Frugel-Horn and Voigt relationship

You'd be unlikely to bother, because they're not working the same way. The Voigt pipe (ML-horn) you picture is a standing-wave generator. It's not impedance-matched over any part of its usable operating bandwidth and is simply employing resonant action: by design you don't want to be limiting that internally except by damping as it's essentially their raison d'etre.