Phase plug question

Interesting design, thanks for the link.

I can't help you with the science of phase plugs (though, like you,
I'd love to learn). However, I'll share a few points on the philosophy behind them.

First of all, there appears to be no strict definition of what a phase plug is. I've seen widgets suspended in front of compression diaphragms, mounted on the pole pieces of cone drivers (piercing the cone), suspended in front of cones and occluding the center of dome tweeters (to roll of ultrasonic breakup)--all called phase plugs.

Originally, I believe phase plugs were so named because they can be used to alter phase relationships in the passband. AFAIK, they accomplish this by physically diverting the paths of higher frequencies. In this capacity, they are also useful as waveguides. Look at the nice wide polar response plots of those TurboMid drivers. I suspect the phase plug has something to do with that.

Also, phase plugs can physically constrain the space over a diaphragm, thus horn-loading certain portions of it, such as those that create the higher frequencies. That's what I was trying to do in the design I present in this thread.

Wgeiger posted these on the high-efficiency speaker asylum:

[013] Title: Phase Plug Modeling and Analysis: Circumferential Versus Radial Types
Author: Clifford A. Henricksen
Publication: AES-P, No. 1140, Cnv. 55 (1976-10)
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
Author: Clifford A. Henricksen
Publication: AES-P, No. 1328, Cnv. 59 ( 1978-02)
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
Author: F. M. Murray
Publication: AES-P, No. 1384, Cnv. 61 (1978-11)
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
Author: Boaz Elieli
Publication: AES-P, No. 2780, Cnv. 86 ( 1989-03)
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
Author: Clifford A. Henricksen
Publication: AES-J, Vol. 24, No. 8, Pg. 639 (1976)
Publication: AES-P, No.1126, Cnv. 54, (May-1976)
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
Author: Bob H. Smith
Publication: ASA-J, Vol. 25, No. 2, Pg. 305-312, Mar-1953
URL: none
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.

[019] Title: Tweeter Design Considerations
Author: George W. Sioles
Publication: AES-J, Vol. 4 No. 3 (Jul-1956)
Abstract: Constructional and Design characteristics of various types of tweeters are presented. The fundemental design problems of a moving coil, horn loaded transducers are addressed and include: air-chamber requirements, magnet and gap design, and horn characteristics.
The phase plug on the Turbosound TMS-3 is symmetrical, and by reducing the volume in front of the woofer, it horn loads the output.

Basically the phase plug reduces the volume at the throat of the horn, and by doing so, raises the acoustic impedance, increasing output.

The shape of the phase plug should also equalize the pathlengths, if properly designed. By changing the pathlengths, it changes the shape of the wavefront at the throat.

Also, I resurrected a twenty year old thread because I was looking for info on one of the other Turbosound phase plugs, the "axehead."


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