OK, now I am happier as I did my duty and the back chambers are ready to be attached to the horns and to be lifted up. Yesterday I spent 5 hours trying to build the back chamber displacement that Jessie and my carpenter recommended me. I bought the 2 large sheets of 2” foam, cutting it on table saw and by foam cutter. I was able to make a fine displacement but then I asked myself how effective was it. If you remember when I was experimenting with resonant frequency in basement I was taping the entire back of the horn and it was where I was able to get the rise of the resonant frequency that I was needed. The displacements that I built allowed a bit more air around the driver. I measured resonant frequency in the driver with and without displacement. It was 4Hz difference - obviously not enough. I re-did the displacement to make it as tight as I could but surprisingly it did not affect the resonant frequency rise.

I was in near panic mode as it was 6PM and I have no solution to finish my back chambers and I have only a day left.  Probably if I had softer foam then it would be possible to press the foam to the driver but I had relatively hard pink Owens Corning phenoplast. I decided that there is no way to do it properly without going directly to the driver.

At that time I learn the spay foams that I use relatively well – I knew how it expends and how it cures. So, I covered the basket vents with very thick fabric, the driver with masking tape and begin to apply the foam directly to the driver. This time I did it one thin layer of foam a time with 40 minutes delay between the layers. Between minute 5 and 10 of foam curing I used hard surfaces to punch foam down in order it to fill all inner gaps.  I did it pretty my up to 5AM, with both horns and it looks as it worked very well. The foam built a true sarcophagus around the drivers, converting my 15-40 to a true compression driver. The resonant frequency on the driver in sarcophagus is 67Hz – much heir rise than it was with phenoplast displacement (54Hz). To remove the driver for this type of displacement will be a bit more difficult but not impossible – foam is very easy to cut with knife. My only fear was the foam might push the fabric out and touch the driver cone. I made some precisions, like let the layers that fill the fabric to dry longer   and do not puck them down to hard.  I still have no 100% assurance that it did not happen. I hope not as the distance from the basket window to the diaphragms is large. Also, I did the drivers sweep and Fs measurements.  The sweeps is clear and the Fs are identical. I presume if one of the cones would be touched by foam then it would impact resonant frequency very dramatically.

Now some considerations.  I do not share the Scooter’s and jessie’s fear about the toxicity of petro-based products. Yes, petro-based products are not good near driver but I do not think that I spayed driver with gasoline. I did a test and confirm that foam doe not penetrate the fabric I used. Foam does not make it soggy or wet. I used fabric from think tarp. Foam does not have fume or gas and when it cures it absolutely inert to any organic things. It is not to mention that the cone was under the foam (in case of fume) and separated with very thick fabric. So, I do not think that any damage or impact was done to driver. I think that sitting form years in weather non-stabilized storages might make more damage to it. Of course you never know, but I do not think that 10 min proximity to uncured polyurethane foam did anything to the driver diaphragm.

Anyhow, now the back chambers are done and the 40 bags of sand (300 pounds) are prepared to be loaded to the horn’s top. We are back on the schedule. When the horns will be installed I will fine-tine the Fs by pocking the small holes in the foam.

guy sergeant wrote:

Can you remind us how & why you arrived at the resonance frequency you have set for these drivers in this particular horn?

67Hz is the resonance frequency of the complete back chamber and driver loaded ONLY to the first section. What it will be load to the rest 6 sections then the new mass of the diaphragm will be much higher and the resonance frequency will be much lower, hopefully around 42Hz but I will fine tune it later. The rest is here:

http://www.GoodSoundClub.com/TreeItem.aspx?PostID=14341

Romy the Cat wrote:

Anyhow, now the back chambers are done and the 40 bags of sand (300 pounds) are prepared to be loaded to the horn’s top. We are back on the schedule. When the horns will be installed I will fine-tine the Fs by pocking the small holes in the foam.

Romy the Cat wrote:

OK, now I am happier as I did my duty and the back chambers are ready to be attached to the horns and to be lifted up. Yesterday I spent 5 hours trying to build the back chamber displacement that Jessie and my carpenter recommended me. I bought the 2 large sheets of 2” foam, cutting it on table saw and by foam cutter. I was able to make a fine displacement but then I asked myself how effective was it. If you remember when I was experimenting with resonant frequency in basement I was taping the entire back of the horn and it was where I was able to get the rise of the resonant frequency that I was needed. The displacements that I built allowed a bit more air around the driver. I measured resonant frequency in the driver with and without displacement. It was 4Hz difference - obviously not enough. I re-did the displacement to make it as tight as I could but surprisingly it did not affect the resonant frequency rise. I was in near panic mode as it was 6PM and I have no solution to finish my back chambers and I have only a day left.  Probably if I had softer foam then it would be possible to press the foam to the driver but I had relatively hard pink Owens Corning phenoplast. I decided that there is no way to do it properly without going directly to the driver. At that time I learn the spay foams that I use relatively well – I knew how it expends and how it cures. So, I covered the basket vents with very thick fabric, the driver with masking tape and begin to apply the foam directly to the driver. This time I did it one thin layer of foam a time with 40 minutes delay between the layers. Between minute 5 and 10 of foam curing I used hard surfaces to punch foam down in order it to fill all inner gaps.  I did it pretty my up to 5AM, with both horns and it looks as it worked very well. The foam built a true sarcophagus around the drivers, converting my 15-40 to a true compression driver.

I have been following your mid-bass horn project with a mixture of fascination and horror for a little while and am awed by the remarkable amount of creative thinking that has gone into it.

I am sure there is no need to worry but I would hate for you to do some really serious damage to the drivers you are using. The 'vents' at the back of the Vitavox drivers are presumably there to ensure there is some amount of convection present to cool down the coil as 95% of the power used by the driver tends to be transformed to heat. I may be talking out of ignorance but I would be concerned with the 'sarcophagus' method you have employed to reduce rear chamber volume to virtually nil.

Also, when cooling by convection cannot take place, the heat will be transferred to the rest of the unit by conduction so that that chassis temperature
might rise considerably and I do not know enough to comment on the effect of high temperatures on the fabric and masking tape you are using.

In any case, operation of the coils at high temperatures, if present may well adversely affect the performance of the drivers when they are called into use for extended durations depending on volume and musical content.

oxric wrote:

Hi Romy: I have been following your mid-bass horn project with a mixture of fascination and horror for a little while and am awed by the remarkable amount of creative thinking that has gone into it. I am sure there is no need to worry but I would hate for you to do some really serious damage to the drivers you are using. The 'vents' at the back of the Vitavox drivers are presumably there to ensure there is some amount of convection present to cool down the coil as 95% of the power used by the driver tends to be transformed to heat. I may be talking out of ignorance but I would be concerned with the 'sarcophagus' method you have employed to reduce rear chamber volume to virtually nil. Also, when cooling by convection cannot take place, the heat will be transferred to the rest of the unit by conduction so that that chassis temperature might rise considerably and I do not know enough to comment on the effect of high temperatures on the fabric and masking tape you are using. In any case, operation of the coils at high temperatures, if present may well adversely affect the performance of the drivers when they are called into use for extended durations depending on volume and musical content. Good luck (you may need it) Rakesh

I agree that sarcophagusing the driver into the foam cocoon I reduced the capacity of the driver to dissipate heat. I would not say that I did it deliberately –I just did not think about it and it was the only way to do the back chamber within the time frame. Still, analyzing the problem now I would bring to the table the following arguments.

1)  Putting the driver into foam reduced or eliminate the exposure of the driver to attic heat (very hot on summer) and cold

2)  Considering the very high efficiently of the channel the driver will use very low power (probably no more than 3W) and with this power the heating up of the VC will not be a subject at all.

3)  The horn has a large front chamber that will be able to cool down the driver

4)  The Vitavox driver use Alnicko magnet that has higher temperature resistance then other magnets

(table 5.2 at http://www.magnetsales.com/Design/DesignG_frames/DesignG_2.htm)

So, I think, or more precisely to say hope, that the het from VC would not be a problem. On summer the attic is so hot that you can cook pizza in there. The VC would generate this heat long after the VC will be melted. So not forget that this is very rare and very low power driver with no cone excursion. The driver has name Vitavox 15/40, 15 is impedance, 40 is max power that the driver can handle. So, considering very low power that I will feed the driver I think the VC’s hear will not be a subject.

Romy the Cat wrote:

Ric, I will not be using sand for decoupling. In fact I fill that sand is not good load bearing decoupler as I will have more or less point loading and sand will not work. I will be using sand for damping the horn surface, primary atop of the horn. As a damping material I fill sand has no rivalry.

Romy the Cat wrote:

My carpenter proposes silicon but it not good for painting. Is anybody have any interesting idea how to separate horn and drywall?