Why don't ported subs blow cones on low freqs?

[Gregory]

I have a gap in my knowledge, and whilst I can make up a credible answer. I'd love to know if I am correct.

Fully sealed subs protect against excessive excursion of the driver by using a physical high-pass filter - the air behind the sub acts as a spring.

Infinite baffle speakers have no such high-pass filter naturally, so use multiple drivers/large cones to protect against excessive displacement (and to stay linear).

My confusion comes with ported subs. AFAIK, the port re-enforces the sound pressure around the tuning frequency whilst also acting as a high-pass filter at the same frequency, so protecting the driver down to that range. However, below that frequency it feels like the high-pass goes away, so there is no restraint on the driver. What confuses me is why that doesn't allow massive driver displacement like an IB.

I can come up with :-
1) There's no power in frequencies low enough for this to be an issue
2) The driver isn't really unloaded enough until much much lower frequencies
3) The amps used have an electronic high-pass filter. Initially the port will keep response flat-ish, but when you get lower, the high-pass will be cutting the power a lot and so protecting the driver

The reason it all becomes interesting is that I am trying to think why an IB sub should need more than twice the piston volume of a ported sub. Twice allows for the port re-enforcement. Protection beyond that could be part of the normal electronic EQ process rather than just 'natural'.

Greg

[wookie]

[FONT="Arial Black"]number 3[/FONT]

[Gregory]

Soooo,

with electronic EQ protection, total driver piston volume shouldn't need be more than about twice the volume required for a ported design. But, convention for IB's is to have much ,much more piston volume than this (like quad 12" drivers or more where something like a Monolith uses a single 12" driver). I'm sure I'm still missing something!

Thoughts?

[IronGiant]

My home made ported effort doesn't have any in built EQ, does the Monolith?

Dave

[Nimby]

An IB has almost zero enclosure compression. This is also its Achilles heel. It needs lots of cone area and longthrow drivers to cope with high levels and low frequencies. Despite this it suffers a slow (typical closed box) LF roll off. It tolerates considerable bass boost to achieve a reasonably flat frequency response down into the single digits of Hz at remarkably high levels.

Any AV or stereo hifi system has a collection of roll-off slopes from each of its constituent components. These are all additive having the effect of a high pass filter. This helps to protect reflex enclosures to some degree even if a separate high pass filter is not used. The driver suspension and spider are also a restoring force with increasing excursion. A rising impedance below system resonance also helps. (the double impedance hump of the reflex design)

I remember the cones on my old Tangent TM1 reflex speakers moving to record warp on a Japanese amp. They suffered terribly from all sorts of rumble. This was much reduced by changing to an A&R A60 which had a higher roll off point. Happy days!

[Gregory]

Originally Posted by Nimby

It tolerates considerable bass boost to achieve a reasonably flat frequency response down into the single digits of Hz at remarkably high levels.

Ah - that might explain. If the bass is not only not cut, but is actually boosted, then I see why the piston volume is required over a ported design. Happily, I have also worked out how to get a lot more driver area under the floor, using the bedroom underneath, so a proper IB is back on the cards. And, my wife thinks its a good idea Moving from 1 10 inch XLS10 driver to 2 or 4 12" drivers of some sort should help enormously with piston volume (about 6 times more I think)!

Greg

[kstrain]

Originally Posted by Gregory

What confuses me is why that doesn't allow massive driver displacement like an IB.

I can come up with :-
1) There's no power in frequencies low enough for this to be an issue
2) The driver isn't really unloaded enough until much much lower frequencies
3) The amps used have an electronic high-pass filter. Initially the port will keep response flat-ish, but when you get lower, the high-pass will be cutting the power a lot and so protecting the driver
Greg

I'm not sure that there is a general answer to this question:-all of your points can be true in one or other example, but are certainly not always the case. It is very likely that a different driver will be chosen for the 3 enclosure designs (even if the same diameter and similar excursion, the cone mass, Qts, resonant frequency are likely to be different).

I suspect too that expectation plays a part: who designs an IB without making it go lower than can be achieved in a much smaller box? Even though the IB is most efficient (sound output for electrical input at low frequency) the cones still have to move that much more. At low enough frequency it should take more excursion-limited ported drives than excursion-limited IB drives to reach a given level in a given room (assuming they have the same area and excursion limit).

Ken

[Gregory]

The expectation/design point is a fair one, but doesn't explain why the drivers don't blow on ported designs unless there is a high-pass filter somewhere to protect them. That isn't expectations, it's just physics - as I see it if the low frequency power was there then the lack of physical constraint at low frequencies would cause a similar excursion you'd get in an IB.

Greg

[Nimby]

Gregory

Don't waste your time (or your drivers) on a couple of 12" for an IB.

Thy will just hit the endstops and be destroyed on the first LF Effect at any reasonable output level. There is no air resistance at low frequencies in an IB to limit cone movement. 4 x normal 15" with 15-18mm Xmax is good starting point. Or (perhaps) two very longthrow (Xmax of 25mm+) 15" American drivers used with care.

It isn't just coincidence that you see these numbers and large sizes of IB drivers. Read through the FAQs and look at the galleries on The Cult of the Infinitely Baffled specialist forum. Click on my signature to go straight to the IB gallery. Note the lack of single (or even pairs) of 12" drivers?

8 x 12" is more like it to protect the drivers from overexcursion. The area of a driver increases as the square of the radius. Greater Xmax is a linear increase as is doubling the number of drivers.

Do the maths or look at the recommended driver list on the IB Cult. Aim for 8 liters (or more) of total displacement. That's not easy or cheap with 12" drivers. Build yourself a box sub and forget about IBs until the funds are available to do it properly. You will need a powerful pro-amp as well.

[Gregory]

Just done some more research based on the space I have with my new under-floor mounting approach. And (hurrah) I think I get away from size constraints at all, so can go for 2-4 18 inch drivers or similar (e.g. Mach5Audio MJ-18) . Rather stunningly they are only $80 each! (lord knows what shipping will cost though). Already have a rack-mounted amp that will handle 600W continuous which feels like it should be enough. Need to defeat it's internal 25Hz high-pass filter though. Enough - it's off topic and it's my own thread!

Greg

[Nimby]

Mach5 has a new specialist IB 18" driver with Xmax around 22mm due out soon.

Two longthrow 18" drivers would be easier on the freight charges with VAT on top of everything than 4 more modest drivers. I priced delivery of 4 MJ18s but decided they offered no more displacement than my present drivers due to their limited 12mm Xmax.

[micb3rd]

1) No there is some of signal information down low, there are films and music with significant frequnecy output below 20hz.

2) A drivers have a mechanical fs which is where speaker moves from a motor controled motion to a suspension (spider surrouind soft parts) controlled motion. this had a very large effect on where you can tune your enclosure and gain from doing so.

Vented enclosure is all about a balance, In general 1/4 octave below vent tuning frequnecy is where a driver will have very little support from the vent.

A larger enclosure will allow you subwoofer to move with less restriction and ajusts the effect the port is having.

How much power you are throwing into the driver makes a differece, if above FB you are quite near the driver limits the it won't be long below FB (vent tune) before the driver is straining as it will be coming closer to a freeair situation.

Having high displacement drivers crusing along is a good thing as you have reserve for dynamics and for extra throw in hand below where the vent and box is no longer giving support.

3) High pass subsonic filters can be used to chop the very bottom end off but they have disadvanatages like actually increasing group delay, vents already have a effect on GD and adding more will not help!

The low FS drivers and 16-25hz tuning point seems to be sweetspot most of the time as it extends frequnecy response to cover most of our materal we are playing.

[kstrain]

Originally Posted by Gregory

The expectation/design point is a fair one, but doesn't explain why the drivers don't blow on ported designs unless there is a high-pass filter somewhere to protect them. That isn't expectations, it's just physics - as I see it if the low frequency power was there then the lack of physical constraint at low frequencies would cause a similar excursion you'd get in an IB.

Greg

I'm sure you can blow ported ones if you want to. There does not seem to be any protection in, for example, a Monolith (I use a 16 Hz brick-wall high pass, with zero group delay as it happens, but the cone motion can still be a bit frightening at times).

I certainly see your main point - near DC the ported sub does not know it has a box, as the air transmits freely through the port, and provides no additional stiffness. So why is that different from an IB? That is what I tried to answer before, but I don't think there is a single neat answer (some subs have high pass filters, others don't, some have stiffer drives with higher resonances and more damping all of which makes them more robust, etc., etc.)

Having a sub (IB or otherwise) that can produce large air movements must tempt the user to EQ down to low frequency and drive it even harder. I stop turning up my Monolith when I hear distortion. That is what I meant by expectation.

Perhaps taking concrete examples (looking at the parameters of particular drives suited to ported and IB) will help you answer your question. Ported subs probably vary in design more than IBs, so it might be easier to ask why (or under which circumnstances) IBs can blow, rather than why ported ones don't (supposedly).

Ken

[Gregory]

Thanks everyone - very helpful.

Greg

[Gregory]

Originally Posted by Nimby

Mach5 has a new specialist IB 18" driver with Xmax around 22mm due out soon.

Two longthrow 18" drivers would be easier on the freight charges with VAT on top of everything than 4 more modest drivers. I priced delivery of 4 MJ18s but decided they offered no more displacement than my present drivers due to their limited 12mm Xmax.

IXL-18.4 is due out next month apparently. Mach5 say it will cost about twice what the current driver does, but that is naff all (like ~£45), so twice doesn't sound scary (and given much larger Xmax, works out ~the same in terms of £/piston volume). For others who may find this via a search, the quote for two of the MJ-18s plus sea-freight was £141 (plus VAT on import I guess) - and about £50 more if you want 6 day air freight.

Greg