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• to be effective at bass frequencies, a BLH needs to be as long as a FLH would be; I've yet to see a BLH, with the possible exception of the Tannoy Westminster, that actually has a long enough horn path for supporting bass frequencies down into the real low bass.
  
  
• the horn mouth needs to be the correct size for bass frequencies; few, if any, are.
  
  
• sound needs a finite time along the BLH. It then mixes with the direct sound from the driver. The sound from the BLH will always be out of phase with the directly-radiated sound. At certain frequencies, it will be 360° out of phase, or multiples thereof, and will reinforce the directly-radiated sound. At other frequencies, it will be 180° out of phase, or 540° etc, and lead to a cancellation. At most frequencies, it will be somewhere in between. At all frequencies, it will lead to time smear. It's debatable how much that matters at low frequencies, but it'd be better to avoid this time smear.
  
  
• most BLHs fire the bass towards the back wall, and almost all at a different height from the direct radiator, which results in a very different way of loading the room.
  
  
• all existing BLHs are folded. There's a very good argument to be made that the different sections of the BLH actually act as resonators at specific frequencies; if you look a the frequency response of real-word BLHs, they all show a series of peaks and valleys.
  
  
• another way of looking at BLHs is to see them as transmission lines; they would probably be more effective if built to the rules of transmisson lines.
  
  
• transmission lines are really only needlessly complicated bass reflex alignments. Instead of a BLH, you'd probably get better results from a properly sorted BR alignment.

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