Two small woofers = One big woofer?

[Bobby Dipole]

I notice a lot of the new model speakers are tall and thin (too thin for a large woofer). They usually have two 8" or 10" speakers. Are two 8" woofers equivalent to one 12" speaker?

Bobby Dipole

[singa]

Originally Posted by Bobby Dipole

I notice a lot of the new model speakers are tall and thin (too thin for a large woofer). They usually have two 8" or 10" speakers. Are two 8" woofers equivalent to one 12" speaker?

Bobby Dipole

That's what marketing people want you to believe.Be grounded in science.There is no literature that say so unless I am wrong.

[pjanda1]

I'm assuming you are looking for something to fill helper woofer role you see on the fullrange forum. Efficiency is especially important to our purposes. If you are looking for a dipole sub and are going to use a big amp, then displacement is more important and drivers with huge excursions are the way to go. Or if you are building a 2.5 way, a single woofer isn't going to help you much.

Some pairs of smaller speakers could equal the performance of one bigger woofer. But, there aren't that many drivers that work well on an OB. When I was modeling (with the albeit simple XLbaffle) before building my current pair, I was interested in the slender look of dual 12s instead of one 15. There aren't that many twelves that work well at all. The Eminence custom 12s in the swap meet (now sold out) were about the best I saw. And of course, the two will often cost more than a single larger driver. It is tough to make a general rule, but I'll bet that most of the time, you are better off with a bigger speaker. Especially for a dipole. I'd go 15 or 18, myself.

It all boils down to your design goals. The Eminence Alpha 15 and the Peerless XLS 12 are both used on OBs, but usually for quite different reasons.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[AEIOU]

Originally Posted by Bobby Dipole

I notice a lot of the new model speakers are tall and thin (too thin for a large woofer). They usually have two 8" or 10" speakers. Are two 8" woofers equivalent to one 12" speaker?

Bobby Dipole

Do the math. Find out the surface of two 8" woofers and compare it to a single 12" woofer. I believe that two 8" woofers are closer in surface area to a 10" woofer than to a 12" woofer. Anyway, larger woofers will usually have a lower fs, that is one of the reason why they can go deeper in the bass.

[Ty_Bower]

Originally Posted by Bobby Dipole

Are two 8" woofers equivalent to one 12" speaker?

This isn't really a relevant question for the full range forum. Regardless, in my very limited experience I think I might prefer multiple smaller woofers over a fewer number (or a singular) larger woofer in some types of enclosures. In particular, I'm thinking of the typical vented bass reflex boxes commonly encountered in those systems usually sold to the mass market. I can't quite say what it is, but I feel a pair of six inch woofers sound better (faster? tighter?) than a single eight or ten inch woofer. Of course, there are many who profess than anything less then twelve or even fifteen inches shouldn't even be called a woofer. YMMV.

[AEIOU]

Originally Posted by Ty_Bower

This isn't really a relevant question for the full range forum. Regardless, in my very limited experience I think I might prefer multiple smaller woofers over a fewer number (or a singular) larger woofer in some types of enclosures. In particular, I'm thinking of the typical vented bass reflex boxes commonly encountered in those systems usually sold to the mass market. I can't quite say what it is, but I feel a pair of six inch woofers sound better (faster? tighter?) than a single eight or ten inch woofer. Of course, there are many who profess than anything less then twelve or even fifteen inches shouldn't even be called a woofer. YMMV.

You may feel that way, but you'll never really "feel" the bass from a couple of 6 inchers! Try and get some rib shaking bass form a couple of small woofers. Regardless of the cone size, acceleration/velocity is just a matter of applying enough force.

[Scottmoose]

Usually the problem when a large woofer is used & people think it sounds 'slow' is that the XO needs more work -the big driver is being run up higher than it's really capable of going, so the transient response is suffering.

[DBMandrake]

Originally Posted by Bobby Dipole

I notice a lot of the new model speakers are tall and thin (too thin for a large woofer). They usually have two 8" or 10" speakers. Are two 8" woofers equivalent to one 12" speaker?

No.

In most cases this trend for narrow speakers is a purely aesthetic pursuit IMHO - wide speakers don't sell well in the general market place, due to a lack of WAF. To try to make up for the lack of bass of the smaller drivers, multiple drivers are used.

Sometimes the multiple "woofers" are used as a 2.5 way design instead of a 3 way. The advantage is you get the extra cone area for bass of the driver that is also handling mids, the down side is you still have bass/midrange IM problems like a 2 way.

As AEIOU points out, two 8 inch drivers have less cone area than a single 12". If you work out the actual cone diameter (including half the surround width) of a typical 8" driver and convert it to area, you'll get about 213cm^2 while a typical 12" will be about 507cm^2 - so on cone area alone a 12" woofer is worth 2.4 x 8 inch drivers.

It doesn't stop there though, with few exceptions a good 12" woofer will have a greater Xmax than a similar 8" driver, increasing the potential volume displacement even further. Suspension is generally more linear due to much larger width spiders, and distortion is lower.

The voice coil of a 12" driver can be made much larger in diameter and more robust than an 8" driver, so assuming roughly the same sensitivity there will be less thermal related compression at higher volumes. (It's not uncommon for the larger driver to be more sensitive too, for a similar Fs)

Another factor is radiation impedance - it's a little bit hard to grasp on an intuitive level, but I remember reading a paper on the design of the B&W Nautilus 801's - the one with the 15" woofer, where they went into the theory behind the coupling between a woofer and the air in a room, and it turns out that a single large round woofer couples more efficiently to the room at bass frequencies (IE there is less acoustic impedance mismatch) than two smaller round woofers stacked above each other sized such that the total cone area is identical to the single 15".

Apparently it's due to the fact that the single large driver has all of its cone area in a contiguous circle, while the two smaller woofers have areas of "dead space" between them, especially to the sides of the gap, which alters the radiation impedance compared to the larger cone with the same total cone area. (In other words the shape of the radiating surface is important)

Bottom line is that all other things being equal a larger woofer can perform better than multiple smaller woofers of similar cone area for bass. That's not to say it's impossible to get good bass from multiple smaller drivers, but you have a lot of practical and theoretical issues conspiring against you.

Obviously a larger woofer won't go as high in frequency, so based on your crossover frequency this sets practical limits on the maximum diameter of woofer you can use.

For example in the aforementioned B&W design they are actually using that 15" woofer up to 350Hz which I think is a bit high for a driver so large (especially when crossing over to a 6" mid) although they have paid careful attention to optimizing the midrange characteristics of the woofer.

However a 12" woofer with a paper cone and clean midrange up to 1Khz can comfortably cross over as high as 300Hz IMHO, so for a large speaker requiring the cone area of a 12" woofer to get the target amount of bass, and a crossover frequency of 300Hz or lower, I can see no reason to use multiple smaller woofers instead of the single 12" woofer, other than trying to achieve WAF by making a narrow cabinet.

"Slow" bass from a 12" woofer to me is problems with the integration of the woofer and midrange driver, (for example the midrange driver being pushed too low in frequency for it's size and not having the dynamic range necessary to reproduce high volume bass harmonics) and/or a bass alignment that is anechoically flat to a low frequency, causing a big rise in the bottom end of the bass range due to room gain. I tend to favour a low end response that has early gradual roll-off to help avoid this, and then equalize any low end rise that is still left.

[aptquark]

Originally Posted by AEIOU

You may feel that way, but you'll never really "feel" the bass from a couple of 6 inchers! Try and get some rib shaking bass form a couple of small woofers. Regardless of the cone size, acceleration/velocity is just a matter of applying enough force.

Personally, I would rather have the advantage of smaller speakers paired with a sub if I was after rumbling bass. Most of the peeps here are after quality of sound and not Cerwin Vega style bass.

[Scottmoose]

You can have both you know.