Home
TP Audio
TP_Audio Links Page
TP Audio Show Calendar
Tim's Live Sound Page
Tim's Computers Page
Tim's Hi-Fi Page
Tim's Misc. Stuff Page
Tim's Personal Page
Tim's Pool Page

 

 

 


Subwoofer Info


Timing

Owing to the cabinet design (especially if it's a long horn) and the filters employed (protective high-pass and crossover low-pass), subs will most always be time-delayed compared to the top boxes.  Time-delaying the tops to match the subs will make the subs sound and feel much stronger, as their output will no longer be heard and felt after the sound made by the tops.  The required amount of delay can be surprisingly large.  My system is all front loaded, yet the required delay on the tops is 12.5 feet.  

I've seen folks ask about putting the sub against the rear stage wall along with the backline amps.  Not good, even if it does not cause other problems with stage resonance, bleed into microphones, etc.).   With the subs back there, once the top boxes are delayed to align them with the subs, they will be way behind the backline and WAY behind the mic line time-wise.  This will create chaos both on stage and off.


Placement 1: Should they be together or split?

The common wisdom is that when the subs are split (there are subs on both sides) they "add" along the center line of the listening area.  Actually, the subs randomly cancel everywhere in the listening area except along the center line.  As such, placing the subs together most always gives the best performance (at least out of doors - anything can happen indoors, but always try them together first).  This is explained in "The Power Alley".  Of course with the subs together the time/phase alignment with the tops will be different everywhere in the room.  But this will usually not be nearly as bad as the response anomalies caused by the Power Alley.  If the subs are centered between the tops, the timing will at least be right at any distance straight out from center stage, and the error will not be too great anywhere else.  If the subs are placed off to one side, the error will be O straight out from the stack with the sub, but get pretty large as you approach the other side.  But it may not be any worse over there than if the sub was with the top box, but the top was not delayed to the sub.  (For an interactive visual of  "The Power Alley", click here.)  I've worked in several rooms in which having the subs together on one side gave the best overall performance, so don't be afraid to try it.


Placement 2: Where should it/they go?

Boundary Cancellation throws another wrench into the cogs.  If the subs are not in a corner, there will be a notch in the frequency response that is related to the distance from the sub to the wall:

2 feet will notch at ~140Hz
2.5' at ~112Hz
3' at ~95 Hz
3.5' at ~ 80Hz
4' at ~ 70Hz
5' at ~ 57Hz
6' at ~ 47Hz
7' at ~ 40Hz
8' at ~ 35Hz
(For a more complete list of distance vs. frequency go  here.)

So, no matter where you put the subs, try to place them within 2 feet of a room boundary (wall, floor, ceiling), or at least 8 feet from a boundary.  (This means that the subs must not go on the stage, unless it is no higher than 2 feet.  Even if it's a short stage, it is a bad idea to have the subs in direct contact with the stage, as the stage is in direct contact with the mic stands, drums, standup bass, etc.) 

Boundary cancellation can actually do you a favor , especially at home with your stereo.  If you have an 8' ceiling, you have a big response bump at 70Hz (see the chart).  If you place the subs (or speakers, if they are running full range) 4' from a boundary, the resulting 70Hz dip will smooth the overall response quite a bit.   (In the case of one's stereo, said boundary should usually be the side wall, if the room is wide enough to still give you a nice stereo spread.)


The Dynacord Bass Arrays white paper offers further information on the above, and more.  Also see the Mogdale/Void Practical Guide to Bass Arrays.


High pass your subs

Very few PA "subs" have much if any useful (clean) output below 50Hz (which isn't very "sub") (Typical bass amp cabinets are even worse - most start dying at 60Hz or even higher).  As such, there's no use feeding in what's not going to come out.  If you put say a strong 40Hz in you get output, but it's not 40Hz that comes out, it's mostly 80Hz and 160Hz.  This is why a lot of systems sound ok quiet but when cranked up get "thick" and the vocals get buried. (Or they have a great kick drum when loud but lose it when quiet).  At high levels, a lot of the "beef" is distortion products from the subs.  This is part of why the Danley Sound Labs, and LAB subs are becoming so popular - they go deeper (when used in sufficient quantities) and, more importantly, their distortion is comparatively low  (They are also very efficient, which is a nice bonus).  For standard ported boxes, drivers such as the GCN 1808 and Worx Audio TL1801SS offer similar advantages (they are less efficient, but don't need to be used in quantity to go deep).


How low do we need to go?

"How important can this under 50Hz stuff be - a four string bass only goes down to 41Hz?!"  Some years ago I worked with a very high quality active Hi-Fi system.  Its Isobarik low frequency section gave it very low distortion and good output into the 30's.  The manufacturer added electronic tweaks to the active crossover that took the response to around 20.  The improvement was plainly audible on music of all sorts, even when there were no bass instruments of any sort in the ensemble.  (Some say that instruments have sub-harmonics in addition to harmonics.  Perhaps this was a demonstration of the validity of that viewpoint.)

In a live application, musically relevant content below 50Hz can give a greater feeling of power, giving the soundperson and band the capability of delivering a performance of greater emotional impact at a lower (= more pleasant and safe) volume level.  Getting it from common subs is the hard part ;-(


Subwoofer "Speed"

It has been said that some subs, especially those with smaller, lighter cones, are "faster" than others (such as those with large, heavy cones).  It just ain't so.  The rise time of the signals produced by subs is very long (in other words, the signals are "slow").  Driver speed is just not an issue.  If subwoofer 'A' "sounds faster" or "has more punch" than subwoofer 'B', it is almost certainly because of one of the following:

'A' is producing more distortion than is 'B'.  The distortion products are more audible than the fundamental frequency.  Most PA subs are junk - 40Hz in at a decent level gives as much 80Hz and 160Hz out as 40Hz out.  (Many Hi-Fi subs aren't a lot better.)

'A' has better phase/time alignment to the rest of the system than does 'B'. (Owing to the various box designs, most subs - especially folded horns - will require that the top boxes be delayed to the subs to align properly).

'B' has a badly tuned port (or a badly designed horn) and is resonating excessively (it is 'ringing' or has a lot of 'overhang').


"Aux-Fed Subs"

Want to clean up your system's sound and get more power from your subs?  Don't feed stuff into the subs that does not need to be there.  Usually, the only things that need to feed the subs are the bass, kick drum, floor tom, maybe the largest rack tom (or two), and perhaps the keyboards.  Feeding anything else into them just reduces clarity.  Aux-fed subs is the ticket.  It's a bit of a nuisance, but thanks to my DriveRack I've been able to do a direct comparison, and the results are worth the trouble.  (If, like me, you don't have any spare auxes, you can use a group instead.  In my case, everything that's going to be in the sub is assigned to groups 3 and 4. 3 is assigned to the main out(s), 4 is unassigned, and feeds the subs input of the crossover. Stuff that's not wanted in the subs (snare, hat, overheads) are panned to 3. Stuff that's wanted in the subs (kick, floor, racks, bass) are panned evenly to 3 and 4. If your mixer does not have subgroups, you can use L Main for tops and R Main for subs. Pan channels L for no sub, pan center for subs. Want more subs on a particular channel? Pan the channel more to the right and readjust the channel level accordingly.  (This of course works for the groups method as well.)  (For basic block diagrams of the system configuration, click here.)  This is actually very handy for another reason: If you turn up the sub aux send on the bass, you are boosting a bunch below your crossover frequency (usually around 100Hz) and very little above the crossover frequency.  If you boost the typical "60Hz" bass EQ control, you get a lot of boost at 60Hz and below, but you also get a lot of unwanted boost as high as 500Hz, which adds to the mud coming from the bass rig.


In the End....

None of this is a guarantee that you will get the quality of sub response that you desire, or that what you get will be even throughout the room, as all enclosed spaces will have hot areas (usually near walls, behind the bar, etc.) and cold areas (null points owing to standing waves).  Experimentation with placement may help.

 

Home TP Audio TP_Audio Links Page TP Audio Show Calendar Tim's Live Sound Page Tim's Computers Page Tim's Hi-Fi Page Tim's Misc. Stuff Page Tim's Personal Page Tim's Pool Page