourself


thanks again
Old 26th September 2012 | Show parent
  #57
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🎧 10 years
Quote:
Originally Posted by Red Mastering ➡️
Boggy,
hvala ljiepa!
Molim i drugi put!
Quote:
Originally Posted by Red Mastering ➡️
I am really limited by the room dimension and items within the room (desk, sofa, etc)
also i got original stands by pmc,
actually I have 2 pairs, hifi ones - much lower (14inches)
and studio stands, 33 inches
funny thing is both seems wrong,
the studio ones are too high, when i sit on chair my ears are little below midrange driver
and I am kind of tall guy (185cm)
on hifi stands is in theory bit better, as my ears are bit over the highend driver
but after initial check, higher stands, are much better, so I stick to them
.......
.
Try to find appropriate stands for your IB2s, also, try to find acceptable vertical position, because response may be better at this position... this may be a couple of centimeters lower or higher than your listening position... use books and something to raise your speakers and to find best height... of course in range where you can change height of your ears, with height of your chair.
Old 26th September 2012 | Show parent
  #58
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3 Reviews written
🎧 10 years
Quote:
Originally Posted by boggy ➡️
Probably, but Gernot said that this things are already self adhesive...
I made contact with the firms that make Basotect and Isobond about US suppliers and was directed to a Sonex supplier. Then I made a call to BASF USA and they also pointed me at Sonex. So I talked to an engineer at BASF and he told me it was the same polymer as Basotect. Long story short, I couldn't find any in the US with an adhesive layer. That's OK, I've glued Sonex to many different surfaces so it's not difficult to do.

Regarding the use of other materials like fibrous pads, etc., I don't think you'd want to use anything too dense or non-compliant. The BASF engineer, once he knew I was talking about VPRs, pointed me at a melamine foam / limp mass vinyl / melamine foam sandwich product and said their tests showed great performance. At the least one could build heavier traps with it!
Old 26th September 2012 | Show parent
  #59
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🎧 10 years
Quote:
Originally Posted by Syncamorea ➡️
I made contact with the firms that make Basotect and Isobond about US suppliers and was directed to a Sonex supplier. Then I made a call to BASF USA and they also pointed me at Sonex. So I talked to an engineer at BASF and he told me it was the same polymer as Basotect. Long story short, I couldn't find any in the US with an adhesive layer. That's OK, I've glued Sonex to many different surfaces so it's not difficult to do.

Regarding the use of other materials like fibrous pads, etc., I don't think you'd want to use anything too dense or non-compliant. The BASF engineer, once he knew I was talking about VPRs, pointed me at a melamine foam / limp mass vinyl / melamine foam sandwich product and said their tests showed great performance. At the least one could build heavier traps with it!
Sometimes is better to be more specific with questions... thanks for the info, btw!

Old 26th September 2012 | Show parent
  #60
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🎧 10 years
I need to have something constant,
stands stay as they are, without putting books under them,
not to mention 1 monitor is almost 40kg...my poor back

the peak at 32Hz and dip around 110Hz - is almost not changeable with monitor/mic positioning,
so I am giving it up for now,
I will start setting VPR's when I got them, rockwool doesn't cut the mustard, as one super humble GS'er said ones

anyway, below is last measurement I made,
there are few ugly peaks and dips
32Hz peak is laughing at me seeing me carrying rockwool panels from left to right
this really bad dip at around 110Hz - I tried to find most offending places and put some panels there, but unfortunately, the difference is minimal

it seems I have to go for at least 2 big VPR panels and maybe 2 smaller,
rockwool and under 100Hz issues aren't good company

still I am not sure where to buy steel and what kind....
how to eventually mount panels
would it be better idea to put foam/isobond on both sides of steel,
or just on...

questions, questions...

link for .mdat file
for some reasons I couldn't upload it here
http://wtrns.fr/mwojor3n2GY2V8M
Attached Thumbnails
build a VPR bass trap.uk-jpeg-red5.jpg  
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