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Author Topic: Fostex FE166En phase plug mod - bullshit or no bullshit.  (Read 4296 times)

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Marvey

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Fostex FE166En phase plug mod - bullshit or no bullshit.
« on: August 13, 2013, 03:04:31 AM »
I've always wondered if the phase plug mod for various drivers (especially the Fostex) were bullshit or not, so I decided to do a rudimentary phase plug mod take some before and after measurements.

SEAS FAQ

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

In the case of the FE166 driver, there wouldn't be much loss in cone area. The dust cap already has a hole in it for venting. See pic below. If we cut the dust cap off, we probably lose about one square inch of total area.
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #1 on: August 13, 2013, 03:13:02 AM »
Here are two baseline measurements of the stock fe166en driver mounted on the BK-16. The microphone is aligned with the driver and 24" away. Graphs are 1/3 octave smoothed. First graph is directly on the driver 0 degrees. Second graph, the microphone is 30 degrees off.

We can see that the speaker has a slightly rising response when dead on. However the driver exhibits beaming behavior - we see roll-off starting at 3kHz when offset slightly from the driver.
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #2 on: August 13, 2013, 03:21:26 AM »
So the idea behind the phase plug is that high frequencies would bounce off the phase plug and therefore we would get a more uniform polar response - less beaming. Here is a picture of the hack job I did - just for testing purposes.
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #3 on: August 13, 2013, 03:30:04 AM »
Here is a 0 degree front on measurement of the speaker with the phase plug mod.

What I find interesting is a decrease in 2nd and 4th distortion in the bass. This is something I noticed after I took the measurements. I did take the average of three measurements however.

I strongly suspect the dust cap is the culprit for the higher distortion found in the un-modded driver. Even though the dust cap had a vent in front, I don't think the vent was enough to prevent an air cushion when low bass was playing. I figure this air cushion presented a varying load on the cone, hence the higher distortion.  I noted during frequency sweeps that the driver displaced like crazy when low bass tones were being played. Still bass measurements are funny. I will need to take a few more to confirm.

I also noticed the area around 2k is a little smoother.
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Anaxilus.

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #4 on: August 13, 2013, 03:37:14 AM »
When I was big into car audio I always preferred phase plugs.  Most of the speakers I end up buying myself usually have them. 

Is that a DIY phase plug sawed from one of those rods at Home Depot?  Not anything I've ever seen.  Used to bullets.
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OJneg

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #5 on: August 13, 2013, 03:40:53 AM »
Interesting, what SPL are these sweeps blaring at in absolute terms?
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #6 on: August 13, 2013, 03:43:34 AM »
Quick answers:
  • Home Depot wooden dowel. Too lazy to carve into bullet. Bullets aren't totally necessary. I've seen cylindrical phase plugs. Would be interesting to measure different types.
  • 0 db = 120 SPL. So we are talking around 90db
« Last Edit: August 13, 2013, 03:49:27 AM by purrin »
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #7 on: August 13, 2013, 03:47:17 AM »
So here is the 30 degree measurement of the phase plug mod.

The second graph is a comparison between the stock dust cap and the phase plug modded driver at 30 degrees. Conclusion is that this mod is not bullshit. At 30 degress of, the area at 2kHz is more even. We also see significantly less roll-off ~5db in the treble.


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firev1

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #8 on: August 13, 2013, 08:11:01 AM »
Any subjective differences in the original sweet spot or it just better overall?
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Marvey

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Re: Fostex FE166En phase plug mod - bullshit or no bullshit.
« Reply #9 on: August 13, 2013, 03:50:54 PM »
It actually makes a difference. The primary reason for the experiment was for HT / movies, where I would usually sit off axis and the HF roll-off was notable. Last night I ended up resting on the couch listening to music, and I usually never do that. The new room I put the speakers is much more sensitive to off-axis response because the couch is much closer to the speakers.

The next step is to design a phase plug to tone down the rising response on-axis and maybe even further flatten out the response off-axis.
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