[2005-12-13]

I am really enjoying my Fostex FF225K fullrange speakers, but I have been itching to build something new. The only thing I don't like about the ff225k speakers is the need for baffle step correction. It's a shame to use 96dB efficient speakers and then knock 3dB off above 300Hz or so to get them to sound good. So I started to think about other ways to get more output under the baffle step cutoff frequency, which is around 325Hz for a 14" wide baffle.

A little background on the baffle step effect... High frequencies are more directional than low frequencies, and they tend not to bend around corners easily because of thier relatively short wavelengths. Low frequencies are happy to bend around corners because the wavelength is significantly longer. It sounds goofy, but it really does work that way. So basically the high frequencies are coming from the speaker and only radiating into the space in front of the speaker, while the low frequencies are radiating around the corners of the speaker cabinet and radiating in to the entire space around the speaker. Since the low frequencies are effectively filling the whole room - they call this full space - while the high frequencies are only filling the room in front of the speakers - they call this half space - the high frequencies sound 3-6dB louder. So to compensate for this, we need to find a way to make the low frequencies louder by 3-6dB.

I have been listening to my "loaner" system at my friends house lately. This is a single ended triode-mode 6v6 amp and a pair of Large Advent speakers with the 10" woofer. These speakers are not very "revealing" but the tonal balance is superb. They have great bass and lots of lower midrange weight that is really appealing. Most fullrange speakers are the opposite, very revealing, but lacking in weight. I wanted to achieve the best of both worlds. The Advents have a tweeter that is designed to play relatively low, ad so the woofer crosses over into it at about 1400Hz. The crossover is first order, which I think is essential to great sound. My plan was to do something similar, but using a true fullrange driver instead of a tweeter, use a woofer that is 3-6dB more efficient than the fullranger, and cross the woofer over at the baffle step correction frequency. My hope was that this would give me the full frequency range, with a minimum of crossover parts.

The Eminence Alpha 10 is a very neat woofer. It is designed as a PA driver, but goes much lower than most PA type drivers without sacrificing efficiency. Unfortunately we have to contend with the triangle of bass extension, small enclosure, and high efficiency - pick 2. We have chosen high efficiency and bass extension, so we cannot have a small enclosure. The boxes for these speakers will be roughly 14" wide, 16" deep, and 40" tall.

The fostex FX120 is the baby brother to the fantastic FX200 8" driver that is sadly discontinued. The FX120 has a very fancy cast frame and very flat frequency response. I'm a big fan of these fullrange drivers with no whizzer cone. The whizzer seems to add an offensive upper midrange peak that drives me crazy. The FX and FF series of Fostex drivers have no such whizzer and are much more laid back. The FX120 does sacrifice a bit of efficiency, and is rated at 90dB. This works out well for this design though, since the Eminence 10 is around 95dB.

Since these are both relatively fullrange drivers, the crossover point could be just about anywhere, and can be first order. The Enminence has a 3.9mH inductor in series with it to cross it over at about 325Hz. In an open baffle, the FX120 starts to roll off naturally at about 200Hz, which is close enough to not use any sort of crossover at all. I experimented with a 50uF capacitor in series, which would be the first order crossover value, but the difference was hardly noticeable, and the sound was much smoother without the cap. Less is definitely more in this case.

Don't you hate it when you have new drivers you want to hear, but the weather is too nasty to build proper speaker boxes for them? Behold the amazing "box it came in" speaker enclosures!

The cool thing is that they actually sound pretty darn good like this. Really good in fact. If a proper enclosure gets a bit more upper midrange from a solid open baffle, and bass down to 40Hz or so, these will be really great.