I don't quite follow you... the SD of the driver is 506 sq. cm., are you talking about measuring the cu. cm. so that I have enough open cubic centimeters under the driver to allow for all the air it can push? I was hoping to put the Parts Express cone spikes with the little disks for a hardwood floor on it, and they would keep the driver about 1.75 inches from the floor. I guess I just don't quite follow you.
To get really picky, assuming your driver is mounted externally, the edge of the surround is usually the closest thing to the floor, and that's where the flow 'choke point' occurs. And you have to allow for the surround to extend during max cone excursion 
Let's work one out with some generic numbers for a 12" woofer.
Lets say the diameter from one peak of the surround roll to the other is 10.5", and approximate Sd as (10.5 / 2) squared x 3.14 = 86.6 square inches. So you need a minimum of 86.6 square inches in the 10.5" diameter 'ring' between the surround and the floor, a ring that has a circumference of 10.5 x 3.14 = 40 inches. 86.6 / 40 = 2.16 inches minimum distance between the surround and the floor -- at rest. But you need to add in something for when the surround is extended outward -- use 1/2 one-way Xmax as an approximation, we'll use 0.5", so 2.16 + 0.5 = 2.65 inches. And that's from the surround, which may extend (at rest) 0.5" or more from the face of the cabinet. let's say 0.75" for a big roll 12", so 2.65 + 0.75 = 3.4" between the bottom of the cabinet and the floor.
Let's work one out with some generic numbers for a 12" woofer.
Lets say the diameter from one peak of the surround roll to the other is 10.5", and approximate Sd as (10.5 / 2) squared x 3.14 = 86.6 square inches. So you need a minimum of 86.6 square inches in the 10.5" diameter 'ring' between the surround and the floor, a ring that has a circumference of 10.5 x 3.14 = 40 inches. 86.6 / 40 = 2.16 inches minimum distance between the surround and the floor -- at rest. But you need to add in something for when the surround is extended outward -- use 1/2 one-way Xmax as an approximation, we'll use 0.5", so 2.16 + 0.5 = 2.65 inches. And that's from the surround, which may extend (at rest) 0.5" or more from the face of the cabinet. let's say 0.75" for a big roll 12", so 2.65 + 0.75 = 3.4" between the bottom of the cabinet and the floor.
Some sub designs purposely limit this space; A.K.A "slot loading". It has the benefit of lowering Fs at the cost of efficiency and standing waves.
The Acoustic Research ARS115PS is a cheap example of this. It has the driver and port firing down and out a slot on each side of the box. Sounds like c**p.
The Acoustic Research ARS115PS is a cheap example of this. It has the driver and port firing down and out a slot on each side of the box. Sounds like c**p.
An externally hosted image should be here but it was not working when we last tested it.
... of course if you get the gap 'tween floor and woofer small enough, the whole box can be floated up on the acoustic wave ... just like those big machine shop tools that are moved around on air pressure ...
I believe there was a thread about someone in Europe complaining about their woofer box "jumping up" off the floor under certain conditions = gap too small ...
I believe there was a thread about someone in Europe complaining about their woofer box "jumping up" off the floor under certain conditions = gap too small ...
Hey Jan...
at the frequencies involved here I don't imaagine that there would be much of an issue but one can experiment easily listening at different levels to see what they prefer. The hight suggested was for a reasonably safe minimum as it sounded as it the owner wanted to keep a low profile.
Personally I would not fire a dynamic woofer up or down as without a DC centering bias the cone is going to sag in time that is unless we are talking HT and auto drivers with massive suspensions.
at the frequencies involved here I don't imaagine that there would be much of an issue but one can experiment easily listening at different levels to see what they prefer. The hight suggested was for a reasonably safe minimum as it sounded as it the owner wanted to keep a low profile.
Personally I would not fire a dynamic woofer up or down as without a DC centering bias the cone is going to sag in time that is unless we are talking HT and auto drivers with massive suspensions.
Re: Hey Jan...
Interesting point - is there some "simple" math involved to determine the optimal bias, and how about compensating for long term drift in compliance?
I've heard musings that the mechanical suspension never stops changing (until in the case of old style foam, the surround rots out entirely)
all in all, it's probably simpler to just vertically mount the driver?
moray james said:
Interesting point - is there some "simple" math involved to determine the optimal bias, and how about compensating for long term drift in compliance?
I've heard musings that the mechanical suspension never stops changing (until in the case of old style foam, the surround rots out entirely)
all in all, it's probably simpler to just vertically mount the driver?
" ... it's probably simpler to just vertically mount the driver? ..."
If you have two subs, then yes, a vertical mount might be considered.
For my money and listening pleasure, I prefer the horizontal mount in the bottom of a good sized box, aimed down with a nice gap of greater than several inches between the box bottom and the floor ... the sound does disperse 360 degrees from the box and (because it is all low freq bass) the sound does not seem to come from a "point source", thus In My OP there is no need for two sub woofer boxes ... (Of interest, see the subs at GR-Research.com = vertical mounted, yet aimed to the side rather than straight ahead !)
If you have two subs, then yes, a vertical mount might be considered.
For my money and listening pleasure, I prefer the horizontal mount in the bottom of a good sized box, aimed down with a nice gap of greater than several inches between the box bottom and the floor ... the sound does disperse 360 degrees from the box and (because it is all low freq bass) the sound does not seem to come from a "point source", thus In My OP there is no need for two sub woofer boxes ... (Of interest, see the subs at GR-Research.com = vertical mounted, yet aimed to the side rather than straight ahead !)
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