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Fostex FE166En phase plug mod - bullshit or no bullshit.

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Marvey:
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.

Marvey:
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.

Marvey:
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.

Marvey:
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.

Anaxilus.:
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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