Non-forward-firing Woofers

I plan on putting together a sealed subwoofer using the 15" adire tempest driver, probably in either Bessel or Butterworth alignments (does anyone have a preference?).

I noticed that all of the reference designs mount the driver vertically; they are all downward-firing subs. Is there a particular reason/advantage to this? Why not a standard forward-firing sub?

Also, some tower mains designs feature side-firing woofers. At what frequency is it "acceptable" to mount woofers this way? When/how does bass become non-directional?

Thanks, Won
 
Hi All,

Mainly, any higer frequency components in the sound (distortion harmonics, rattles in the driver etc.) is attenuated by being fired into the carpet, and only the low bass reaches your ears. The other reason is you can make the sub look like a nice coffee table (Or other piece of furniture), very important for the oh so important 'significant other acceptance factor'

Be careful, apparently the suspension on some subs (although I can't give examples) will 'droop' after a long period of being mounted vertically, causing distortion and reducing maximum output. Maybe others will know of offending units.

It's generally said that bass sounds become directional around 100Hz, although I beleive it's lower than that. Cross over your subs in the 60 to 80Hz region and you will be fine.

Cheers, Adrian
 
Won,

In some cases the orientation of the woofer may simply have to do with the woofers mechanical construction. Classic case would be the NHT-1259. The 1259 has a very soft suspension that will allow the voicecoil to move partially out of the magnetic gap should the driver be mounted vertical. This off-set then limits the one way X-max for that driver.


Kent
 
I think a "coffee table" with a Tempest in it would do a good job of shaking whatever books/flatware/baked goods you put on it. Maybe with something to drape over it?

Being so close to the bass is probably a plus, too. And I need to get back at my lower-neighbors for their loud parties anyway! :)

Any opinions on side-firing woofers?

Thanks, Won
 
I agree with something Adrian said above: That bass directionality is more easily perceived than most people believe.
If you use a side-firing woofer and cross it over at too high a frequency, you're going to lose imaging capability, etc. The only reason I can see why people use side woofers is to keep the frontal width of the cabinet down. This is good as far as it goes, but you're into a set of tradeoffs that I prefer not to mess with.
That's just my take on things. Cabinet configuration is subject to more compromises than the usual audio discussion, and can lead to people getting emotional about their pet views.

Grey
 
Well, being an engineer by training, I'm a bit distrustful of unjutsified statements -- not to say that there isn't one, in this case.

So downward firing woofers are better for music because they are less "punchy?" This is a term (like "wooly") that I often hear/read describing bass. What exactly characterizes "punchy" bass, and how would I "correct" for it assuming that I don't build a downward firing woofer? Is it something hard to deal with, like high pressure levels leading to nonlinearities in the response of air? What if I just chose to point the woofer to a wall? I suppose these all depend on many other conditions.

I'm sorry but I'm just a sucker for precision and unambiguity. I guess I might have picked the wrong hobby, then...

Thanks, Won
 
Being an engineer also, I understand, but audio is 50% science and 50% art. That's where the 'warm fuzzies', 'slam', 'punch', 'presence', etc. etc. come in. I do have a couple of comments, tho.
Downup-firing woofers do have the drawbacks as mentioned before (cone sag; dampening factor: discussed later). Cone sag will shorten the life expectancy of the driver over side-firing design, simply by the fact that you can rotate the side-firing driver every couple of years to extend driver life. Facing the driver directly into carpet will cause significant output loss. Most down firing systems use a reflector base for wave propogation. Care should be taken in the design allow enough distance between the reflector base and the woofer to prevent loading of the driver (as GRollins stated). A connical shaped reflector base would be best for wave propagation, but construction is far more difficult depending on your construction skills and access to tooling.

I do have some thoughts on side firing woofers. GRollins stated that side firing woofer could keep the frontal width smaller and is obviously true. The performance suffers as the proximity of the two sides increases, requiring more sophisticated defraction baffling within the enclosure (such as BMW's matrix system) to prevent rear woofer loading. This is where deeper is better.

I would like to address David Thatcher's statement about the loss of 'punch' (the artsy term) in a down firing sub. True, this is not a very scientific descriptioin, but if you think about the musical spectrum, it might give this simple terminology a little more credibility. Take the hit on a timpani drum for example. This contains both very low frequency (worble) and higher resonances (the 'slam' of the skin giving definition or liviliness to the sound). Because of the inherent damepening of higher frequencies (As Kiwi-abroad stated), steming from loss through reflection from the base, some of this 'slam' of the drum is lost. This assumes, of course, a xover point above where this is prevelant. Careful xing over at a lower frequency (60-80Hz-which I think is better for this design) then puts this 'slam' within the mid-woofers realm, requiring consideration of using a large enough cone size to produce the 'slam' factor. This is where side-firing is better giving the 'slam' back to the woofer while xing over at a higher point (80-100 Hz).

Just my take on things.

Jeff
 
I too am planning on making a sub from the adire tempest (once I finish my xovers for my main speakers). And including it in a coffee table is what I'd like. Everyone talks about down and side firing positions, what about top firing? I was thinking of having it top firing (if it's even possible) with a gap between the sub enclosure and the coffee table top, and have a hole in the coffee talbe top filled with plexiglass or glass, so that from the top the driver is viewable, but isn't open to damage or completely covered by the top (as I said there'd be a gap).
 
THE SAGGING PROBLEM

Just because it doesn't sag at first is no indication of what might happen down the road. I built one set of down firing woofers for someone and everything is fine at first - 2 years later the cones had sagged so much that the woofers were for all practical purpose acting a rectifiers. Not good. Gravity will always take its toll.

The best solution for coffee table use is to have one woofer downfiring onto a panel, one woofer upfiring onto a panel. The enclosure is vertically symmetrical. Every few months, flip it over!
The panels? Absolutely rigid, no flexing.

THE COFFEE TABLE PROBLEM

But, the coffee table idea is no good in the first place because of the exaggerated time alignment/phase problem! The woofers belong at the same listening distance as the satellites. This really is not negotiable. Else, why bother with a phase coherrant crossover like the L/R for the woofer to mid? And, on top of that there is the . . . .

THE SLOT LOADED WOOFER PROBLEM

Which is what downfiring woofers are. Qts changes plus there is a resonant cavity in front of the woofer. This must be accounted for in the overall design of the system. A down firing woofer acts much like a band-pass box and if you don't know what you are doing, it's a poorly designed bandpass box.

PUNCH (or whatever you wish to call it)

This is not soley the domain of the woofer. You will get maximum punch if the system is phase coherrent, has a flat frequency response at the listening position and the speakers and listener are properly positioned in the room so as to minimize the standing wave and early reflection problems. It's as simple as that. Square wave reproduction AT THE LISTENING POSITION in the first 30ms is what you are looking for. There are magic words here; SQUARE WAVE. Repeat after me, SQUARE WAVE, SQUARE WAVE. This is a concept that seems to be ignored. Square wave reproduction is a valid criteria.

SIDE FIRING WOOFERS

No problem and no big deal. It's OK up to about 200 or 300Hz with normal sized woofers. It's pretty much related to dispersion.

At the "listening positon". A key phrase. And the listening position isn't very big either. In fact, if you take EVERY single thing into consideration, the listening position is probably no bigger than a 2 foot diameter circle. Get things perfect for this circle and everything outside starts to fall apart. Try to expand this circle and everything inside it starts to fall apart.

You can't have everything.
 

Two Moons

Member
2001-12-06 10:47 am
Bill Fitzpatrick said:
THE SAGGING PROBLEM


THE SLOT LOADED WOOFER PROBLEM

Which is what downfiring woofers are. Qts changes plus there is a resonant cavity in front of the woofer. This must be accounted for in the overall design of the system. A down firing woofer acts much like a band-pass box and if you don't know what you are doing, it's a poorly designed bandpass box.

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How high should the woofer be from the ground to avoid this problem?

Thanks,

Dave
 
My efforts in this area used a 2"x2" leg at each corner of the baffle board. The legs were usually about 9" long. This reduced the cavity effect to pretty much of a non-problem and the Q change was somewhere in the area of 5%, if I remember correctly - it's been a long time - 1970.

One thing I do remember quite vividly was that a 150 pound coffee table with 4 downfiring 12" woofers actually raised itself off the floor by a good 2" on a particularly loud bass note!

It certainly would be easy enough to build a box with interchangable legs and run some tests outdoors.