Rules for Subwoofer placement and stacking

Frequently asked questions and the answers to them.

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Bill Fitzmaurice
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Rules for Subwoofer placement and stacking

#1 Post by Bill Fitzmaurice »

Sub Placement:
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:

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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:

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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.

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.

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If you have more than two boxes stack them higher.
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You can go wider too, stacking eight cabs in a 2x/2x (or four cabs in a 1x/2x) stack as shown here.
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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.
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If there's a corner available aim the assembly at it.
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With more than 2 cabs stack them above the first two.
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Or go side by side:
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Room Size:
The usual placement rules don't always apply in small rooms and home theaters. In small rooms there are numerous boundary reflection sourced nulls, which vary in frequency at every listening position in the room. Placing multiple subs close to each other won't reduce these nulls. One possible cure is to place multiple subs as far from each other as possible. In home theaters that can even mean on both the front and rear of the room. The only way to find out what's best it via experimentation.

Using Horns 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.

Phase:
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.

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