Physics of downward firing woofers


Ok ... this question will show my complete lack of knowledge about physics ... but here goes anyway:

Every now and then I come across a speaker with a downward firing woofer. I wonder: why don't the sound waves bounce off the floor back towards the speaker, rattling the bejeezez out of it and / or messing up the woofer itself?

~Jim
79ee45c9 73de 479a 9f6b ab4f75b05b18jimspov
The sound waves do indeed, at least to some extent bounce back from the floor, go through the driver cone and back into the enclosure and, while that's not necessarily the greatest strategy sound-wise, it will not really mess things up all that noticeably if there is sufficient airflow at volume. Were all the walls of the enclosure to extend down to the floor cutting off the woofer to the outside air then at volume such a sub would sound absolutely terrible and probably blow itself up. But sound waves at low frequencies tend to be more omni-directional, so having them pointed directly at the ear is not thought of as critical when crossed over low enough in frequency. 

While not treating the woofers like higher range drivers and pointing them at the listener is not as critical, doing so, for a number of reasons technically, is still a good idea and all else being equal the direct setup will sound better in terms of things like phase response, frequency response...even dynamics.

This was likely a design trend that was originally employed to offer better integration into a decor by giving it a more "furniture" look, sometimes in conjunction with a reduced size enclosure for a smaller footprint. Otherwise in general there is nothing particularly good performance-wise about that kind of approach, but they are not really egregiously bad sounding or anything.
If I recall, it has more advantages than merely cosmetic.

The main advantage is you eliminate any sort of "floor bounce" effect, where the direct woofer sound is affected by the delayed floor response, causing interference, or a noticeable dip or rise in response.

What i don’t remember is whether there is improved acoustic impedance matching as well, adding to the woofer’s efficiency.

It does not eliminate the room response modes however.

Best,

E
One example is the Alison Model 9.

Roy Alison’s later models tried very hard to eliminate boundaries as reflection points, and this was an example of his ideas. While the woofer was down firing, the entire speaker was meant to be placed along a wall, increasing efficiency and eliminating early reflections of rear walls. I’m sure that I do not do Roy’s designs and brilliance justice, but you can find more online.

The woofer was meant to excite the wall and floor at the same time, again, taking advantage of increased efficiency and eliminating the possibility of a late arrival screwing with the perceived response.



Best,

Erik
Thanks for this short discussion 
Just did a little more reading, and there are indeed reasons NOT to do a downward firing woofer, one of which is simply gravity. Gravity will pull a cone downwards, moving the resting point away from the neutral position, and straining the suspension over time.

This may mean linear excursion is better in one direction than another, but I think this is something that could be compensated for with custom drivers.

Best,

E
Jim - thanks for bringing it up

ivan and Erik - thanks for food for thought 
Bass waves are many feet long so the floor a few inches away doesn't have any bounce effect really.  The sound typically radiates out Omni-directionally and this decreases bass resonance in the room.  ATC makes down-firing subs and it's not cosmetic at all.  Even the pro versions that don't take cosmetics into account fire down.  I've had both down and front-firing subs and I think down-firing is a fine decision.  You probably can't get the perfect integration that you could theoretically get by having the sub lined up with the speakers so the whole system is time aligned and all frequencies arrive at the same time but the problems with deep bass in most rooms mean you don't typically have the sub lined up that way anyway.  It's usually placed to be out of sight or placed to minimize bass peaks and valleys caused by the room's resonant frequencies.
Another thing, some speakers have down-firing ports which is the same idea.  A long time back I went from B&W 804s to 802s.  The 804s had front ports and the 802s had bottom ports.  The measured in-room response of the 802s was dramatically better in the deep bass.  It was very close to spec whereas the 804s had huge variances. 

I noticed Revel started doing down-firing ports on the ultima 2 line.  It has its advantages.
Works great with a subwoofer but is totally wrong for proper bass woofer 
How about upward - firing woofer ?
Advantages and did advantages.... Personally, it is rare that a down firing woofer is as accurate as a front firing.  It isn't impossible, but the floor makes an effect on the bass.  The gap between the floor and woofer can act as a slot load to a ported cabinet,  giving a bass increase at given frequencies.  Kinda like adding another port. If the woofer is too close to the floor as the bass rebounds, it can cause some cancellation.... So Getting a down firing right is tougher than a front firing.  Next,  a down firing woofer does offer some cone protection if it were ever needed. Down firing ports are a very different scenario and really are a different conversation,  they are not affected by the same problems of a down firing woofer. 


inna - 

Usually done for omni or semi-omni directional designs, like Linkwitz LX Mini

http://www.linkwitzlab.com/LXmini/Introduction.htm

Floor bounce is the wrong word for bass, but interference is a real thing. Anytime you see a woofer far from the midrange and near the floor you are seeing designers think this is important.   Perhaps this is more related to having an even boundary re-inforcement? 

Best,

E

Downfiring is probably not a very good idea if you have carpeted floor. Unless I am terribly wrong.
Very timely topic for me!

Theories aside, I am interested in actual experiences of others comparing down and front firing subs in practice.

My experiences with front firing bass drivers (conventional box speakers) versus downward firing (as is the case in OHM Walsh speakers) is that floor interactions are perhaps the biggest issue to address first in either case to get bass under control. Its usually a yuge problem with suspended plywood floors found in upper levels of most homes. Not a problem with solid concrete foundations found at floor level in most homes.

I only have experience with front firing subs to-date and same problem there.

Have been debating getting a new sub and not sure about whether to get front versus down firing. There are very good quality subs of both persuasions out there it seems.
"....actual experiences...." *G* OK...

Agreeing with Erik @ 11:25, my modest bottom-firing sub is up against the wall.  If I opt to listen to my front-firing speakers, they pass up to a pair of 12" +/- 15" above the concrete floor. They in turn pass up to a 6' active with a 6" passive physically above the 12".  Both are aligned +/- 30" from the back wall to accommodate the dipole ribbon above them that takes it to top....

...when I'm not listening to omnis, anyway. *S*  Not your typical approach, but I like the results.  All the bass and lower mids one could want, unless you're into 'burping' in the car.  Not the sort to hang out here @ AG... ;)
A successful design with downward-firing woofers were the Snell Type A's. Excellent bass extension and impact! No need for a subwoofer! I own the Type AIII's and can attest to this - some of the best dynamic bass available! And the against-the-wall placement frees up a lot of space in the listening area!
Anyone have or compared the Sumiko subs? They seem well received in home audio circles. They use a downward firing active driver and a front firing passive, which is said to help make them integrate easier. Seems to make sense.
Maybe the only real listening-oriented problem I could say that I’ve ever really run across in downward-driver designs is that, as a rule, I find there can be a big difference really between "bass impact" (or "wallop") and "bass dynamics". It’s possible to have lots of impact and yet still have (sometimes) even dreadful bass dynamics. Yet many people might be inclined to say that that kind of sound is as good or even preferable to one that tics all the traditional boxes in bass reproduction (???). Although it’s usually not to quite that objectionable degree audibly, that to me may be about the worst thing you can find yourself getting into with downward designs, that and the slight loss of timing that seems to go with it. But, it’s possible these designs may work as good or better in some "problem rooms" where there may not be enough space to properly present a direct setup. But, that’s one reason why I say above that dynamics can be better with a direct setup. They are not necessarily any stronger, they are just presented more fully intact.

I know when trying to assess downward subs that there tend to be sooo many other variables involved (placement, room size, etc) that establishing the exact audible effect the downward arrangement actually has can end up getting pretty intractable.
mapman, now that (the Sumikos) sounds like the best of both options...although now it'll drift into 'the driven should be the direct, the passive the downer'.....;)
One could always void the warranty....*L*
jimsprov

What a dumb question. Anyone can see that all those downward firing waves only rattle the floorboards and disturb the renters below you. 


Why call each other names? Especially for no good reason.
Well, downfiring bass doesn't sound like a great idea to me either but I would not judge it until I hear it in each particular implementation, including room.
Bass is difficult to manage, anyway, so sometimes solutions may look counter-intuitive.
Kef Reference 3~2.  I have a pair.  Two bass speakers in each cabinet.  One firing down, the other upward.  Both held together with 'force canceling rods.  An OK speaker, they have provided me many years of happy listening.  If the idea was so good I believe Kef would have continued on with the technology.
http://us.kef.com/explore-kef/kef-museum/1990s/reference-series-models-one-two-three-four

Gravity has absolutely no audible effect on a downward firing woofer's performance. None. A speaker cone is extremely light and its performance relative to the pull of gravity is meaningless unless it's a 172 inch cone, and if it wasn't, cones facing forward would also be distorted due to gravity as they'd sag ("cone sag" isn't a thing unless maybe you get a paper cone wet from something like spilled beer or groupie vomit)…or facing up…facing anywhere. Also, I own both front firing and downward firing woofers and they both sound great on the wood floors where they woof...
+1 @wolf_garcia  I liked the groupie vomit reference😲
Trust me…you don't wanna know the details.
Downfiring is probably not a very good idea if you have carpeted floor. Unless I am terribly wrong.
I've never seen it make a difference. The Classic Audio Loudspeakers models T-1 and T-3 have down firing woofers. Somehow the bass sorts out how to get to your ears whether there is a carpet or not.

I think there is something to the height of the speaker off the floor, but my Fulton Premieres had down firing woofers and it was a lot closer to the floor than my CARs are. Overall the Fulton was less efficient, but I don't think that had anything to do with the woofers in particular.


inna 

I just make a joke. I learned long ago at a motivational seminar that "there are no dumb questions" 

Not sure what motivations they were using. The only thing that motivates me know is a good night sleep. 
Ralph, did you remove the carpet to compare or move everything to identical room ?
Now, why do I get tired of hearing about Classic Audio speakers? Seemingly no reason, right?
The main advantage of a downward firing sub is to avoid forward driver modulation that would compete for your attention by adding confusion to the highly perceptive mid and hi frequencies.
try reversing your subs and experiment with 180 switches and this is for many folks an improvement in focus.
Downward subs, when used with neutral speakers, will be complementary
 in balance.
Ralph, did you remove the carpet to compare or move everything to identical room ?
As you know I have a set and have shown with them at shows since the early 1990s.

I've had carpet underneath mine and also bare wood floors (went from wall to wall to tribal rugs about 10 years ago). At shows we've often had carpet and it seems like other factors of hotel rooms are issues- usually we are trying to tame the highs in the room. Things like false walls and false ceilings are far more problematic for bass. Such things act like bass traps but usually you don't run into problems like that at home.
Sound travels at about 1087 ft/second so the length of a 40 hz bass wave in 1087/40=27 feet.  Carpeting won't have any significant effect on a wave that long.  It's why bass traps have to be so big. 
The Vandersteen 2Wq sub has 3 eight inch woofers facing downward at the bottom of the cabinet with no holes for woofer exposer. It seems to have slots cut around the bottom outer edges of the cabinet where sound comes out from the back side of the internal woofers. All you have to do is listen to these subs properly setup and make your own decision as to how they sound.
I have 2 REL's and have wondered if carpet, or a pad under the drivers is better for sound (to reduce relection) or not. I have not expermented with it yet to see. Any thoughts?
I’m not sure here, but I’ve always assumed that the idea is to get the driver as close to the primary reflective surface - in this case, the floor - as possible. An equivalent strategy for a front firing subwoofer would be to place it facing away from the listening position and close to the wall opposite the listener.

In either case, the theory is eliminating quarter wave interference in the output from a subwoofer. This is the most destructive cancellation and will cause dips in output at the frequency where the first (powerful) wall reflection encounters subsequent output from the subwoofer.

Imagine that you hold a synth key to continuously generate output over the octave between 25 hz and 50 hz. The sub is continuously pouring this output into the room. Also bear in mind that these long waves are omnidirectional.  As you first hit the synth key, the the initial wave migrates across the room and bounces back from the nearest reflective surface (a round rip measured in milliseconds). The reflected wave then encounters the subsequent output from the sub because you’re still holding the key down and generating new (subsequent) output at 25-50 hz. If this reflected wave is at the same frequency as that output, the reflected wave will cancel the new wave and create a dip in frequency response.

If you search for it, you can find the math to calculate how the frequency for quarter wave cancellation changes with distance to the nearest reflective surface. It "shifts" with time/distance and won’t "come back" at the same frequency it left the sub.

In a nutshell, the closer the driver is to the primary reflective surface, the higher the frequency of the quarter wave destruction. If you get the distance low enough, the destructive interference occurs above the subwoofer’s high cut and the sub’s output should be very smooth. In a downward firing woofer, the distance will always be small (inches from the floor) and the quarter wave cancellation will be high enough in frequency to spare the sub’s subsequent output.

I think the idea is to save you from yourself. In theory, it should be harder to place a downward firing sub "improperly". I can’t swear that this is the best theory to employ in sub design, and I might be wrong in assuming that this is behind the idea of a downward firing sub, but it’s always been my assumption.
Would you put downfiring speaker or sub on a spiked stand instead of the floor, or it would make it worse, or no difference? Carpeted and not carpeted floor. Some put speakers, without downfiring woofer, on stands and claim the improvement, at times big one.
 My two Vandersteen 2wq downfiring subs. are decoupled from the listening room floor.  The subs. original spikes were replaced with brass audio points which sit in Herbie's Audio Labs Giant Cone/Spike Decoupling Gliders which are .67" high.  Raising the subwoofers up off the carpet yielded a bit tighter deep bass but reduced the perceived in-room bass volume necessitating a slight increase in sub. output level.  

The best thing about it is that the deep bass is now only in my dedicated listening room and not being transmitted throughout the house!