I think we are talking about two different approaches. Let me add some more to the discussion.

Lets say one is starting from scratchand the new full range drivers just arrived on the doorstep, the question is what to build.

Path 1 might be a back loaded horn. So the back loaded horn is built and the hope is that the bass will be outstanding and no subwoofer is required. The full range driver is all that is used and the entire audio spectrum is covered. Great intent but this is not what I see often when reading the audio forums. The builder puts together the back loaded horn and then starts looking for a sub because the very bottom end is disappointing. I have seen this many times.

Path 2 would be to design a system including a sub right from the start. The full range driver is only required to play from say 200 Hz up. No bass from the full range driver. Why build a back loaded horn? If you build a OB then a crossover is needed for the sub and for the full range driver on the open baffle. The full range driver will need a Zobel and a crossover which places passive circuit elements in the signal path. If you build a sealed box for the full range driver, you have an acoustic 12 dB/octave crossover that can be tuned by the box volume. A ported box produces a 24 dB/octave roll-off. The closed box does have an advantage of controlling dirver deflection so Xmax is less of a concern. Then all you need is some form of crossover for the sub. Either of these approaches is an easier build compared to a back loaded horn. The full range driver never sees bass frequencies so destroying the driver is probably not as big of a concern. If a full range driver in a sealed box is paired with a high efficiency woofer to produce a system that is 95 to 100 dB efficient, for 1w/1m, from 30 - 40 Hz up I believe it would play plenty loud and not risk distortion or driver damage. A 15 or 18 inch woofer will move a lot of air. Room placement is no longer a requirement to get bass. This system might be very compact.

So I am not trying to compare a full range driver in a back loaded horn to one in a sealed box. I am comparing a back loaded horn and sub system to a full range driver in a sealed box and sub system and wondering why the simple build would not be as effective as the complex build. What extra does the back loaded horn add to this type of system?

Martin

Dynamics and speed.Efficency is there also. Sure a horn will not hit the 30s, but if it can hit the 50s there is just so much more there than a vented cab ( we are talking low Qts drivers here). I have started a new project in running some sims in a loaded low Qts driver in a MLTL. IMO and from what i have seen in the sims is that by loading a driver in the form of a filter chamber/throat and installing it into a MLTL is that greater SPL can be produced without breakup and still hit the low frequencies and at the same time provide for a cut off at a point (still working on this) so that the upper harmonics of the TL action have less effect so stuffing can (possibly) be eliminated.
It still probably wont have the dynamics of a horn though. Although with a Flare at the end it appears to lessen ripple and may provide for more apparent dynamics. Dynamics is something i cant sim at the moment.
ron

It is useful to compare how the Lowthers mate with the Hedlund Horns compared to open baffles. I will refer you to a post which Paul Joppa made on the Magnequest forum because he knows a lot more about the theory why BLH work so well:

http://www.audioasylum.com/forums/magnequest/messages/2737.html

And Paul is right, with the Lowthers in Hedlund Horns, the response is flat. Thus there is no need to add crossover components. The added energy and efficiency of the backhorn down to below 100 hz allows for using subwoofers for what they are designed for, reproducing the lowest frequencies.

Now I am experimenting with open baffles. I need to work on my baffles more, but there are differences which I can comment on. The open baffles are more transparent and for sounds above the horn region of the Hedlund Horn (which is around 250 hz), the open baffle seems to be better. I might give the nod to the Hedlund Horn below this frequency, but it is too soon in my experimentation to say all this with conviction (I still need to finish my experimentation with the open baffle first and then put the Hedlund Horns back in place before I can decide for sure). The open baffle system needs an inductor to roll off the higher frequencies starting at about 500 hz. I am only using a single inductor instead of the much more complex crossover which Dick Olsher has in his plans for the Basszilla project. I am highpassing the Lowthers in the open baffle about 100 hz, and still the cone moves a lot. But I hope that I can live with this as I have a pair of 15 inch drivers and also a set of Avalanche 18 inch drivers which I hope, together, will allow me to get sufficient bass from 20 hz or below up to 100 hz as dipole woofers. I am a little worried that the Avalanche drivers will be too "slow" crossing them over at 100 hz. If so, I can always revert back to the Hedlund Horns which can go deeper.

Considering that open baffles are fairly easy to implement, and I already have the Hedlund Horns, I would not even consider trying a simple box. I would do the open baffle before trying a box. I have a set of Fried C5s, which are VERY high quality transmission line mini monitors. I thought about trying the Lowthers in those boxes, but I believe that they would not be a good match for transmission lines.

Retsel

With tipped up response of the Lowther DX4s on open baffles I need to use some sort of means to flatten out the frequency response. With the the super tweeters there I need to put a series inductor in place. It would be interesting to do away with the inductors and go back to trying a shunt capacitor instead of the inductor thus placing no passive components in the signal path of the Lowther (which of course would make the super tweeters useless, unless if I provided them their own amps.....).

Retsel

Ed, my guess (and I have MUCh less experience than you do) is that BLH horns are built if someone is not using a subwoofer. If one is using a sub and crossing the "full range" driver at 90 Hz or so, then I have not seen anyone use a horn.
Now, with a real low Q driver that is not really meant to produce a lot of bass, like a 3-4 inch fostex, one may need a horn to get down to even 80 -90 Hz, in which case it is totally justified. But if one is using an 8 inch full ranger, that can achieve 90 Hz in a simple BR all day long, the reason for a horn may become less obvious.

What do you think?
-akhilesh