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(Very) near field sub-woofer

andrew

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#1
I'm tinkering with the idea of setting up a near-field sub-woofer behind the (single seat) listening position. The context is that in-room measurements that I've done with the microphone being 10 to 30 cm from driver has all shown near flat results - so I'm reasoning that this would be a simple method to getting near flat response and, at same time, reducing distortion as a result of running the sub at a lower volume. It seems as if the downsides are the odd looking arrangement of sub right behind the seat and potential for localisation - although the latter would seem manageable given that I'll be using a digital X/O at 60 to 80 Hz. Has anyone got any real-world experience with such a set-up? Or, accepting that this set-up won't work for a lot of scenarios, am I missing anything about it's application for a single set set-up?
 

digitalfrost

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#3
I'm listening in the near field and experimented with subwoofers. I was never happy until the subs were on the desk, next to the satellites. Even under the desk was locatable for me, and I doubt you'll be happy with a setup that puts the sub behind you.

Crossover topology makes a big difference, you want at least 24db/oct LR or possibly more. And you probably want a crossover frequency below 80hz...
 

DonH56

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#6
Many folk do it to get more tactile feel from their sub. I have not expect briefly and it does do that. The rule of thumb in general is to keep anything in front half the diameter of the cone away but many pix I have seen violate that...
 

andrew

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#7
The rule of thumb in general is to keep anything in front half the diameter of the cone away but many pix I have seen violate that...
What does this mean? Is it that, if one has a 15"driver, then there should be 7.5"free space in front of the driver? If so - and presumably the reason that you mentioned it - is that this means that a smaller sub might be better for this purpose?
 

DonH56

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#9
What does this mean? Is it that, if one has a 15"driver, then there should be 7.5"free space in front of the driver? If so - and presumably the reason that you mentioned it - is that this means that a smaller sub might be better for this purpose?
This is outside my area of expertise (as if I had one ;) ). But, based upon some exchanges with folk who do know what they are talking about, and a foggy memory of a grad acoustics class or two decades ago, the rule primarily relates to waveform coalescence and reduction of (very) near-field compression from the boundary. Reading between the technical babbling my guess is that it does not matter for a sub used for tactile response, and I suspect how much it matters even for normal propagation is questionable for a subwoofer. I think you'd want enough to give the cone to move without fear of striking anything plus a local air buffer region, maybe a few inches, but probably do not need more than that.

But we really need someone who actually knows to chime in; I'm too tired to dig up my old text and try to figure out the impact on the driver (cone) from a boundary so close.

I would not use a smaller sub, especially for tactical response, though they do make physical shakers (e.g. ButtKickers) for that.
 

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