216 replies to this topic

[Drizt]

My wife and I have just started putting all the bits and pieces together but we have found it is going to be a bigger job than we anticipated. Oh, and the acoustic treatments are going to be bigger than I really thought they would be. Don't get me wrong, I know what the measurements are, I guess they just look bigger in real life.

Well, I am certainly no handy man and this took my wife and I a little longer than we really should have.... but o well.

Here are some pictures of the progress thus far.


This shows the 2.4m x 1.2m frame that will hold the Megasorber P100 material.



This just is a bit of a close up of the angled brackets we used to keep the thing square and stable.



This shows the location (above the stairs) where the acoustic treatment will go.

This will be wrapped in RIM Trilogy One Gold. I think it is going to look really good once it is up. Here is a link to a thread regarding the fabrics I will be using. -> http://www.stereo.ne...stic-treatments




Now... that is treatment 1 of 5 that are going on the walls. The other four will be free standing units that will at the first reflection points on the front and side walls.

Then there will be multiple smaller treatments on the roof at the first reflection points as well as a cover for my plasma to absorb some of those early reflections.

I'll post more pictures as we make progress.

[LogicprObe]

Looking good so far!
Believe me, you get better at all these projects the more you do!

[Drizt]

Looking good so far!
Believe me, you get better at all these projects the more you do!

Thank you mate. I'm sure if I did this more often I would get more proficient at it. I just hope this all works out (well, I hope it holds together )

[nortonbert]

Looks good thus far.

It isn't mounting flush on the wall is it?
A gap of 50mm or 100mm between the wall and the frame will dramatically improve performance.

[Drizt]

I'm going to use L brackets to give the desired ~100mm air gap between the absorber and the wall.

Harvey from Megasorbers sent me some data showing how much an air gap improves the low end absorption efficiency. It really is quite dramatic.

[rocky500]

Hi Dritz
This ~100mm air gap, is it specific to your traps?
I ask as I have some Blackhole bass traps that I will be putting up soon. Mine do go in the corners so it might be a little different.

[Drizt]

I assume it would hold true for any bass absorber but that really is just a guess. I don't know the technical reason for why it works.

[KenTripp]

It works because these are broadband absorption traps that work best at areas of high particle velocity. The air gap isn't doing anything other than moving the absorbing material further away from the wall where the particle velocity is higher. It's zero at the wall. Same effectiveness, perhaps more can be achieved by using thicker material. At low frequencies you need thicker material and spaced even further from the wall to absorb as the wavelengths are higher. Or you can use membrane traps which work better. Or you can use corner traps, such as tube traps or even super chunks that work in areas of high pressure such as in the corners of the room. Wall placed traps do work but the most effective place to trap bass is the corners of a room. For taming reflection points you're only really interested in over 300hz anyway and not room modes or bass so hard against the wall will work just as well.

Edited by KenTripp, 15 November 2011 - 07:19 AM.

[Drizt]

It is too late now for me, but someone just told me the air gap needs to be sealed behind the absorber. Is this true?

From what KenTripp says above I wouldn't have thought so.

[KenTripp]

It is too late now for me, but someone just told me the air gap needs to be sealed behind the absorber. Is this true?

From what KenTripp says above I wouldn't have thought so.

I pretty sure that if you seal the air gap you turn it into a pressure device. Probably why floor to ceiling corner straddling flat panel traps work so well.

[Drizt]

How much difference you think it will make ? Have I wasted my time on my current frame ?

[aechmea]

Like Ken says, Drizt.

You are making broadband absorption traps which work by retarding the motion of air particles. They work when they have moving particles to retard. Particles are stationary against the wall and speed up as you move away from the wall and reach max velocity at 1/4 wavelength. Therefore by moving the absorber away from the wall you will retard lower frequencies (=longer wavelengths).

100mm thick absorber + 100mm from the wall = 200mm = 1/4 wavelength of about 300Hz; therefore will be very effective from ~300 up and be less effective on a sliding scale down from there.

Also like Ken said, membrane traps work differently - by vibrating a rigid or semi rigid or even limp membrane into a box and then trapping these vibrations - you aren't doing this.

Your wall in the stairwell looks made for trapping!

[I too was surprized at the size of the sheets when they arrived. Mine are semi-rigid fibreglass though. Over the last 6 months I have been moving them (4 full sheets and 10 pre-formed tubes) around the room, leaning them against the walls and in corners to investigate where they sound different and rationalising why. All good fun.]

If you haven't already done so buy a copy of "Master Handbook of Acoustics" by Everest and Pohlmann. Essential reading.

[aechmea]

How much difference you think it will make ? Have I wasted my time on my current frame ?

None and no.

Edit; sorry. My 'none' needs explanation - making a sealed box instead of your frame will have no beneficial effect. Your frame is fine for a broadband absorber.

Edited by aechmea, 15 November 2011 - 08:34 AM.

[KenTripp]

How much difference you think it will make ?

I've got a room full of tube traps and superchunks and having never built or used a flat panel absorber I couldn't really tell you.

Fair bit of evidence around though that sealing a corner mounted panel makes a big difference but this is different to a wall mounted panel because of the higher pressure in a corner/greater distance from the wall etc.

Also sealing a wall mounted flat panel is more applicable to a membrane type trap such as Ethan Winer uses.

Have I wasted my time on my current frame ?

Not at all as your priority would appear to be taming first reflection points and in general deadening the room a bit rather than sorting out bass issues.

Edited by KenTripp, 15 November 2011 - 09:05 AM.
spelling

[KenTripp]

...and having never built or used a flat panel absorber I couldn't really tell you.

Actually thinking about it, that's not quite true. My room is solid brick, timber over concrete floor and a really solid old fashioned and very thick laden(sp?) plaster ceiling. Glass blocks and 6mm (x2) double glazed windows etc.

So when I built the the room I put in an offset lightweight metal stud wall with standard 10mm plaster running down one side, 3.1m high and 6.8m long. Vertical studs are only supported at top and bottom so the wall can flex and the entire cavity is filled with Rockwool. Fully sealed, so this is a membrane trap.

[Drizt]

Thanks for the replies thus far.

I've sent an email to Harvey at Megasorber to clarify what the following measurement is showing. (Is it a sealed air gap ???)

http://www.stereo.ne...54&d=1319798541

[Drizt]

Your wall in the stairwell looks made for trapping!

Just wondering if you expand on this a little and tell me what you would do with it?

If you haven't already done so buy a copy of "Master Handbook of Acoustics" by Everest and Pohlmann. Essential reading.

I have a copy but have never got around to reading it. Lazy maybe

[Paul Spencer]

Aaron,

Aechmea and Ken are on the mark. You are on the right track if you want a broadband absorber. All the plaster and flooring in your room acts as a membrane pressure trap already, although the tuning is probably limiting the effect. A sealed pressure trap wants a completely different design, and a number of them running in parallel with different tuning points. Spacing it out will let it work to a lower frequency, it's based on the wavelengths in question.

[Drizt]

Thanks for all the advice guys.

[KenTripp]

(Is it a sealed air gap ???)

Probably not but look at how big a gap it is.

[Drizt]

Probably not but look at how big a gap it is.

Yeah it is 250 + 50mm for the P50 he used (so 300mm total).

Using aechmea's logic above and looking at this chart -> http://www.hottconsu...wavelength.html

300mm quarter wavelength matches up somewhere around ~250Hz which is where the peak absorption is in that graph.

I note that it still raises the low frequency absorption efficiency substantially below that frequency too.

[Drizt]

Probably not but look at how big a gap it is.

Turns out it was sealed.

Harvey sent me the following:

It is preferred to have the air gap sealed. When we did the test at RMIT, the air gap was provided with a box, ie all edges were sealed.

Also we need to trapped the low frequency sound. Therefore the sealed
Air gap will give you better results.

[KenTripp]

Possibly accounts for the better performance down low but it's a pity they didn't also test it not sealed so you'd know for sure.

[aechmea]

The RMIT tests shown seem to have used a 250mm gap + 100 thickness of material = 350mm = 1/4 wavelength of 240Hz. So the results are consistent with normal broadband absorption numbers.

Sealed construction is not normally part of the broadband absorber scene. But if your were to proceed along the 'membrane' path then sealed enclosures are part of the deal as are membrane thickness and material and distance from wall and ... = another animal entirely.

[Membrane traps are quite different in design and construction. The fundamental principle for membrane traps is the natural frequency of vibration of their membrane and they are targeted at particular frequencies. There is a very precise formula in Everest and Pohlmann to calculate the effective frequency for membrane traps (also called diaphragm traps or panel traps). Gyprock sheeting with a gap and with insulation as a wall or ceiling is a membrane trap as Ken says. Then there are Helmholtz resonators and 'perforated panels' and 'polys' and .... ]

PS. OK the material is 50 thick rather than 100, but the results are still pretty close.

Edited by aechmea, 15 November 2011 - 10:12 AM.
PS

[aechmea]

Geez. I'll have to learn to type faster. 3 more responses by the time I replied to an ancient one. - anyway I'll leave it posted.

[Drizt]

It is actually the P50 which is 50mm thick.

Yeah, I want to stick to broadband absorbers and not go down the membrane trap path (more can go wrong).

The RMIT tests shown seem to have used a 250mm gap + 100 thickness of material = 350mm = 1/4 wavelength of 240Hz. So the results are consistent with normal broadband absorption numbers.

Sealed construction is not normally part of the broadband absorber scene. But if your were to proceed along the 'membrane' path then sealed enclosures are part of the deal as are membrane thickness and material and distance from wall and ... = another animal entirely.

[Membrane traps are quite different in design and construction. The fundamental principle for membrane traps is the natural frequency of vibration of their membrane and they are targeted at particular frequencies. There is a very precise formula in Everest and Pohlmann to calculate the effective frequency for membrane traps (also called diaphragm traps or panel traps). Gyprock sheeting with a gap and with insulation as a wall or ceiling is a membrane trap as Ken says. Then there are Helmholtz resonators and 'perforated panels' and 'polys' and .... ]

[Drizt]

Possibly accounts for the better performance down low but it's a pity they didn't also test it not sealed so you'd know for sure.

Very true, it would be great to know how much affect the enclosed box had. Basically it would tell me if I should go to the effort of extending my frame (or building it again from scratch) to accommodate the enclosed box.

[aechmea]

Just wondering if you expand on this a little and tell me what you would do with it?

I would do exactly what you plan to do - hang that ginormous 1200 x 2400 frame with the fluff, right in the middle of that blank wall and its not even in the way. Assuming that you get up to 50% coverage and that that wall is behind your head.

Somewhere I read that you are better off with more smaller traps adding to 50% but I sort of think that that would be splitting hairs and we all have larger fish to fry in our rooms. Currently I have about 50% coverage at the front and about 30% at the back. Most of the 50% is distributed towards the corners but not diagonally across the corners cos I am trying to kill some of the backwave off the Maggies on the front wall as well.

That is the setup I keep returning to, so maybe that is going to be as good as it gets for me. With planar speakers (= cancellation at 90 degrees), side walls aren't such an issue for me.

I have avoided cutting and building until I am absolutely convinced I have it about right.

I have a copy but have never got around to reading it. Lazy maybe

Hahaha. Chapter 12 is the one for all of these trapping techniques.

[Drizt]

Edit: yes that is behind the listening position.

Thank you for the reply mate. This has all been very informative. The massive frame will go just about the stair well railings you can see. I can't go any lower than that without impeding the stairs (people will hit their heads on the way up).

[Drizt]

Harvey just replied and unfortunately he hasn't tested sealed vs. unsealed. Bugger.

[Peter the Greek]

FYI - particle velocity at the walls/corners isn't exactly zero, because it moves along the surface. This is why superchunks do add some benefit as bass traps, though not as effective as a membrane (for bass trapping). From what I understand no one has really explained the phenomena mathematically.

Absorbers at reflection points aren't really designed/used for bass trapping...which I think someone said above

Dritz - what's the total cost of your panel project?

[Drizt]

I don't have all the costs on hand but give some ball park figures.

1 Pack of Polymax XHD in black (4 panels of 2400x1200x100) - $450
1 Panel of Megasorber P100 (2400x1200x100) - $200
30 odd meters of Trilogy One and other materials - $300
Wood, screws, glue, brackets etc. - $100

DIY enjoyment - priceless (That is if it works.... hehe.. if it doesn't well I will have learnt something).

[Peter the Greek]

Pretty reasonable considering a room full of off the shelf panels would set you back around $4k (I know its not the same, but still)

24x12 is huge!.........I need to make one this size for a ceiling mount but cant find a satisfactory fixing mechanism (only want a 50mm panel that sits flush)

[Drizt]

Yeah not bad $$$ wise compared to some of the commercial stuff I looked at. I couldn't bring myself to spend that kind of money.

For the roof I'm using multiple smaller panels.. 800mm x 1200mm x 100mm. Not 100% sure on how I am going to fix it to the roof, I'll cross the bridge when I get to it. But my initial thought was to use those white plastic plaster screws and then screw some hooks into them to let the material hang from it. I'll sure up the design later on.

[Drizt]

Grrr.. I'll probably try this bloody sealed air gap thing. I know if I don't I will always be wondering, "what if".

Do you guys think it would be worth adding some more 90mm wide pine to extend my existing frame or should I start again with 190mm wide pine instead of the 140mm pine I did use?

[Jenski]

Wifey thinks you should start again...

[Drizt]

Wifey thinks you should start again...

I guess I have my answer then.

Going backwards sux.

[svenr]

Drizt, sealing the air gap does not affect the function of this resistive absorber. Sealing the air volume is only required for a resonant panel absorber, where the enclosed air is to act as the compliance in a mass-compliance-resonator.

Go ahead as planned, and verify the effectiveness of the treatment by measurements. I suggest reading Toole's Sound Reproduction on where to place absorptive treatments and where to use diffusors. You should avoid killing all the early lateral reflections and get very low interaural cross-correlation / apparent source width. Also helpful is the specification for the ITU listening room - they tell you where to use what.

[Drizt]

Thank you for your response svenr.

I have learnt that as with everything in audio, nobody seems to agree on the best methodology for applying room treatments.

Go ahead as planned, and verify the effectiveness of the treatment by measurements.

Do you have a particular set of measurements in mind? Or just before and after at the listening position?

Edited by Drizt, 15 November 2011 - 04:03 PM.

[Drizt]

I'll be the guinea pig and test this out with some measurements. I'll build a sealed version and compare.

Anyone have some measurements they would like me to take comparing the two. Be descriptive with what measurements you want taken (where/how/why).

[LuckyDog]

Hi Drizt

I assume your are wanting to tame some flutter echo in your room. As others have indicated, the pressure trap you are now considering really is a targeted treatment strategy for low frequencies and would be a waste when incorporated into absorption panels at reflection points IMO. Such a pressure trap is best placed in the corners of the room.

To check the impact of your absorption, an energy time graph/RT60 measurement is a good one to use.

Good luck.

[Drizt]

Hi Phil,

Good to hear from you mate. It won't be that expensive to build another frame, so I'll give that a go, take some measurements and then we will all know if it is a waste of time/money or not

My failures may help others

[Paul Spencer]

Just to throw a spanner in the works ... what about diffusers over the stairs? You are lucky enough to have your back wall far enough away for it to work nicely! They could also look decorative. If I were you I'd try something before actually building something you hope to keep. For example, put a plank across and then put up a foam mattress, or one of your panels. Try it out!

For a simple midrange absorber, you don't need to mess around too much or worry about sealed rear chambers etc. The main thing is to decide if you are doing the right thing! ie do you like what it does?

You can measure these things and look at the energy time curve, that gets done in studio control room design, although the emphasis seems to be on reducing the early reflections, while here you will be influencing more the later ones.

[Drizt]

Alternatives could be tried at a later date if they are required. I have bought all the materials now so the only thing that can change is the implementation / location etc. Anything I try I will have to build anyway's, basically anything I do is going to be a case of 'suck it and see'.

I don't have a foam mattress to try it out I have tried that in the past and found they aren't great (didn't make much difference) and aren't really a good indication for what a real absorber would achieve.

This is all a learning experience. As everyone can see it is hard to find anything in audio that everyone agree's on. It seems learning by doing is the only real way to go.

It is only time and money to lose right.... (The CFO is going to get cranky at that )

[dastrix]

Harvey just replied and unfortunately he hasn't tested sealed vs. unsealed. Bugger.

He also doesnt have flow resistancy figures for his stuff