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2 1/2 way speaker - what does the 1/2 mean?

post #1 of 10
Thread Starter 
I was auditioning speakers this weekend and the B&W 704s are a 2 1/2 way speaker. I notice in various threads some people seem to confuse the number of drivers with the number of "ways" so this may help others clarify some misunderstandings too. With the 2 1/2 way speaker what is the "1/2 way"?
post #2 of 10
It means they have two drivers the same size, but crossed over at different points. The 704's use a 6.5 inch Paper/Kevlar cone bass driver to cover the range from 30 Hz to 125 Hz. They use another 6.5 inch woven Kevlar cone driver to cover the range from 30Hz to 4kHz. One is a bass driver and the other is a bass/midrange driver.
http://www.bowers-wilkins.com/displa...097&terid=1112

Craig
post #3 of 10
Thread Starter 
Quote:
Originally Posted by craig john View Post

It means they have two drivers the same size, but crossed over at different points. The 704's use a 6.5 inch Paper/Kevlar cone bass driver to cover the range from 30 Hz to 125 Hz. They use another 6.5 inch woven Kevlar cone driver to cover the range from 30Hz to 4kHz. One is a bass driver and the other is a bass/midrange driver.
http://www.bowers-wilkins.com/displa...097&terid=1112

Craig

How did you come up with the frequency ranges? The specs say crossover frequencies are 150 HZ, 4 kHz.
post #4 of 10
Quote:
Originally Posted by rdilliker View Post

How did you come up with the frequency ranges? The specs say crossover frequencies are 150 HZ, 4 kHz.

Close enough. What that means is that one woofer rolls off at 150Hz and the other at 4kHz.
post #5 of 10
Quote:
Originally Posted by rdilliker View Post

I was auditioning speakers this weekend and the B&W 704s are a 2 1/2 way speaker. I notice in various threads some people seem to confuse the number of drivers with the number of "ways" so this may help others clarify some misunderstandings too. With the 2 1/2 way speaker what is the "1/2 way"?

A 2.5 way is just a 2 way speaker with an extra driver that rolls off starting low - typically around 200hz - at a 6db slope. The extra woofer can be called the .5 woofer and handles the BSC, Baffle Step Compensation. Baffle Step losses are theoretically (but often not in practice) 6db/octave, so instead of of compensating for those losses with response shaping of a woofer alone like would be done in a 2 way, the 2.5 way has a dedicated driver whose job is to reproduce these losses. Both woofer btw have no high-pass filter, and reproduce the full range of bass.

The two woofers in a 2.5 way can be anything, they don't have to be the same size, they can be completely different or exactly the same. There's nothing in the design that dictates what the woofers must be.

The primary advantage of a 2.5 way is increased sensitivity. As far as a corporation is concerned the advantage of a 2.5 way is they're much easier to market and sell than a 2 way, simply because they're more impressive to look at. As a designer though, 2.5 ways suffer from unacceptable performance issues - the same drivers put into a 2 way MT or MTM will almost always sound better.
post #6 of 10
Quote:
Originally Posted by rdilliker View Post

How did you come up with the frequency ranges? The specs say crossover frequencies are 150 HZ, 4 kHz.

They also say the -6 dB point is 30 Hz. Hence the ranges.

Craig
post #7 of 10
Quote:
Originally Posted by craig john View Post

It means they have two drivers the same size, but crossed over at different points. The 704's use a 6.5 inch Paper/Kevlar cone bass driver to cover the range from 30 Hz to 125 Hz. They use another 6.5 inch woven Kevlar cone driver to cover the range from 30Hz to 4kHz. One is a bass driver and the other is a bass/midrange driver.
http://www.bowers-wilkins.com/displa...097&terid=1112

Craig

That's not really correct ... the .5 woofer covers up to about 800hz actually, since that's when baffle step losses typically begin it must, and it'll only be crossed at 6db/octave.
post #8 of 10
Quote:
Originally Posted by TurboFC3S View Post

That's not really correct ... the .5 woofer covers up to about 800hz actually, since that's when baffle step losses typically begin it must, and it'll only be crossed at 6db/octave.

Then why do they have the crossover at 150 Hz, (which I just realized I incorrectly transposed as 125 Hz in my first post)?

Craig
post #9 of 10
Quote:
Originally Posted by craig john View Post

Then why do they have the crossover at 150 Hz, (which I just realized I incorrectly transposed as 125 Hz in my first post)?

Craig

The crossover for the .5 woofer is nothing more than a big inductor, something like an 8mH iron core. That gives you the 6db/octave rolloff - to match the rolloff from baffle step losses. So while they list the crossover point as 150hz, it's a very shallow slope which allows the .5 woofer to be heard much higher into the frequency range.
post #10 of 10
I always looked at it this way:

A 2-way speaker has a single low-pass filter for the mid-woofer and a single high-pass filter for the tweeter (a single complete crossover).

A 3-way speaker has a low-pass filter for the woofer, a high-pass filter for the midrange, a low-pass filter for the midrange, and a high-pass filter for the tweeter (or two complete crossovers).

A 2.5 way speaker lacks the high-pass filter for the midrange causing it to being a mid-bass driver.
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