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Romy the Cat wrote:

Last night Levine and BSO played a phenomenal Mahler 5, truly great play.  I had some issues with BSO sound and I probably will bring it up to them. Tonight I decided to listen the M5 broadcast for the first time and I realized that my bass is gone. I mean it became not as interesting as I remember it was 3-4 days ago when I was listening it last time attentively. I have no time to look into it now but something is not right….

As I predicted the Vitavox drivers, as they start working after years of sitting in the box, got softer and the resonance frequency dropped. Sine I recognized that my midbass lost its quality very aggressively I anticipated that the resonance dropped in 20s. I was wrong. Measuring resonance not I got 37.2Hz. That is very interesting as I did not expect the sound go so week with juts 5Hz off the mark.

I decided to move resonance back to 42Hz stopping it to work with open back chamber. Here I had another surprise – the driver was practically did not response to the presence of the back chamber cover. Before it moved resonance for 12Hz but now the driver was so soft that presence of the back chamber lid it recognized as almost open air! Funny but now I need to displace the back chamber again.

I took dense, non-compressible foam and fill ~ 30% and then 50% of the space in the back chamber with back chamber sealed from outside.  The resonance went to 38.6Hz. Not a lot I figured. However, the Sound got surprisingly better. Paying more with it I was able to drive the resonance to 39.4Hz and this is how I finally closed the back chamber for now. The sound is nothing like the last week, the sound is very good but it become good only if I set resonance over 38Hz -38.5Hz. I have no explanation for this phenomena and I did not expect that the 1-2Hz will do such a huge difference. I was thinking and thinking about it and then I got bored from it. If for whatever reason my horn and my driver want to have resonance over then I will give it to them and let them choke from their sick pleasure.

Anyhow, I have my midbass back and get some idea how my driver behaves as it ages. The moral of this story is that if you use vintage drivers with paper suspension in your horns and if you demand high level of operation and performance then let your vintage drivers to burn-in a lot and only then lock the operation of your horn.

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