Car audio drivers for open baffle bass...

I'm wondering why we don't see more car audio drivers suggested for open baffle bass.

I see the Eminence Alpha 15A suggested a few times ($60 at PE), but that has a low Xmax of 3.25 mm. For similar cost, you can get an Image Dynamics CTX128, that has the same impedance, but it's a 12" driver with a 16mm Xmax, which results in 2.5 times the linear displacement capability of one Alpha 15A, which should mean more linear SPL capability and cleaner bass at lower frequencies.

Another option is something like the Infinity 1262W. This is a DVC that's slightly more expensive, but it does provide the option of being able to "tune" the Q by simply not connecting one coil, or shorting one coil with a resistor to achieve a particular Q. Oh, and with its 12mm Xmax it has almost double the linear throw of the Alpha 15A.

Many of these drivers are designed for stupid people behind the volume controls so they should be sturdy enough for OB use.
In general car woofers are the worst choice for open back baffles since they are designed to produce sub low frequencies in shoebox sized boxes.

That's an over-generalization of car audio drivers. The cheaper ones, like the ones I've listed, might actually be a pretty good fit for OB use (fairly low Fs, medium-high Qts, decent Xmax, decent Vas.

The t/s parameters can give a pretty good idea of what the drivers are capable. Vd, which is Sd*Xmax, should provide a pretty good indication of how much air they can move linearly.

Even the Infinity Kappa 122.7Ws I have in my cupboard at the moment (left over from an older car audio install) might be suitable. Fs=18 Hz, Qts~0.5, Xmas=12mm, and needs a pretty huge box to get Qb below 1.
I hope the means of the thread are to discuss and not to start a flame war.

I have yet to hear a car audio driver that can sound ok. I measured a Focal 5V2 two years ago, came from the highly praised Focal Polyglass V2 130 system ( was in my car so not abused and bought BNIB ). The was obliterated in every means by the SB13PFC which is very modestly priced, both on measured data and listening experience. And from the subwoofers I have heard, none is even close to a hi fi driver but all come at ridiculous cost. And if you`re ready to pay $700-800 a driver - there are many choices such as AE, GPA, Fostex and others.

Scott L

Paid Member
2008-12-27 12:32 pm
Knoxville, TN
Is there a Doctor in the House ?

Please, Oh please (and I am dead serious here)

What is this fascination with open baffled bass ?

The reason I ask is, it seems to me, based on everything I have learned,
that the goal for a bass driver to actually produce bass, is to separate the front and rear waves such that they don't cancel.

I do realize because if the way I posed the question, an easy answer would be to say,

"Well, I guess you just haven't learned everything"
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I have yet to hear a car audio driver that can sound ok.

I base my position mainly on measurements, rather than subjective opinion, particularly when it comes to bass.

For example, the current Alpine Type-R 12" driver is based on a home audio design and with its flat BL curve and 20 mm Xmax, it's possibly the cleanest I've heard so far, and my measurements of its performance show why, with measured distortion considerably lower than the drivers I previously used.

However, it is the cheaper car audio drivers I've referring to, not expensive ones like the Focals, whose design makes them suitable ONLY for car audio use. With these cheaper drivers, while the manufacturer recommends very small boxes, when you run the t/s parameters, you find that a large box or IB would be better suited.

And it only stands to reason that a driver with a Vd over double that of the Eminence Alpha is likely to produce cleaner bass at lower frequencies.
Ah, but how linear will the drivers be up to Xmax?
I have a pair of JBL GTO1214 drivers here, with a claimed 14.5mm Xmax. They get to around 10mm one-way free-air before distortion becomes apparent. This is one of those drivers that's built for abuse, and I think things have gone very non-linear by the time you get them close to Xmax.

People like the Alpha-15A because it doesn't need EQ to do OB bass. You just cut a hole in a board, add a 3-8" wideband unit, and cross over at 200Hz. The Alpha-15A is also fairly efficient, even with the dipole rolloff.

Ah, but how linear will the drivers be up to Xmax?

The same could be asked of the Alpha :)

In any case, even at 10mm they have 1.5x the Vd of the Alpha. This opens up the opportunity of using the same baffle size and getting more low bass capability, or narrowing the baffle a bit for the same low bass capability. I actually like the latter option - so the build ends up look a bit less like someone just took a 4x4 piece of ply and cut holes in it for speakers.

People like the Alpha-15A because it doesn't need EQ to do OB bass. You just cut a hole in a board, add a 3-8" wideband unit, and cross over at 200Hz. The Alpha-15A is also fairly efficient, even with the dipole rolloff.

The car audio drivers are less efficient yes. If you use one coil on a DVC, that should boost Qts significantly. But I'd think more of active EQ and filtering for the low end, using something like an iNuke, which allows for additional niceties like dynamic EQ, which should work out pretty well for an OB system.
Isn't Xmax the maximum linear excursion?

Also Xmax is usually peak-to-peak. With 14.5mm Xmax one way would 7.25mm so with the 10mm you mentioned you are already well above Xmax and heading towards Xlim where damage occurs.

Supposedly maximum linear excursion, yes. In practice, I don't think there are any solid standards. Some are based on voicecoil geometry, others distortion measurements.

I have some PA drivers where Xmax is 9.5mm and Xlim is 26mm (both one-way). They're pretty linear out to 15mm one-way.

The JBLs are, I suspect, designed so that the motor assembly can do 14.5mm one-way travel, but the suspension etc won't allow it, so that people in small hatchbacks can't bottom the drivers out in a too-small ported box.

Brian, with some DSP, then yes, I expect car audio drivers can be made to work quite well. Just keep an eye on wind noises from the ventilation around the voicecoil.
FWIW, I used a pair of Beyma 15P1200Nd on OB for a short while. Went to 20Hz with a mere 25dB of boost at the bottom end. Cone excursion was scary when the low bass came along. Around 30mm p/p, and those were moderate levels. Needed an NU6000DSP, and was hitting the -6dB light, even at those levels. Aiming for 35Hz extension might be prudent ;)

I've always been curious about autosound drivers like the ones that have been mentioned here. But each time I pull up a prpoduct page I find very few specs, and what I find seems very dubious (e.g. overinflated power ratings, etc.).

To me a driver is a driver no matter what "label" the manufacturer wants to give it. Provide me with a complete set of specs, and some plots of frequency response and distortion, and I'll make my own judgements.

Too often I see autosound products rife with marketing hype without anything to back them up. Very rarely do I see something I would trust.

Anyway, to the O/P, if you mean a driver with high Xmax, Fs around 35-40Hz, and Qts above 0.5 then yes this would be a good candidate for an open baffle woofer. IMHO frequencies below about 35 Hz with any real SPL are a real challenge with an open baffle speaker (even a short line like a U- or H-frame needs a lot of bass correction). Choose the natural resonance of a driver (Fs around 35Hz and higher Q) so that it works for you, not against you. A lower Q (e.g. meant for a box, Q=0.35) has a falling response that is only pulling the low bass SPL down even further.
The difference with car audio is that a lot of the customers do not understand the technical specifications so it is easy to sell them some cheap chinese junk rebranded by a 'big brand name' along with some ridiculous 'max power' claim, oversized magnet and eye catching design such as a kevlar cone and sculpted surround.

What I tend to find is that with car audio woofers is that the Fs is way too high due to stiff suspensions to prevent bottoming out when mounted in a door and and the volume knob is abused. I guess this is insurance against the "it claimed it was 100W but it blew up on my 50W headunit, it's defective!". Usually any bad woofer breakup nodes go unnoticed as they point off-axis and get muffled in the carpet foot wells of the car.

It takes a bit of work (and active processing) to actually build a car audio system that sounds half decent, therefore it is easy to sell someone some junky speakers because even decent speakers will sound like **** when installed in the factory locations and driven straight from the headunit with no processing.

With car subwoofers, even the better ones are usually only useful for subwoofer duty because very little thought is given to cone geometry to deal with breakup nodes and often the sensitivity on high-xmax subs is pretty poor which leaves you no option but to use a dedicated amp and active crossover. This is in comparison to the home/pro audio world where there is a huge overlap between 'woofer' and 'subwoofer' territory and you can find woofers with decent extension and still decent sensitivity. Often there are other considerations with car audio subwoofers which limit their performance such as being resilient to very high ambient temperatures, abuse by exceeding xmax and taking impacts from loose objects in the trunk of a vehicle - this usually means that most subs have thick poly cones and flat/concave dust caps which don't necessarily help sensitivity or frequency response up the top end.

For instance, I have a pair of Boston Acoustics 10" car audio subwoofers and while they are great as subs, they have a huge cone breakup node at 1kHz which makes them pretty well useless as a woofer because you want them to be pretty far down by 200Hz to avoid generating distortion products on the breakup node of the cone.

There are still some 'diamonds in the rough' to be found in the car audio space. Over the years certain car audio manufacturers have rebranded some quite expensive european drivers - the ridiculously low resale value of car audio gear and the fact that most people buying car audio gear don't actually know what they are buying except for the brand name means that there are bargains to be had if you look hard enough ;).
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No. It's usually one-way. Though some manufacturers would quote a peak-to-peak rating for Xmax. I always refer to one-way Xmax in my posts when doing comparisons.

You're right, I got that **** over tits somehow.
It is the other way around with many manufacturers: Xmax one way but Xlim ptp but all in all it is all over the place.
@Brian Steele have you tried building any OB subs using any of the drivers you've mentioned?

Not yet. However, I've got two Infinity 122.7W car audio drivers sitting unused in my garage at the moment, so I'm tempted to try a dipole woofer design with them. I've also got some spare 5/8" ply available to build the necessary H baffles...

I actually tried and measured a dipole bass alignment some years ago. I wasn't impressed. The supposed "relatively free from room modes" attribute of dipole bass systems turned out to be rubbish when actually put to the test. See The Subwoofer DIY Page v1.1 - Projects : An INF10 Dipole Subwoofer

I am willing to try again, if I don't have to invest too much to do it.
I base my position mainly on measurements, rather than subjective opinion, particularly when it comes to bass.

For example, the current Alpine Type-R 12" driver is based on a home audio design and with its flat BL curve and 20 mm Xmax, it's possibly the cleanest I've heard so far

You should base your position both on measurements and listening with no crossover. Would you say that two drivers that measure the same would sound the same?

The Type R has a somewhat flat BL curve to 10mm, then it goes to almost 45 degrees change. Sensitivity of 85db ( god knows whether its at 2.83V or 1W, as its typical for car audio companies to produce near zero T/S data ) and only 10mm of motor force factor linearity ( I have to see other measurements, such as resonances, suspension behavior, distortion products ) doesn`t look very impressive. Where did you get your results? I saw Klippel data published on diymobileaudio.