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I was listening recently my Carl Schuricht’s Bruckner and I again noted what I am been noting for a while. I feel like my Midbass horns begin to sound a bit saggy. It is not too but and the degree of change is very minor. It is not even auditable at normal dally listening but if to observe the composite experience from multiple week then I say that the effect is there. Being me, if I acknowledge the problem then I do pay attention to the problem with much attention. Yes, I do feel that there was some very gradual change in my Midbass sound for the last 2 month or so.

Thinking what would it be I can propose two factors:

1)    My new Vitavox 15” drivers that were sitting in box for 60 years finally got broken-in completely.

2)    The high temperature at attic has softened the suspension of the drivers.

In both cases most likely the resonance frequency dropped for a few Hz that lead to a very mild sagginess of my midbass sound. I did not measure the resonance frequency nowadays, probably I shall… Most likely two processes worked together and second one strained the first on.

I have a remote humidity and temperature sensor sitting atop of the midbass horns back chambers. At the hottest day of this year so far with temperature of 94F I have at my attic 121F with attic venting off.  My attic does not have venting roof ridge and use side venting. I installed a powerful exhausting fan with thermostat, rated for twice size of my attic.  It cool attic very well but I did not note any improvement in sound since the exhausting fan went up. Indeed the difference is VERY mild…

What it boils down is that I think I need to revise the size of my back chamber in respect to new lower resonance frequency. It will take a little filling of the back chamber to drive the Fs up back to 42Hz. If I recalibrate the thing for summer, let say for 75F then during the winter I might put in the back change a very small heating element with thermostat and to stabilize the temperature. I still am contemplating it and not so egger to proceed. I like what I have now – the back change is empery as it would allow me to rotate the driver. The vintage 15-inshers with paper suspension need to be rotated 180/45 degree once a couple years if they are mounted vertically.

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