> One possible application for something like this would be to design a loudspeaker where the time-of-flight of the soundwave travelling from a tweeter to the l

> One possible application for something like this would be to design a loudspeaker where the time-of-flight of the soundwave travelling from a tweeter to the listener would be the same as the same sound sent from a midrange speaker. Most designs do not adjust for such differences. Most designs, true. Very high-end speakers do sometimes incorporate all-pass filters in parts of their crossovers to compensate for the set-back of the apparent sound source in woofers. But very few people tilt their speakers so that the distance from the midrange and woofer to their ears is exactly the same as that from the tweeter at the listening position, and getting complicated crossovers to be reliable at 100 watt power levels is very expensive. Time-of-flight compensation is trivial in the digital domain, so some active crossovers (which operate on line level signals, before the power amplifiers) also do this. The downside, of course, is needing one power amplifier per speaker cone, not just one per box, and the extra wires if using separate amplifier blocks rather than active speakers. The consensus seems to be that it's not worth the trouble in ordinary listening situations.