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[Stephen Thorpe]

I'm a live guy doing a DJ system club install right now. We have 20x16 dancefloor and the sub is going to be built into the top left corner and fire across (16ft) to a brick wall. The sub is an EV Deltamax 2x18 box.

If a 63hz wave is about 17.9 feet, is it a good thing or a bad thing to be shooting it at this wall? Should I push the owner to sacrafice dancefloor space so I could fire it away from the wall?

[Randy Pence]

which top is getting aligned to the subs and how are you deciding which other 3 will not?  You need more sub.

[Stephen Thorpe]

All of this gear was from an club that the guy had before building this one, plus a few pieces bought at auction. I tried to get him to do an install with a new system but he spent all of his money on building the club. So basically I'm just putting it together the best i can and leaving it open for easy upgrades. The tops are 4 EV PI15-3's. 2 covering the dancefloor and 2 filling the back of the house. Processing is a DRPA. I don't even wanna talk about the amps.  

I know that we're a little underpowered but I have to use what I got. So adding more subs isn't an option.

[Randy Pence]

gotcha.

If it isnt already, get the sub more wall loaded. It might solve your wall issue and the output of one double 18 really isnt enough already.  Is the current plan to install it right near hte dj?  That is where dancefloors seem to fill up first.

[Douglas R. Allen]

Bob Diegel wrote on Mon, 06 February 2006 23:40

I'm a live guy doing a DJ system club install right now. We have 20x16 dancefloor and the sub is going to be built into the top left corner and fire across (16ft) to a brick wall. The sub is an EV Deltamax 2x18 box. If a 63hz wave is about 17.9 feet, is it a good thing or a bad thing to be shooting it at this wall? Should I push the owner to sacrafice dancefloor space so I could fire it away from the wall?

16 feet is very close to one wave length so that will be ok.
What you want to avoid is 1/4 wave length or 4 1/2 feet if you think 63 hz is going to be a problem. Allot of the "Thump" is more around 80hz with 3 1/2 feet that you want to avoid.

What happens is if the wall is 4 1/2 feet away the 63 hz note will hit the wall and come back and reach the drivers just as the next wave is released. Traveling 9 feet total. But it will be out of phase and cancel the next wave.( I am unsure if this is the best wording for this so perhaps others can chime in here.) Here is something to try so you can see. Put on a 80 hz tone and have your sub almost touching a wall facing it. Now start pulling the sub out from the wall. At around 3 1/2 feet you will notice a big drop in spl. As you move it farther out it will increase again. To avoid cancellations have the subs closer than 3 1/2 feet from a wall and not on a stage that is 3 1/2 feet high. Touching a side wall or on the floor is best most of the time. Of course you have to consider the other walls and ceiling as well. If you don't have access to software like Smaart use an 80hz test tone and check spl around the room as you move the sub to different spots. Sometimes an unexpected location can give you the best results.

Kindest regards
Douglas R. Allen

[Antone Atmarama Bajor]

     Phase Cancellation.  If the sub is 1/4wave of a frequency from a boundary the reflected wave will be 1/2wave late recombine with the incident wave and cause phase cancellation.  The ascending portion of the wave will overlay with the descending portion of the wave essentially 180degrees out of phase.  Though the reflected energy may have lost a few dB of Umph so Total cancellation probably wont happen but quite significant cancellation will occur.

    I'm guessing that similar things happen at 3/4 Wave and any 1/4 wave multiples but to a diminishing degree.  The further the distance.  

    So the moral of the story is to avoid cancellation make sure that your sub is less than 1/4 Wavelength from the walls at its High crossover point and make sure your top boxes are more than 1/4 wavelength of their low cut distance from a boundary or soffit mounted?

Antone-

[Tim Padrick]

Frequency/distance chart: http://www.padrick.net/LiveSound/CancellationMode.htm