Active EQ and Cone Excursion

[david teltschik]

I recently bought an NS15-992-4A woofer from Aurasound because I loved the sound of it's sibling in the MDesign's Godfather subwoofer. If you have a chance to check that sub out....i would so go do it. It was rather impressive. The driver specs have been attached below. I am currently trying to decide what allignment to use and I"m torn between the vented/sealed allignments. The vented allignments require massive vents which makes the boxes a bit awkward. Not that I don't mind large subs...my current one is about 8ft3, but I can't seem to make the dimension look good. This has brought me to consider a sealed allignment, which is what the Godfather subwoofer uses anyway.

My question is this....can anyone tell me how adding EQ to driver effects it's excusrions plots? I did some reading last night in all my audio books, and could only find a paragraph in passing about it, but was hoping to get more details. What equations govern the increase/modification of the cone excurions with additional EQ?

I also read that adding EQ changes the phase of the response rather dramatically as well......how does this affect the sound of a driver in practical terms? Can this me mediated by setting the subwoofer to a nearer position in the processor?

Just FYI...I will be using an Adire ADA1200 amp to drive the system.

[ThomasW]

The primary differences between sealed and vented is that with vented you get some free boost that the tuning frequency, and they require slightly less power.

The Adire plate amp has a built-in LT circuit and plenty of power. I recommend a sealed box with a Qtc of 0.5 (aka critically damped) then use the LT circuit to boost the bottom end. LT circuits make for better sounding boost as compared to simple parametric EQ.

You won't know whats really happening until you build a box and measure it's performance in your room. Trying to hedge your bets with modeling programs is just like 'bench racing' cars designs, doesn't really tell how things work on the track. So build a test box and see how it performs.

[Davey]

David,

As you probably guessed there's no free lunch with equalization. If you want to extend the response lower you'll need more excursion....which means more power. (This is all assuming that your driver is capable.)
Sealed box systems roll-off with an approximate 12db/octave slope so (as an example) if you want to extend your response one octave lower you'll need 12db of EQ boost, and the excursion/power requirement increases by a factor of 4 to achieve this.

The driver you mention would require a box of approximately 8 cubic feet for Qtc=0.5. You can make the box smaller but the Qtc will rise and so will the power requirment. You seem to have plenty of power available so maybe this is an option?

Read the Thor pages on the Linkwitz website for an excellent description of your future design work.

http://www.linkwitzlab.com/thor-intro.htm

I'd say your first order of business is to decide how big a box you can tolerate/build.

Cheers,

Davey.

[Dennis H]

What Davey said. The SPL of a sealed box at a certain frequency, say 20 Hz, only depends on how far the cone moves. A smaller box will take more amp power to get the same cone movement at 20 Hz but the max SPL is the same as a bigger box. Max SPL is determined by the Xmax of the driver. The size of the amp you need is determined by the box size which tells you how much EQ you need. A 3 dB EQ boost needs twice the amp power rating.