What are the advantages of having a phase plug for a mid/woofer?
I cant think of a reason to have one unless you are playing kilohertz stuff. The reason for a phase plug is to redirect high frequencies produced near the center of the woofer, strait out into the room instead of bunching up before they leave the apex of the cone.
You can make the speaker look even more phallic
But seriously, it can help very slightly with power compression. Probably not significantly unless the phase plug is designed specifically for that usage (see Kilomax drivers or Parham's mod).
Could I rephrase that, and tell me if its correct?
Without a phase plug, the high frequencies from the center of the cone will tend to ripple outwards on the surface of the cone as going forward. But, with the phase plug, it avoids this, and the signal tends to go straight out without this distorting effect?
In practice, phase plugs on home hi-fi drivers don't have much effect. Look at how the best manufacturers do it.
There is an other effect that is easy to measure : on some badly designed drivers the dust cap creates a resonating cavity that will add a nice peak before the roll off. Many times this seems to be due to break up modes of the cone (and it is...), but many times also, when removing the dust cap, no more peak...
So this happens not because of the phase plug but because of removing the dust cap.
Any confirmation/contestation about this ?
What’s the difference between a dust cap and a phase plug?
Why use a phase plug?
• Reduced moving mass
• No gluing or other interaction with the cone
• Better cooling of the voice coil
• No cavity resonances
• More consistent high frequency response
Why use a dust cap?
• Avoid air leakage and “chuffing” noise
• Larger cone area
• Conceal a small voice coil with a big dust cap
• Cosmetical reasons
Mission: Phase Plug
The purpose of a phase plug is 4-fold:
1) Reduce front cavity volume [Vc]
2) Reduce effective throat area [St] 'seen' by the driver through use of radial or circular slits placed in close proximity to the driver's diaphragm.
3) Mitigate standing wave modes in the front cavity through the judicial placement of a select number of these slits.
4) By these means, a high compression ratio [St]/[Sd] may be achieved to improve conversion efficiency, flatten frequency response, and extend it as well.
P.S. Here are the abstracts for the three most important articles I have found to date. There are additional articles by Dodd, Henricksen and others, available at the AES website.
Title: An Application of Bob Smith's Phasing Plug
Author: F. M. Murray
Affiliation: Jamse B. Lansing Sound, Inc., Northridge, CA
Publication: AES-P, No. 1384, Cnv. 61 (Nov-1978)
URL: AES Publications: Preprint Search
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 optimization of the annular ring type, little did he realize how studiously he would be ignored.
Abstract: His design is now incorporated into a large compression driver capable of operating to the high frequencies where this design is important
Title: An Investigation of the Air Chamber of Horn Type Loudspeakers
Author: Bob H. Smith
Publication: ASA-J, Vol. 25, No. 2, Pg. 305, Mar-1953
Affiliation: Division of Electrical Engineering, University of California
URL: Cookies Required
Abstract (1): 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.
Abstract (2): The following specific cases are analyzed: (1) the case in which the horn throat enters the air chamber by means of a single orifice, (2) the horn throat enters the air chamber by means of a single annulus of radius [r] and width [w], and (3) the horn throat enters the air chamber in [m] annuli of radii [r1],[r2],...[rm] and widths [w1],[w2],...[wm].
Abstract (3): The analysis reveals that the radial perturbations caused by the horn throat excites higher order modes. At the resonant frequencies of these modes the horn throat pressure becomes zero and the loudspeaker does not radiate. By suitable choice of annulus radii and widths the first [m] modes may be suppressed and the corresponding nulls in the output pressure eliminated.
Title: A New Methodology for the Acoustic Design of Compression Driver Phase-Plugs with Concentric Annular Channels
Author: Mark Dodd
Affiliation: GP Acoustics, Ltd., Maidstone, UK
Author: Jack Oclee-Brown
Affiliation: GP Acoustics, Ltd., Maidstone, UK
Publication: AES-P No. 7258, Cnv. 123 (Oct-2007)
URL: AES E-Library A New Methodology for the Acoustic Design of Compression Driver Phase-Plugs with Concentric Annular Channels
Abstract(1): In compression drivers a large membrane is coupled to a small horn throat resulting in high efficiency. For this efficiency to be maintained to high frequencies the volume of the resulting cavity, between horn and membrane, must be kept small. Early workers devised a phase-plug to fill most of the cavity volume and connect membrane to horn throat with concentric annular channels of equal length to avoid destructive interference .
Abstract(2): Later work, representing the cavity as a flat disc, describes a method of calculating the positions and areas of these annular channels where they exit the cavity, giving least modal excitation, thus avoiding undesirable response irregularities . In this paper the result of applying both the equal path-length and modal approaches to a phase-plug with concentric annular channels coupled to a cavity shaped as a flat disc is further explored.
Abstract (3): The assumption that the cavity may be represented as a flat disc is investigated by comparing its behavior with that of an axially vibrating rigid spherical cap radiating into a curved cavity. It is demonstrated that channel arrangements derived for a flat disc are not optimum for use in a typical compression driver with a curved cavity. A new methodology for calculating the channel positions and areas giving least modal excitation is described.
Abstract (4): The impact of the new approach will be illustrated with a practical design.
I agree with most of the above comments. On a woofer a "phase plug" isn't really a phase plug in the performance sense. The original ussage and the name comes from compression drivers where wavefront phase uniformity is achieved by careful manipulation of the path lengths from all parts of the diaphragm. For woofers there is a bit of high end fashion to it because some classic drivers (the Lowther) had one, so "it must be good".
The addition of a dust cap gives a variable for adjustment of the woofer's high-end response. You can usually tame the top peak with the right dustcap.
Nearly all of my woofers are made with a phase plug. There are several benefits in the way that it is done. First, it is part of the cooling system. The VC windings sink heat into the aluminum former. The former sinks heat into the full copper sleeve on the pole quite quickly. There it is dissipated into the large steel pole. The phase plug then pulls heat from the steel pole where it can be sent to the external air instead of into the enclosure like in a traditional vented pole. This whole process increases the power handling of the driver and lowers power compression.
The biggest benefit though is to extend the clean upper end response. This is more about eliminating the dustcap than adding a phase plug though. The dustcap is always responsible for some kind of breakup mode. The frequency it is at depends on the size, thickness, and stiffness of the dustcap. I've measured drivers, the 18sound 6nd410 for example which had a breakup peak of nearly 15dB at 4500hz due to the dustcap! Removing the dustcap got rid of that resonance. Even if crossing a full octave below with a high order crossover, the resonance is still problematic. Especially in a driver with high 2nd harmonic distortion. The harmonics of the fundamental tones below the crossover will excite the resonance to levels higher than the fundamental in some cases. EQ can do nothing to prevent those resonances from being excited. Damping the dustcap will shift or lower the magnitude of the resonance but it will still be there. Eliminating the source of the resonance, dustcap in this case, is the only clear solution.
When a dustcap is put on, it also creates a chamber of air at the top of the pole. This chamber of air has a resonance to it which often times can be in in the area the driver is intended to be used. With driver excursion this chamber of air changes. It gets larger on the outward stroke and smaller on the inward stroke. With a solid non-vented pole piece this chamber of air expanding and contracting creates a continually varying loading on the cone which leads to very high distortion. The solution is to typically vent the pole. The problem is now you have a chamber of air that is "tuned" to a certain frequency by the "port" or pole vent. This creates an even more significant resonance. Then as the cone moves in and out the chamber of air is changing in size while the port stays the same size. The resonance created by this tuned port is continually varying in frequency based on the excursion of the driver. Inward excursion the tuning goes up. Outward excursion the tuning goes down. It is a continually variable resonance that nothing can be done with EQ to correct. The most logical solution is to get rid of the chamber of air and any resonance is now eliminated.
There is also some benefit in a wideband woofer to help smooth the upper end response. The TD15M for example shows a smoother response from 3KHz to 4.5KHz with the phase plug than with simply removing it completely. There is also slightly better off axis response in the 1KHz to 2KHz range with the phase plug on.
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