Subwoofers - Front Firing vs. Downward Firing

Acoustically there is likely to be only a small difference in the bass. The downfiring sub will be close to the floor surface at a high pressure point - so you may get slightly more acoustic output (think how loading helps a horn) - although this will lower the F3 of the design and reduce the sensitivity (need a bigger amp) but the resposne will be flatter. The downfiring will also reduce the audibility of any small amounts of midrange that come through.

If you want to run the sub up higher in frequency then a front firing will have the advantage as you can place it close to your speaker and use it like a bass bin. (if you do that with a downfiring sub then you will tend to smear the midrange)

For a real laugh look at HT Shack, where they test
subs intended to be woofer downward next to a wall [RELs] in a parking lot
with the woofer pointing to the side.

FWIW - Ilkka Rissanen, who conducted those "laughable" tests,
recently got a job working in the research department for
[url=http://www.genelecusa.com/products/]this[/url]pro audio
manufacturer. The bad thing is that working at Genelec gives him access to
a top notch research laboratory equipped with the very finest measuring
instruments in the industry (Klippel Analyzer, Audio Precision, MLSSA) and an
anechoic chamber. I suspect even Genelec still go outside from time to time
to make calibration measurements in a halfspace (parking lot) or suspend a
speaker from a crane and then repeat the measurement to get a calibration
for their chamber...just as Dr. Floyd Toole used to do at Canada's National
Research Council Laboratories before he got a VP job at Harman (since
retired).

Definitely a different crowd.

Bear in mind that by moving the woofer around you are altering the position of the bass driver. This is similar to moving a speaker or raising it on a stand. If your subwoofer is 2 feet high then if you turn it upside down you have rasied the woofer by two feet.

Two feet corresponds to a quarter wave cancellation of around 130 Hz ( a suckout ) - since most subs have plenty of output at 130 Hz then you will get the smoothest response by keeping it downfiring. If you tuck it up against a wall then you'll get the smoothest response.

Of course room modes will be more severe with a wall or corner placement - so "in theory" you can try to play around with position to get the most even response at the listening position by balancing quarter wave cancellation with room modes. In practice this is very hard to do as they hardly ever line up so that they cancel eachother over the sub bandwidth and you lose dynamic range in doing so (the sub is most powerful placed up against a side wall or corner). Generally a corner placement with down firing sub will give you the most dynamic range and smoothest response but this comes at the expense of overly strong room modes. Often a side wall placement with downfiring and pulled out from the wall where you have the speakers seems to be a suitable compromise - in this case the quarter wave cancellations from floor and side wall are eliminated and the cancellation from the wall behind the main speaker is not severe or deep because of the odd geometry between you and the sub.

As a general rule - as you move the sub out into the room you'll need to use a sharper low pass filter and lower cut off to achieve proper integration and don't forget to add delay based on the sub position to the listener => 1 foot closer = 1 msec of delay (roughly).

Here is an anecdotal explanation on why downfiring gives a smoother response in the 100 to 300 Hz range. This is quite an old discovery. The technical explantion is "quarter wave cancellation", which I gave above.

I checked out REL's design (uses downfiring). Richard Lord uses a 12 db/octave filter on the sub amplifier close to the corner frequency of the sub. This is very clever. It allows him to make a small subwoofer sound musical. Small boxes have higher Q (resonance) which is the main cause of HT boomy sound. By rolling off the driver before it hits resonance means that REL can maintain low Q across its output bandwidth even in a small box. The compromise with this design is that the main acoustical output is below resonance and SPL output is limited and distortion will be higher than alterantive designs. The added benefit is that the natural roll off below resonance will tend to integrate better with the in room bass boost and furthermore the 12 db/octave roll off from the amplifier will mate very well with most full range speakers. All this to say, REL makes a sub that will be highly musical (low Q) and one that integrates well with full range main speakers in a convenient small package. It may not kick the proverbial butt of the JL F113 in measurements and pure SPL output but for many audiophiles with full range speakers it may immediately work much better for them in their room from the get go. REL's design is less suitable for use with small satellite speakers. YMMV