While it’s customary with PA to have speakers to either side of the stage, that’s usually not the best way to place subs. Subs work best when they’re placed either close together for mutual coupling, or spread very wide to cover large areas. The basic rule is to have them either less than a quarter-wavelength apart or more than two wavelengths apart for their pass band, which for 40 to 100 Hz means less than 2.8 feet or more than 56 feet.
Boundary loading should be used whenever it’s practical to do so. Having subs next to a wall gets you 6dB of additional sensitivity, and putting them in a corner an extra 12dB. In most cases you’ll have best results aiming the subs towards the wall or corner with the mouth about a foot away from the boundary, using the room corner as an extension of the horn as shown here:
The next diagram shows wall placement with two cabs. In effect this creates one larger sub from the two smaller ones, with the room walls and cabinet sides creating a large horn mouth:
If there is a 'prime directive' when it comes to sub placement it is this: Never place subs with their radiating planes between 2.5 feet and 8 feet of a boundary.If you do at some frequency the within the 35 to 100 Hz passband, depending on the distance, the reflected wave will be 180 degrees out of phase when it meets the original wave again, cancelling it out.
Outdoors is the toughest challenge, as there are no boundaries for reinforcement and no room pressurization (cabin gain). In this case create a virtual extension of the horn by facing it downward, the upper edge supported to place the mouth at about a 30 degree angle. The picture below shows a Titan, upright and ground-angled.
Coupling the horn mouths with a V coupler plate, details of which are shown in the plans, increases the effective horn path and mouth area, giving an extra 3dB for free.
If you have more than two boxes stack them higher.
You can go wider too, stacking eight cabs in a 2x/2x (or four cabs in a 1x/2x) stack as shown here.
With T48s the spread between the two sets of mouths means they'll couple only below 100Hz, so you need to crossover at 100 Hz or lower. T39s laid out this way can crossover as high as 125Hz.
If there's a wall available aim the assembly at the wall as shown; this can be done indoors or out.
If there's a corner available aim the assembly at it.
When using wall or corner placement with a stacked array pull the array 6 inches further out from the wall/corner for every additional layer of cabs.
Here's the same treatment applied to Tubas.
With more than 2 cabs stack them above the first two.
Or go side by side:
This shows a pair of coupled Tubas using wall loading. With two cabs the distance from the wall is 9 inches for T24s, 12 inches for T30s, 16 inches for T36s. For each additional layer of cabs add another 4 inches distance.
With a pair of corner loaded coupled Tubas the distance out is 6 inches for T24s, 9 inches for T30s, 12 inches for T36s. Add another 3 inches for each additional pair of boxes high.
The usual placement rules don't always apply in small rooms and home theaters, as explained here:
Small rooms cause boundary cancellations close to the subs, so they don't seem loud there, while further away where there are fewer or no cancellations they pound. In this case they'll work better split, and the only way to know where they'll work best is via experimentation. You need to do this when the venue is not open for business.
Using Tubas with other subs:
Mixing horn subs with direct radiators is a recipe for disaster. The response of the two formats is so different that it's impossible to EQ either one of them properly, while differing phase can easily lead to blown drivers. Never mix even different models of subs, including Titans or Tubas, let alone different formats.
It’s critical that your subs and mains be in phase at the crossover frequency, or their outputs will cancel each other out. The best way to check it is with a signal generator or test CD, running a signal at the crossover frequency through the system. If your crossover, sub amp or system controller has a phase inverter function try it with the feed to the sub amp in both positions; you’ll be able to hear one position is louder. With a simpler system make up two sets of interconnects from the crossover to the sub amp, one regular, one with reverse polarity, marking them so you know which is which. Placement of the subs and mains will change their phase relationships, so test it with every new venue or change of speaker placement.
Frequently asked questions and the answers to them.
Moderator: Bill Fitzmaurice
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