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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.

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