Woofer Physics

[fredk]

I have been lurking here for a number of months trying to get my head around sub design. At some point I am looking to build a sub that will reach down to ~15Hz to accurately reproduce pipe organ music.

I am still not clear on what characteristics in a woofer limit or allow it to reach the lower frequencies. It seems that producers of commercial subs are able to make subs that play flat down to 15Hz, but that dosn't seem to be the case in the DIY world. I am wondering why that is.

From some discussions I have read, it also seems that there are some tradoffs between linearity, headroom, and driver size that leave me wondering whether it is better to choose a smaller driver with better linearity and drive it harder, or a larger driver with lesser linearity and retain more headroom (less risk of non-linear distortion).

Some other details. My rig is in a small room - 20x11x8 - and I do not listen at really high volumes - usually 70-80db. Though I will be using this for HT as well, my primary concern is accuracy.

Budget: as little as possible, but realistically $500-700??

So, how do I go about narrowing down my choices?

[Sonnie]

I can tell you that in the DIY world there is absolutely no problem reaching 15Hz with accuracy and linearity... especially if all the volume you want is 80db. A regular ported sub tuned to 15Hz will work and if you have room for a larger box or tube, an LLT will exceed your expectations.

OTOH... I have not seen very many commercial subs able to extend flat to 15Hz.

Figure $275 for a Behringer EP2500 and $100 for construction materials... you have $200-$300 left for a driver, which should allow you several choices.

I am curious as to what albums/tracks you are listening to that extends to 15Hz... have you got some names and titles?

[fredk]

The only one I own at the moment is the ELP debut album. I don't know how low the pipe organ reaches on this disc but there is some synth that goes that low.

I was thinking of classical pieces like Bach's Toccata in D Minor. My understanding is that a 16Hz. stop is normal on a pipe organ.

I have always loved pipe organ music, but never owned any. Now that I finally have a decent system, I have an urge to go low and pick up some good pipe organ pieces.

The budget numbers you give are about what I figured from all the reading I have done here.

There may not be many, but the SVS PB13 and the JL F112/3 are two commercial offerings that seem to get there with a minimum of distortion. On the other hand, I don't see anybody here talking about reaching that low with a 12" driver doing DIY.

The reason I mention 12" is that the Infinity Cappa 12" seems to be a very good driver for the price, but the port tuning for designs floating around here is 20Hz, so I am not sure how low it would reach.

[fredk]

... especially if all the volume you want is 80db. A regular ported sub tuned to 15Hz will work...

OK, now this statement interests me. With an average listening level of say 75db, I can expect some peaks up in the 90-95db range on dynamic material, but my volume requerements are modest compared to the norm around here, hence the topic name. I want to understand what sort of driver is optimum for me and why.

[Sonnie]

Yeah... the Kappa Perfect is exceptionally linear as long as you stay at 90db or less. You could probably make it work if you kept the volume low enough, but distortion does start to rise below 20Hz.

I paid $150 each for my pair of VQ12's. You could tune a pair of these to 15Hz with a Low Q and if you kept the level at 90db or less... it would be pretty clean.

[Sonnie]

ELP huh... surprising. I never knew this. I'll have to check it out.

[ccdoggy]

for the price/performance/requirments you cant go wrong doing DIY.

I have heard the JL Fathom 113 directly against my LLT. I paid maybe $800 for my 15" sub LLT. The Fathom is pretty amazing, but a few things you have to remember about it. One is that its using its own built in EQ to flatten it out, thats really only why its so flat so low in a sealed box, not really a bad thing but could give it a little advantage. it costs quite a bit more then what you could spend on a diy sub that does the same thing. Keep in mind that all of those tests are done under controlled rooms and placement that helps them out allot. corner loading or not, shape of the room can change everything.

The main problem about going low with a smaller driver (12") is the surface area (i know there is more but simplifying). sure you can get it to go low, but the limiting surface area means it will not have as much output as a 15" with the same material. Also many on here are of the thought "Go big or go home", a 12" crossed my mind but why not just get the 15" for the extra headroom and effect.

I had a Kapper Perfect in my car and thought it was a great little driver 10". almost enough for my car so i ended up selling it. Would have no hesitation to suggest it, a pair would be great.

[fredk]

The main problem about going low with a smaller driver (12") is the surface area

Thats what I want to know. Is it just surface area or is it also excursion, or the amount of power required...?

I guess I am curious to know the limits of the Cappa perfect. If you are right Sonnie, then I would be pushing the sub to its limits in HT use or for organ music with a lot of deep bass.

The alternative is to go to a less expensive (relatively) and less linear larger driver and hope not to get too much distortion.

[ccdoggy]

more distortion is not assumed with a larger driver. remember with a larger cone you are moving it less for a given output. Thus less distortion as its less excursion on the driver. moving a 15" 1 inch will make less distortion then the same driver in a 12" thats moving 2 inches. (assuming the 15" has relatively 2 times the surface area of the 12")

Whats the largest size box you are willing to make?

[fredk]

more distortion is not assumed with a larger driver

Agreed, but I am assuming that to keep the cost reasonable for me, I need to compromise and choose a lesser quality driver. Hence the trade-off of a better driver driven to or slightly beyond its limits vs a larger driver of lesser linearity, driven well within its limits.

As an example, the Cappa 12 perfect vs. a Mach 5 ixl's 18.

I have no WAF to worry about, so I don't have any restriction on box size other than being able to physically fit it in the room.

[SteveCallas]

fredk wrote:

At some point I am looking to build a sub that will reach down to ~15Hz to accurately reproduce pipe organ music.

I am still not clear on what characteristics in a woofer limit or allow it to reach the lower frequencies. It seems that producers of commercial subs are able to make subs that play flat down to 15Hz, but that dosn't seem to be the case in the DIY world. I am wondering why that is.

Because DIY subs are too busy being flat down to 10hz! I really don't know what you are basing your comment on, but commercial doesn't stand a chance against DIY for any type of subwoofer unless your only goal is a very small sub with a nice looking enclosure for only a couple hundred dollars. That's fine if you're a home theater in a box kind of guy, but if you want a sub that can stay flat into the low teens with plenty of headroom and minimal distortion, DIY is absolutely the way to go.

From some discussions I have read, it also seems that there are some tradoffs between linearity, headroom, and driver size that leave me wondering whether it is better to choose a smaller driver with better linearity and drive it harder, or a larger driver with lesser linearity and retain more headroom (less risk of non-linear distortion).

Don't assume a larger driver is less linear than a smaller one - it's usually the other way around when we talk about distortion at a given output level. If we take a 12" driver and a 18" driver, and both have 20mm excursion, the 18" driver is going to be able to displace significantly more volume because the surface area of the cone is much larger. The 18" driver has to move much, much less to displace the same amount of air as the 12" driver. If budget allows, focus on 15" or 18". Mach5 IXL 18" should do nicely.

and I do not listen at really high volumes - usually 70-80db. Though I will be using this for HT as well, my primary concern is accuracy.

I have a very hard time believing you don't listen to music any louder than 80db. But if that's the case, this is an easy project. I'd design it capable to at least 105db though.....you'd be suprised how much louder you may listen as your system gets more capable.

[fredk]

DOH! It seems I posted a few words shy of a complete thought yesterday. What I meant to write was:

There are a few commercial vendors building 12/13" subs that play flat down to 15Hz, but you don't see that in the DIY world.

See, because my goals are modest, I am wondering If I can get away with a small driver like the Cappa 12.

Does that make more sense now?

Yes, I really do listen that 'quietly'. When I was a kid, I used to run the other way as soon as my father fired up any power tool. Me and high volume have never gotten along.

I do see your point on handling peaks though.

Don't assume a larger driver is less linear than a smaller one

Maybe I don't understand driver design and function. It seems to be agreed that the Cappa 12 is an exceptionally linear driver. I always assumed that this referred to its linearity when run within spec, not just as it approaches its limits. Does linearity only become an issue as one approaches the limits of the driver?

[robertcharles123]

Which small commercial subs play down to 15 hz with authority. Illka tested tons of subs that did not come close to what was claimed. Are these "claims " or actual measurements?

[SteveCallas]

fredk wrote:

There are a few commercial vendors building 12/13" subs that play flat down to 15Hz, but you don't see that in the DIY world.

That's primarily because the commercial industry doesn't want to have to ship large subwoofers, and the average guy may not want a large subwoofer, so they have to work to cram as much in a small enclosure as possible. That's one way to go about getting performance, but it's not very cost effective. The drivers get significantly more expensive, the power demands increase, and built in electronics with EQ boosting and cutting become essential. In DIY, we question why enclosure size has to be a limiting factor just as we question every potential limiting factor.

Does linearity only become an issue as one approaches the limits of the driver?

Not necessarily. But even the most linear 12" driver can only do so much. Let's look at the Maelstrom X 18 and the Kappa 12 as an example. The Maelstrom can displace ~7.8 liters peak to peak, the Kappa can only displace 1.64 liters - that's a huge difference.

Now let's say the Kappa is ultra linear throughout it's whole 14.5mm excursion - which it clearly isn't, but for the sake of argument we will say it is - it can still only move so much air in a feeble attempt to reproduce lower frequencies. It will hit that 14.5mm peak excursion early and ride it all the way down. What you end up with is a driver that can't produce much output at all down low, and when it does try to, it will exceed its excursion limits and probably bottom out. This equates to a lot of non linear distortion and a poor attempt at reproduction of the input signal.

Now let's say the Maelstrom is not a very linear driver - which I have no reason to believe is the case, it's probably quite linear, but let's say so for the sake of the argument. To reproduce the same output levels as what the Kappa can, the 18" driver barely has to move at all. The 18" is also capable of actually reproducing lower frequencies (at a very high level), and it has a lot more headroom to handle bass peaks without the risk of bottoming. It may not be as linear when it reaches 13mm of excursion as the Kappa (probably not true, but sake of argument), but since it never has to use 13mm of excursion to outdo anything the Kappa can, it doesn't matter. If it ever does reach 13mm excursion, it's going to be playing significantly louder or lower than anything the Kappa can hope to achieve. This means that in nearly every scenario the 18" is going to play much more cleanly and track the input signal MUCH more faithfully.

Lastly, Ilkka has measured this Infinity Kappa 12 you are referring to, and it doesn't look to be anything special for a 12".

http://www.hometheatershack.com/foru...fect-12-a.html

[ccdoggy]

Persoanlly I think you have a few preconceptions about distortion (mainly larger drivers have more) that are incorrect.

Like steve said, those commercial offerings have expencive drivers with huge motors, large amps, and EQs that drive up the price.

You could get something like a AudioPulse Revo driver and put it in a smallish sealed box, put ~1200 watts to it with an EQ and get better results then the JL. but its going to cost a pretty penny, in diy terms it would still be cheaper then the JL. in DIY we figure why not taylor the box to exactly what we want (sometimes making it pretty large) and get the performance out of the sub without having to EQ the outta it, without needing a billion watts (although we usually get the bigger amp anyways), and very low distortion.

If anything is pushed outside its limits distortion raises quite quickly, yes. So it would make sense that a larger driver would be be pushed to those limits nearly as soon as a smaller one.

We are not telling you to not go with the Kappa 12", just that there are options out there for the same or better output volume. The Kappa is a good driver i really liked it, but just not enough output for me.

I vote you take a look at the LLT threads around here and look in the subwoofer testing area and look at the distortion graphs of allot of different setups. Might see something you like in there in your price range.

[fredk]

Persoanlly I think you have a few preconceptions about distortion (mainly larger drivers have more) that are incorrect.

No, but maybe I am not communicating my assumptions very well.

I assume that within a given size driver there will be better (more linear) and worse drivers, and that this quality is usually directly related to price. I am also assuming, or more correctly, choosing to believe someone else (for now), that the Kappa 12 is an exceptionally linear driver.

So, if you have a smaller 12" but exceptionally linear driver being pushed to its limits, and a larger, but less linear driver operating well within its limits, is there a clear choice?

I also have a clear performance expectation: flat (or as flat as my room will allow) to around 15Hz.

If I had the money, I would just plunk it down on the best 18" driver and be done with it. Unfortunatley, in my world, every penny counts.

...better output volume.

I don't care about bigger better more, just enough for my modest requirements. I know, "what an odd fellow"...

Robert: The PB13 certainly does and Ilkka has the measurements showing it and its ported. I had thought the Fathom113 does as well, but revisiting measurement, it does not.

ccdoggy. You are right, some of the commercial subs I looked at do involve larger amps and eq.

Steve, what I hear you saying is that a moderately priced large driver, because it is hardly working, is more likely to perform better and reach down where I want it to.

Let me ask this another way as well. If the Kappa 12 is capable of playing flat to 20Hz, given average rolloff and average room gain, where is my -3db point going to end up?

Thanks everyone for your replies.

[Blaser]

Fred,

The best thing you could do is a modeling of that driver with win ISD or unibox.... It will answer so many of your questions. Give it a try.

[SteveCallas]

I am also assuming, or more correctly, choosing to believe someone else (for now), that the Kappa 12 is an exceptionally linear driver.

Let me ask this another way as well. If the Kappa 12 is capable of playing flat to 20Hz, given average rolloff and average room gain, where is my -3db point going to end up?

You've got Kappa on the brain. You must not have clicked on that link I provided, because again, it's been tested and it's nothing special - it's not a driver that is going to get you clean, flat extension to 15hz. I won't push this point any farther after this post, but simply put, within the budget you mentioned, I would not spend any more time considering that driver. Move up to a 15" or 18".

[DrWho]

What's all this talk about smaller drivers being more linear??? Ignoring polar response, smaller drivers are always going to be less linear. Surface area is your friend when it comes to linear displacement of air. Nonlinearities in drivers result because the cone has to move. To achieve the same SPL at the same frequency, a larger driver doesn't need to move as much...thus it naturally tends towards better linearity.

[robertcharles123]

fredk,

I thought we were talking about woofers and cabinets that were of the kappa 12 price and build. The woofer in the svs is a monster more on the tc2000/revo level. The box is also not very tiny either. Also, the price is in the 1500.00 and higher range- not cheap. I thought you were looking at systems that were of comparable $ to what you wanted to spend. A commercial sub at 500-700 will find it hard to do what you will do for the same money. As a matter of fact, you will be able to get what the svs gives for what you want to spend, but it will require work building on your part.

Thanks and good luck,

Robert

[WmAx]

[quote=SteveCallas;134650]

Now let's say the Kappa is ultra linear throughout it's whole 14.5mm excursion - which it clearly isn't,

but for the sake of argument we will say it is - it can still only move so much air in a feeble attempt to reproduce lower frequencies. It will hit that 14.5mm peak excursion early and ride it all the way down.

Actually, the xmax specification of 14mm is very conservative. It's completely linear within this range. The xmech is well over 20mm for this driver. This may not be a SPL driver, but it is one of the most linear drivers I have worked with, with the exception of JL W7 drivers.

Lastly, Ilkka has measured this Infinity Kappa 12 you are referring to, and it doesn't look to be anything special for a 12".

First, the Kappa Perfect should never be used in a sealed configuration. It is not suited to this application. It will end up leaving it's linear excursion range under 30Hz at any substantial SPL output level. Also, if you pay attention, the amp used in that test would be lucky to output 200 watts into 4 Ohms bridged. The amp used was not even rated for 4 Ohm bridged use. Looking up the manufacture specs on that amp, it is clear that it was grossly inadequate. I have no doubt the amplifier was distorting in those tests. Avaserfi has powered the Perfect 12" with 2000 watts and achieved 120dB output at 20Hz in a ported cabinet system in an average sized room. I have two of the 10" versions on my computer speaker system powered by 650 watts each, and together they easily hit 120dB at 30Hz(the cabinets are only designed to respond down to about 28Hz on my computer system) with no significantly audible harmonics with pure sine waves.

-Chris

[WmAx]

Sonnie wrote:

Yeah... the Kappa Perfect is exceptionally linear as long as you stay at 90db or less. You could probably make it work if you kept the volume low enough, but distortion does start to rise below 20Hz.

I paid $150 each for my pair of VQ12's. You could tune a pair of these to 15Hz with a Low Q and if you kept the level at 90db or less... it would be pretty clean.

Actually, given the correct amplifier, a single 12" VQ in a properly built ported cabinet can produce around 120dB at 20Hz with no substantially audible level of harmonics with a sine wave test tone in an average sized room, corner loaded.

-Chris

[WmAx]

DrWho wrote:

What's all this talk about smaller drivers being more linear??? Ignoring polar response, smaller drivers are always going to be less linear. Surface area is your friend when it comes to linear displacement of air. Nonlinearities in drivers result because the cone has to move. To achieve the same SPL at the same frequency, a larger driver doesn't need to move as much...thus it naturally tends towards better linearity.

I think this was a mis understanding. I think this resulted from my comment on another forum in regards to the OP. That was simply a comment that one should not automatically assume a larger diameter driver is superior to a smaller one. Given an extraordinary linear driver of 12" and an average 15" driver, it is possible the 12" driver can exceed the performance of the 15" device. Of course, if you have two drivers with somewhat comparable motors, and one has a larger surface area, then of course the larger device will have superior linearity within a given SPL range.

Also, the OP is on a strict low budget. It's not like his budget will allow for a Maelstrom-X or something.

-Chris

[SteveCallas]

I call shenanigans.

20mm excursion with a 12" driver still isn't anything to get excited about. 2000 watts put into this driver in a 19hz tuned sub can potentially drive excursion usage up to 37mm - unless one used some type of sever limiting both above and below tune, the 20mm could be exceeded very easily. A few hundred watts seems appropriate for this driver in a ~100 liter sealed sub or a 19hz tuned ported sub. Ilkka's test is valid.

I can assure you the claimed 120db achieved, if actually achieved, was far from clean. Far as is Seattle to London far. There is no magic bullet, a 12" driver can only do so much. It's comforting to think an inexpensive 12" can do it all, but let's be real here, it can't.

[WmAx]

SteveCallas wrote:

Hmmm, how can I put this without getting in trouble with Sonnie?




20mm excursion with a 12" driver still isn't anything to get excited about. 2000 watts put into this driver in a 19hz tuned sub can potentially drive excursion usage up to 37mm - unless one used some type of sever limiting both above and below tune, the 20mm could be exceeded very easily. A few hundred watts seems appropriate for this driver in a ~100 liter sealed sub or a 19hz tuned ported sub.

I can assure you the claimed 120db achieved, ahem, was far from clean. Far as is Seattle to London far. There is no magic bullet, a 12" driver can only do so much.

Avaserfi has built this and measured it - I'm sure he will be glad to discuss specifics with you.

Again, it's not a SPL sub, but far more linear as compared to most drivers, and will still produce cleaner high SPL as compared the majority of commercial subwoofers around. Want a linear 12" sub with super high SPL? A JL W7 or TC Sounds TC2000 or TC3000 based driver is what you need. I would rather have 14mm of extremely linear travel than the non-linear 25mm of the typical SPL sub that has substantial BL change after just a few mm of excursion.

I actually use the 10" units on my ultra-linear computer sound monitor system, and I have a pair of 12" units I was going to use on my new mains, and they would have also performed perfectly. But it is true I am not using the 12" units now, as I could not refuse that 1/2 price deal on a pair of AP AXIS 15" drivers from pe a while back.

-Chris