Orthophonic reproducers - "phase plug" ?

[De Soto Frank]

What is the purpose / effectiveness of the "phase plug" in the throat of the Orthophonic sound-box ?

[Dave]

I came across this ..."The phase plug extends high frequency response because it guides waves outward toward the listener rather than allowing them to interact destructively near the driver. They serve to equalize sound wave path lengths from the driver to the listener, to prevent cancellations and frequency response problems. The phase plug can be considered a further narrowing of the horn throat, becoming an extension of the horn to the surface of the diaphragm. The later Orthophonics included the "phasing plug" in the throat to further improve the compression air flow which has the effect of slightly flattening the midrange peak in the frequency response. "

Personally i've never been able to "hear" the difference in sound quality between one with..and one without.
But my hearing is not what it used to be.

[Phototone]

The use of a phase plug is common practice in horn drivers for loudspeakers right up to today. If you have a speaker system that has horns driven by drivers that use a diaphragm (such as the ever-classic Klipsch models) then you will have phase plugs in the drivers.

[Uncle Vanya]

One doesn't need very acute hearing to appreciate the difference in reproduction afforded by a properly restored Orthophonic reproducer fitted with the phasing plug from that of one which lacks this modification, when said reproducer is used on a machine with no leaks in the horn. Bass response is somewhat improved, and treble response appears to be improved. The overall effect is not quite as loud as that of an earlier reproducer, for a group of strong resonances in the mid-range is damped quite substantially.