 rowuk wrote: |
Impossible. A passive radiator can NOT change its tuning frequency on the fly. It is a Helmholz resonator - one single tuning frequency. It is also NEVER in phase, it is always late as it takes several cycles to get resonant. It is possible that at the tuning frequency and its multiples (55Hz, 110Hz, 165Hz), that it adds a maximum to the pressure. If the Q is low enough, it will resonate at its tuning frequency even if exposed to frequencies slightly higher or lower. THis is one of the reasons for lower pitch definition in resonant systems (port/PR/transmission line.
I agree with Paul about vintge spekers sounding best when the front and rear are equally loaded - like with an infinite baffle (box bigger than VAS). |
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When I said it "locks in" and is in phase I do mean it with all this in mind. I'm not trying to redefine what a PR is. It defiantly is always late. But there is a point where it is not so late that it does not decouple the active diver. That was the point I was referring to at 55hz or so.
| rowuk wrote: | |
If the passive is tuned too low (which is the case here), the box behaves more like an infinite baffle - no LF increase but relief of box pressure. |
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Sounds pretty-much like the situation to me. It is not LF increase but wide band dampening.
If what u say about the equal loading of the front and back of the Tannoy is true, then Dunnoy is probably benefiting from this.
I listened again last night to the Dunnoys. I haven't had them running fro a while. Maybe a few months. One thing I observed when listing was - It's like they are not just generating pressure but also suction. They do the front of the note so well, but they also do the back of the note so extremely well. It's like they can hold negative space at the back of the note for how ever long they want. It's not just negative space though, It's like a cascading, shrinking spiral of space that as it shrinks and diminishes in energy it still fizzes and fires off energy as it goes. It made me think of Viktor Schauberger's work on suction.
N-Set, I invite you to make your own recording of the phase of your Dunnoy. I used my phone camera to do it and a free online sweep generator. I had Tannoy, Scanspeak and laptop screen all in shot when I filmed and shot from as side on as possible. I didn't observe another phase flip like you mention below 20hz. Definitely out of phase from 1hz to 20hz.
If you do film your Dunnoy you will get insight into the conundrum of integrating more bass. I'll give you an example. For the sake of this argument let's pick a design frequency of say 40hz to crossover. When I watched my footage I observed the phase drift start around 55hz. Im calling from 55hz and up, in-phase and from 55hz down out-of-phase. It's not technically in phase yes, also it not just suddenly out of phase either, and for that matter it doesn't start at 55hz but there is an observable shift around there, both visually and acoustically enough for me to choose that frequency for this argument.
So at 40 hz what driver do you want to interface with? That is which driver do you want to time align with? If we choose the tannoy then we will be out of phase with the PR. If we choose the PR we will be out of phase with the Tannoy. As we go lower and the difference of phase between active and passive radiators increases, the issue gets worse. Which ever way you cut it, if you add another driver to your room delivering sub 55hz energy, it will be out of phase with Dunnoy.
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