In particular, I'm looking at Monitor Audio Gold Reference 10's, which have this alignment, and trying to figure out whether they would be suitable for wall-mounting. The description does not say anything about ports.
Fourth order refers to the design of the crossover network.
This is possible but not likely. Fourth order passive crossover networks are quite rare.
In this context, "fourth order" means that the woofer-cabinet system is a fourth order mechanical system. Actually all simple vented (or "reflex") speakers are fourth order... so "fourth order reflex" is somewhat redundant.
Recall from physics class that a simple spring-mass mechanical system is second-order. A sealed speaker is modeled as second order where the combination of the driver suspension and air compliance in the box is the "spring" and the cone mass is the "mass". In a vented system, the mass and compliance of air in the port introduce two additional "orders" --bringing the total to four.
I agree that a rear-ported speaker shouldn't be mounted on the wall, but in reality, as long as you're at least a port diameter away from the wall, the tuning won't be affected. If your front speakers are used for stereo listening, you will experience much better imaging if you keep them 2-3' away from the wall.
Monitor audio products appear to be well made and get consistently good reviews. SGHT was impressed with the Monitor system they reviewed recently. I haven't heard them, though, so I can't offer much. I personally prefer sealed-woofer satellites for home theater. Group delay is lower -- which generally equates to better transient performance --while the extra bass extension of a ported system is unnecessary when mated with a subwoofer. A popular and effective home theater approach is to use a 12dB/oct high pass crossover for the satellites and a 24dB/oct Linkwitz-Reilly low pass for the sub. The 12dB/oct rolloff of sealed satellites provides a natural complement and results in a smooth 24/24 symmetrical L-R slopes.
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