Subsonic Filter needed?

McLoki

I just purchased a new (to me) SVS 25-31 cs+ from Mazeroth and had a question about a subsonic filter - basically, do I need one?

I understand what a subsonic filter does but I dont understand why it is neccesary. When you start playing tones below the port frequency I understand that the woofer unloads and the speaker no longer has the backpressure needed to help control the woofer.

My question comes in as to why this "unloading" would be any different than an Infinitely Baffled Subwoofer. Shouldn't the amp be in control of the woofer in both directions and not really NEED the spring of backpressure?

My amp will provide plenty of power (500watts/channel) and current (120 amps/channel - peak) to control the woofer will I still need a subsonic filter? What harm will happen if I don't get one? (woofer bottoming - flames shooting out of my amp - etc.)

If I do need one, do any of you have any leads for where to get one inexpensively? (I was planning on getting a parametric EQ (feedback destroyer) but I don't think it has a subsonic filter either.)

Thanks,

Michael

gatemplin

If you take a look at an excusion plot for a vented sub (I dont know how to post one) the excusion is low at high frequencies, goes up before the tuning point, drops at the the tune, and skyrockets below the tuning point. It takes very little power to reach max excursion below the tuning point.

An IB is not unloaded though, the cone is on its own. In a ported box, below the tune, the port and driver are out of acoustic phase and it will easily bottom.

If the sub bottoms, it usually sounds like a loud clapping noise when the former hits the back plate. That can warp the former and cause it to rub on the pole piece. (dont ask me how I know;) )

You can buy SS filters, www.partsexpress.com will probably have them, so does http://svsubwoofers.com/marchandsubsonic.htm

Another option is buying a plate amp with a SS filter built in. The BFD is a good idea but cant be used as a SS filter in this situation. Also, Adire may be able to custom build you a ADA plate amp with whatever SS filter you want, it already has a PEQ built in. www.adireaudio.com

Dr. Spec

Actually, the correct term is a high pass filter. If it engages below the threshold of hearing, it could be characterized as an infrasonic HPF.

An IB system has no enclosure at all, nor does it employ a vent (Helmholtz resonator) which would damp cone motion at the resonance point. As such, an IB (like a sealed subwoofer) will exhibit a progressively greater excursion to reproduce progressively deeper frequencies.

The difference between the IB and the sealed is that the IB lacks an air spring and will respond to the deepest frequencies with very little amp power. The IB is typically limited by mechanical excursion limits, rather than thermal power handling. If you want more volume/power with an IB, you simply add woofers as needed. A larger amp is rarely required.

The IB system typically involves larger and multiple woofers (twin 15" is usually the minimum). But yes, the IB can be bottomed, just like a vented enclosure can be bottomed below the tune point. An IB system I helped design and build was recently bottomed on Master & Commander. I recommended the install of a 15 Hz HPF.

Because the vented enclosure will exhibit quite a bit more output (than an IB) for a given driver size and driver number, people can get away with say a single 12" woofer. But when you are pushing that woofer hard, the potential to bottom it becomes all that much greater below the tune point as compared to an IB system with comparable overall output.

Installation of a HPF on the CS+ will conserve amp power and help protect the woofer from bottoming. If you never push it that hard, you probably won't need it. If you push 9/10 or higher, then I suggest one.

gatemplin

I found one. See below the tune, you are safe for a while but dont push it too much. Like Doc said, if you dont push it you should be safe. Personally I would use an SS filter.

This isnt the SVS obviously but all non-eq'd plots look similar.

McLoki

My mistake on the IB sub. So what you are saying is below the tuning point, rather than negative pressure like a sealed sub would produce, the sub starts running out of phase with the port creating positive pressure to push the sub further (beyond xmax).

It makes much more sense now. I would prefer not to spend the extra $200 on a SS filter right now. (I will have to wait to see if I get more benefit from that or the BFD)

It sounds like at least initially that I will be ok provided I don't go nuts driving the woofer. Am I correct in assuming that the 20hz tune would provide more protection (lowering the tuning port) or will it really not be that much difference?

Thanks for all your great descriptions of what I will be facing.

One other thing, will I be ok just turning down the volume if I hear the sub bottoming or will it be to late at that point and I will be out a woofer?

gatemplin

It sounds like at least initially that I will be ok provided I don't go nuts driving the woofer. Am I correct in assuming that the 20hz tune would provide more protection (lowering the tuning port) or will it really not be that much difference?

You are fine if you dont push it. By lowering the tuning frequency you decrease the possibility of bottoming below the tune, but increase the risk of bottoming out above, see the first excursion hump on the graph. If you push it too hard either way it will bottom.

My mistake on the IB sub. So what you are saying is below the tuning point, rather than negative pressure like a sealed sub would produce, the sub starts running out of phase with the port creating positive pressure to push the sub further (beyond xmax).

Yes, that is how I understand it.

Usually, you will hear the sub straining before you bottom it out.