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Phase Plug for Planars
Phase Plug for Planars
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Old 14th February 2017, 09:01 PM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default Phase Plug for Planars

Recently, I've had very good results using phase plugs on midrange drivers. (Car Audio | DiyMobileAudio.com | Car Stereo Forum - View Single Post - Imitation)

Phase plugs on tweeters has been (mostly) solved, so I thought I'd examine what happens on a planar.

First off, what's a phase plug for?

I would argue that a phase plug is generally designed to minimize phase differences between different points on a radiator. For instance, if you are listening to speaker with an 8" woofer, and the speaker is rotated 45 degrees, the FAR side of the cone is 5.66" further away than the near side of the cone. That pathlength difference creates a dip at 1193Hz due to the phase difference. This is because 1193Hz is 11.33" long, and one side of the cone is radiating sound that's one half wavelength out-of-phase.

That dip at 1193Hz will basically limit how high you can listen to your speakers.

A phase plug can improve things, by equalizing those pathlengths.

This article covers this well : Phase Plugs

Click the image to open in full size.
I think the BG NEO 8 is a particularly good candidate for a phase plug. This is because the radiator is capable of playing up to 10Khz and beyond, but the geometry of the radiator limits it's upper response. (Because it's so big.)
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Old 14th February 2017, 09:05 PM   #2
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

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Click the image to open in full size.

Here's the frequency response and distortion of the NEO8 with six different configurations.

I'll explain what the different options were shortly.
To my eyes, there are three good solutions here:

1) the bare driver performs fairly good as it is

2) The Costas array phase plug reduces the high frequency peak of the NEO8, but without the cost/complexity of a capacitor. The frequency response is also a little smoother.

3) "Phase Plug 2" extends the highs, but at the expense of frequency response which is lumpier overall

I'm not real excited about the results of phase plug three and four. Basically the distortion goes up when too much of the radiator's surface is covered, just as it does in a horn.

Last edited by Patrick Bateman; 14th February 2017 at 09:21 PM.
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Old 14th February 2017, 09:30 PM   #3
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.
This is the Costas phase plug. A Costas array is an array where the elements are literally random. I considered flipping a coin 48 times to come up with this pattern, but wound up using a random number generator instead.

The appeal of the Costas array can be seen in the patents of Voishvillo at JBL. Basically, dips and peaks are frequently caused by geometry. If you have two points on the radiator that are radiating the exact same frequency, the pathlength difference can sometimes lead to a peak or a dip. (Depending on whether the pathlength difference is one wavelength, or one half wavelength.)

So the Costas array works very simply, it randomly alters the radiator by removing the radiation from some points on the surface.
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Old 14th February 2017, 09:36 PM   #4
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.
Phase plug #2 was designed to mimic the effect of a log spaced array.

Basically 50% of the top and the bottom of the NEO8 is masked off.
25% of the center is masked off.
The idea is to get something like power tapering, where the edges of the array have the power reduced to widen the beamwidth and extend the highs of the array overall.
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Old 14th February 2017, 09:42 PM   #5
Cal Weldon is offline Cal Weldon  Canada
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Phase Plug for Planars
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Old 14th February 2017, 09:43 PM   #6
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.
Here's phase plug number three.

This is simply an evolution of phase plug two.

I don't recommend phase plugs 3-4. If you look at the distortion performance, you'll see that the distortion is rising due to compression that's too high. Phase plug #3 has distortion that's about 5dB higher than the Costas Phase Plug.

Oddly enough, the distortion on the Costas Phase plug appears to be a little bit better than the NEO8 with no phase plug at all.

Last edited by Patrick Bateman; 14th February 2017 at 09:46 PM.
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Old 14th February 2017, 10:00 PM   #7
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

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Here's the polar response of the four phase plugs.

To me, the Costas phase plug is the winner here. In the midrange and the lower treble, the Costas phase plug response is virtually the same as the 'bare' NEO8. But in the top octave, the Costas phase plug is doing a couple of things that's positive:

1) it is raising the output level off axis. This is because of the geometrical cancellations of the large NEO8 surface

2) it is flattening the peak at 11khz. For the same reasons.
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Old 14th February 2017, 10:05 PM   #8
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

I was curious to see how the NEO8 would react to EQ. I've noticed that it is frequently impossible to use EQ to raise the output of a tweeter above 10khz or so. This is because the rolloff isn't electrical, the rolloff is caused by cancellation. So you can pour all the power you want into the driver, it just won't get louder. (And this is a great way to blow a ribbon to bits, literally make it explode.) I've only blown up two drivers during testing in my life, one was a planar magnetic, the other was a ribbon.

But there's good news here! The NEO8 *does* appear to be "eq-able". In the pic above, I've used 10dB of boost to bring up the top end of the NEO8. BTW, be careful with this in the real world, 10dB of boost is pressing your luck, from a durability standpoint.
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Old 14th February 2017, 10:18 PM   #9
chrisb is offline chrisb
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well, the referenced article seems to refer specifically to "horn loaded speakers* - (NOT the silly polished bullets sometimes seen on hi-fi speakers)" -
*which a dipole planar tweeter used as shown here definitely is not.

I'd be concerned about resonances induced in the moving diaphragm by randomly sealing the apertures - up to 50%, if I read correctly

but thanks for yet again another creative use of 3M Blue painter's masking tape.
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Old 14th February 2017, 10:28 PM   #10
Patrick Bateman is offline Patrick Bateman  United States
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I was concerned too.
Resonances would be indicated by peaks in the frequency response and a rise in distortion.

But the Costas array phase plug actually had lower distortion.
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