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The crossover for the Etude TL is quite flexable and is easily adapted towards creating a timbre matched complete surround system. This article outlines some modifications to suit a centre and surround speakers.
It's important to first consider where the speaker will be located. There are three likely scenarios: free-standing, on-wall and flush. They each have an impact on how the adjacent wall reinforces the sound.
When flush mounted, the bafflestep compensation provided by the second woofer is not required. As a result, the response below 200 Hz is boosted by 3 dB. This could result in a muddy sound. This problem is eliminated by removing the second woofer along with its associated crossover network. The box volume can be cut in half and the cost is reduced. On-wall mounting is an in-between option that is probably closer to the flush mount scenario.
Mains - sealed vs TL
In a 5.1 or 7.1 channel system the need for TL mains may be questioned. If you prefer a sealed version, allow 9L of net volume per woofer with the box lightly filled with dacron. A predictable 2nd order roll-off at 80 Hz will be the result. This is easy to cross to a subwoofer.
Often it is worth letting the TLs run lower as having two extra bass sources overlapping with the subs can result in a smoother in-room bass response. This is discussed and illustrated in my bass integration guide article.
If you have the mains under a screen, then the ideal setup includes three idential TL mains across the front which would be freestanding. If they must be placed close to the wall due to space limits, you may consider leaving out one of the woofers. The vent should have 100mm clearance to the wall. In this case, the wall behind the speakers should be treated with relatively thick absorption - 50mm or greater is suggested. As the speaker is moved away from the wall, reinforcement is lost and the second woofer becomes necessary.
A more typical scenario involves a smaller centre sitting horizontally. This is the worst possible arrangement as any seat that is not directly in the middle will result in the mid and tweeter becoming misaligned, as shown below:
The woofer is not critical due to the lower crossover point, but the relative distances to tweeter and midbass are important. The ideal seat is actually slightly towards the right of the centre seat, but when moving further to the sides you can see there is a misalignment, causing destructive interference. Vertical alignment of the tweeter and mid eliminates this problem.
The simplest has a vertical sealed box. Perhaps a more appealing version mounts the tweeter over the midwoofer, then puts the other woofer on the side. The asymmetry can be hidden with a grille. In that case I would have the grille only over the woofer and set the midwoofer forward so that with the grille in place, the front of the grille is flush with the baffle around the midwoofer. This avoids baffle edge diffraction.
Surrounds should be placed on side and rear walls with a low profile and the second woofer removed. The box should be lightly filled with dacron with an internal net volume of 9L. You might also consider flush mounting where a sealed box is installed in the wall cavity. This completely avoids baffle edge diffraction and also offers a neat solution, although sound isolation considerations do also come into play.
With the information provided here, you can now make some choices:
1. Budget vs higher quality crossover
A budget version of the crossover uses electro caps. You might choose this option for surrounds that many consider less critical. Keep in mind that such caps are less stable over time. Otherwise, omit the caps shown in yellow and use better quality caps.
2. Flush or on-wall vs free standing mounting
For a free standing version, build the crossover as shown with two woofers. Otherwise, you can leave out the second woofer and its associated crossover network. As a general guide I'd suggest building surrounds on the wall. With the centre speaker you have to consider the conditions involved as it could go either way.
3. Vertical vs horizontal mounting
For surrounds this depends on the distribution of seating. Choose the option that results in lesser misalignment of tweeter and mid.
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