How far can the driver of a down-firing sub be from the floor?

[The Paulinator]

Just far enough that it doesn't hit the floor? Or are there some issues with air flow if you get too close?

[moray james]

take the Sd of the driver and adjust the open space under the driver all the way around and match that to equal the driver's Sd. More open space is fine less will choke the driver.

[The Paulinator]

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.

[moray james]

that your driver has an Sd of40 sq in. that means that if your box was 10 inches square you would have to lift it off the floor one inch for a total of ten sq inches per side. This would be a reasonable minimum distance.

[ggunnell]

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.

[janneman]

Originally posted by moray james
that your driver has an Sd of40 sq in. that means that if your box was 10 inches square you would have to lift it off the floor one inch for a total of ten sq inches per side. This would be a reasonable minimum distance.

Moray,

Is there no issue with standing waves or resonance in the 'pipe' formed by the relatively narrow opening to the outside world?

Jan Didden

[theAnonymous1]

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.

[FastEddy]

... 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 ...

[moray james]

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.

[Cal Weldon]

ggunnell,

I like your explanation.

Except the last line where you have added the extra surround extension. Should that not be 2.91"? You added the .5" to the .75"