Greets!

I'm only aware of AES papers on the subject. There may be others also, these are just the ones I've browsed. Like these folks and the pioneers of sound reproduction though, I learned most of what I know from experimentation:

[013] Title: Phase Plug Modelling and Analysis:

Circumferential Versus Radial Types

Publication: AES-P, No. 1140, Cnv. 55 (1976-10)

Author: Clifford A. Henricksen

Abstract: Mechanical modeling of a simple phase plug

yields an electrical-mobility equivalent circuit; a

single-frequency notch filter. This applies directly

to "normal" circumferential-slit configurations. The

analysis is then applied to a radial-slit phase plug,

[014] Title: Phase Plug Modeling and Analysis: Radial

Versus Circumferential Types

Publication: AES-P, No. 1328, Cnv. 59 ( 1978-02)

Author: C. A. Henricksen

Abstract: Mechanical modeling of a simple

two-dimensional phase plug and diaphragm yields an

electrical mobility equivalent circuit; a two-pole,

low-pass filter. At higher frequencies, this analysis

becomes incomplete, and a model presented by Merhaut

(1975) is us

[015] Title: An Application of Bob Smith's Phasing

Plug

Publication: AES-P, No. 1384, Cnv. 61 (1978-11)

Author: F. M. Murray

Abstract: The war of the phasing plugs still rages

after more than 25 years. Compression driver phasing

plugs have vacillated between annular rings, salt

shakers, teardrops, and now radial slots again. When

Bob Smith provided simple design criteria for

[016] Title: The Dual Coil Inductively Coupled

Loudspeaker System Performance Optimization and the

Application of an Acoustic Phase Correction Plug

Publication: AES-P, No. 2780, Cnv. 86 ( 1989-03)

Author: Boaz Elieli

Abstract: Analysis of the dual coil inductively

coupled loudspeaker system using an electrical

equivalent circuit is presented, and some of the

practical design aspects relating to the system

optimization are discussed. The application of an

acoustic phase correct

[017] Title: Ultimate Performance of Wide-Range

High-Frequency Compression Drivers

Publication: AES-J, Vol. 24, No. 8, pp. 639 (1976)

Author: Clifford A. Henricksen

Abstract: Performance equations are developed from an

electric mobility model of a typical electromagnetic

compression driver. The analysis is independent of

impedance and relates ultimate performance to

material, air, and phase-plug variables.

[018] Title: An Investigation of the Air Chamber of

Horn Type Loudspeakers

Publication: ASA-J, Vol. 25, No. 2, Pg. 305-312,

Mar-1953

Author: Bob H. Smith

Abstract: The front air chamber design is treated as

a boundary value problem which yields a solution of

the wave equation for the general case in which the

horn throat enters the air chamber in a

circumferentially symmetrical manner.

GM