What do two woofers do that a one doesn't?

[Mitch G]

For example, if you look at the Axiom line of bookshelf speakers. They have a small speakers with a single woofer and then a bigger one with two woofers of the same size. Is the only benefit of this more SPL? Or, do you get other benefits as well?
It seems like if one woofer has frequency response X, then two woofers would have frequency response X but just be able to go louder. Or, am I missing something?

The reason I ask is that I'm considering upgrading from my Atoms as my mains. But, I'm not looking for louder sound. I'm looking for fuller sound. It seems to me to get fuller sound would mean going to a larger woofer and not necessarily more woofers. Is that not the case?



Mitch

[Bent]

I've never looked into the inner workings of Axiom's stuff - but on the surface two tweeters facing the same direction is a very bad thing... you wind up with "combing" problems, but I'm quite sure that with Axiom, being as well regarded as they are, would have taken steps like integrating different crossover frequencies and such to alleviate/reduce/eliminate these problems.

[JCD]

Mitch G wrote:

For example, if you look at the Axiom line of bookshelf speakers. They have a small speakers with a single woofer and then a bigger one with two woofers of the same size. Is the only benefit of this more SPL? Or, do you get other benefits as well?
It seems like if one woofer has frequency response X, then two woofers would have frequency response X but just be able to go louder. Or, am I missing something?

The reason I ask is that I'm considering upgrading from my Atoms as my mains. But, I'm not looking for louder sound. I'm looking for fuller sound. It seems to me to get fuller sound would mean going to a larger woofer and not necessarily more woofers. Is that not the case?

Mitch

Going louder is sort of the reason -- With two woofers the drivers don't have to work as hard to get the same SPL level. Since they don't have to work as hard:
1) they are less prone to distortion
2) the point that they start to roll off is lower,
3) etc..

This doesn't take into account any advantages (or disadvantages) of say an MTM alignment or a TMM alignment's dispersion characteristics. I'll leave that one for the smarter folks..

All things being equal, I would expect more drivers to be better. No science to back that up, just a prejudice or gut feeling.

JCD

[Bent]

just goes to show how well I read, huh?

I was sure I saw you say tweeters as well, but alas, I saw only what I wanted to see.
my appologies,

[azjimmy]

I know I'm probably sticking my foot in it here, but the way it was explained to me was that two 8" drivers are more ridgid than a single 16" (same effective area). The larger driver is more prone to flexing/distortion on dynamic pasages, so two 8" drivers can drive at a higer volume with less distortion.**

**(This is from the B&W Sound Experience DVD)

Jim

[MACCA350]

azjimmy wrote:

I know I'm probably sticking my foot in it here, but the way it was explained to me was that two 8" drivers are more ridgid than a single 16" (same effective area). The larger driver is more prone to flexing/distortion on dynamic pasages, so two 8" drivers can drive at a higer volume with less distortion.**

**(This is from the B&W Sound Experience DVD)

Jim

A 16" driver has four times the displacement area as a 8" driver
8" = 50.26 square inch
16" = 201.06 square inch

So two 8" drivers have half the displacement as a single 16", and thats not factoring in that larger drivers normally have a longer throw which will displace more again.

Whereabouts on the B&W Sound Experience DVD(I have a copy of this) did they say this? Were they talking about bass or midrange drivers?

cheers

[drf]

There are manny factors to consider when deciding which is better, I haven't confirmed yet but i think macca is right about the displacement and throw. Another factor is at which point you want the driver crossed over or its upper freq limit, A single 15" driver will suffer cone breakup issues at a lower frequency than 2x 8" drivers (cone breakup is mainly a ridgidity issue).

On spl, it really depends on the sensitivity of the drivers, e.g 2x 8" drivers with a sensitivity of 84db/W/M will (theoretically) provide 87dB/W/M yet a single 15" driver could easily (nearly always) have a sensitivity of at least 89db/W/M due to its size.

as you can see its not exactely straight forward, however depending on the application and compromises required one option will present itself as being better.

Dr F

[Wayne A. Pflughaupt]

For example, if you look at the Axiom line of bookshelf speakers. They have a small speakers with a single woofer and then a bigger one with two woofers of the same size. Is the only benefit of this more SPL? Or, do you get other benefits as well?
It seems like if one woofer has frequency response X, then two woofers would have frequency response X but just be able to go louder. Or, am I missing something?

A couple of factors come into play here. For instance, a WTW arrangement is typically done that way for controlling vertical dispersion (if I recall – someone help me out if I’m wrong about that).

But since you’re looking at Axiom and they don’t have any WTW speakers, the other factor comes into play. Primarily, the reason to add more or use bigger drivers is to increase bass output and/or extension. This has much to do with the room; the larger the room, the more low frequencies are “lost.” Thus, the larger your room is, the more bass output your speakers require in order to compensate, so they’ll sound balanced and not “thin.”

There is more information on this topic in Part II of my House Curve sticky thread at our BFD/REW Forum.

Regards,
Wayne

[azjimmy]

MACCA350 wrote:

Whereabouts on the B&W Sound Experience DVD(I have a copy of this) did they say this? Were they talking about bass or midrange drivers?

cheers

It would figure I would get my wires crossed on that!
It's been a while since I saw that DVD, but IIRC, it was the part on cone break-up.
-it, now I'm going to have to go back and watch it again...
Jim

[drf]

this dvd sounds interesting, what demographic is it aimed at (e.g begginer or experienced speaker builder). Are there many benefits from traking down a copy?

thanks,

Dr F