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Old 28th August 2007, 04:35 PM   #1
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Default Hole in driver dome

Many owners of Fullrange drivers have addressed or acknowledged that there is some IM distortion going on behind the dome of the fullrange driver. Some address this problem by removing the dome and replacing it with a phase plug. Reading Bob Brines' website suggests to me that while this does make a difference in quality within a certain frequency band, a driver does lose some 'air' in the treble response.

A couple of manufacturors (Audio Nirvana, Lowther and Fostex) produce drivers with a hole in the dome, and with phase plugs.

Is there some documentation somewhere that stipulates that the hole in the dome must be dead center? Could the hole/s be made off to the side of the dome, closer to where the dome meets the whizzer (if there's a whizzer).

I'm asking this before I do it to a driver myself. I have performed the hole-in-dome tweak myself and this did clear up some distortion I was hearing.
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Old 28th August 2007, 06:54 PM   #2
hm is offline hm  Europe
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Hello,

Wikipedia says

http://translate.google.com/translat...&hl=en&ie=UTF8

It thus quite is in the range of the possibility that manufacturers phase Plugs of second kind begin, without it comes to clear improvements, which are provable by measurements. This would be then a kind mode feature.
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Old 28th August 2007, 08:38 PM   #3
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Depends on the driver I suspect. You can get a resonance problem in the hollow pole-piece that adding a plug can solve, or you can at least prevent too much of a build-up by cutting a hole in the dustcap. There are some dodgy claims made though. Best of the lot are DaveP10s, which are proven effective. Bob Brines has before & after measurements of Dave's FE167E plugs for example, in his FT1600 MLTLs & endorses them (not something Bob's prone to do without good reason).
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Old 28th August 2007, 09:39 PM   #4
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Quote:
Originally posted by Scottmoose
Depends on the driver I suspect. You can get a resonance problem in the hollow pole-piece that adding a plug can solve, or you can at least prevent too much of a build-up by cutting a hole in the dustcap. There are some dodgy claims made though. Best of the lot are DaveP10s, which are proven effective. Bob Brines has before & after measurements of Dave's FE167E plugs for example, in his FT1600 MLTLs & endorses them (not something Bob's prone to do without good reason).
I read the Brines article on phase plugs. It sounds like no matter what tweak there is, some people still feel the need to add a super-tweeter.

I have been pondering the hole in dome thing, having performed the the tweak on a RS 40-1354 and found that it did indeed improve the upper mids, or at least eased the distortion problem, but I still felt the need for the super tweeter. I was wonder if I could maintain the 'air' of the dome and still aleviate the IM distortion from behind the dustcap by making a hole/s not in dead center, but on the periphery of the dome. I think I'll have to satisfy my own curiosity. I have a couple of super squakers (40-1197s) that I can experiment with.
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Old 29th August 2007, 12:54 AM   #5
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Not to hijack or anything, I have seen drivers which have holes placed in the cone near where it bonds to the voice coil. Always 4 holes evenly spaced around the vc and close enough for the dustcap to cover them. Would this also be a method of achieving the same goal?
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Old 29th August 2007, 01:47 AM   #6
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Default I've seen that too

But I've seen it on woofers, not full range drivers. The dome on a FR is usually equal in circumference to the voice coil cylinder, so a hole in the cone wouldn't be an option.

I believe the holes in the cone itself (under the dome) is for voice coil venting.
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Old 29th August 2007, 10:32 PM   #7
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Greets!

Hmm, I thought you were active on the old FR forum back when Cyr-Marc did some experimentation on a 40-1197 and got worse results from venting it: http://melhuish.org/audio/response.html#rs

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Old 30th August 2007, 12:27 AM   #8
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Originally posted by GM
Greets!

Hmm, I thought you were active on the old FR forum back when Cyr-Marc did some experimentation on a 40-1197 and got worse results from venting it: http://melhuish.org/audio/response.html#rs

GM
I remember that, but I'm more about getting rid the distortion caused by the dome, which I have myself aleviated on another driver. Yes, the hole in the dome reduces higher treble response, but almost every FR driver I've ever used needed a super tweeter anyway.

I have this nutty idea that if the vent were near the edge of the dome, instead of dead center, it might retain some upper frequencies. Oh well, we shall see...soon.
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Old 30th August 2007, 01:57 AM   #9
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Well, Panasonic made a FR driver with an inverted, side vented, DC with phase plug:
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File Type: gif panasonic fr driver.gif (48.3 KB, 150 views)
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Old 30th August 2007, 10:01 PM   #10
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Default Well, I did it

Turns out I only have three good 40-1197s (I had four - one was inop out of the box).

So I tried my idea - I burnt three holes, in a triangular pattern, at the edge of the dome in one driver, a hole in the middle of the dome on one, and no hole on the third.

Just from a pure listening perspective, the one without the hole is just so harsh and shouty. The one with the hole in the middle is not as shouty, but has lost a little 'air'. The one with the three holes near the edge has less shout, some of the shimmer is still there, though less shimmer than the control driver. However, the three-hole driver has more shimmer than the one with a hole in the middle and is less shouty as well.

I went ahead and removed the dome from the one with hole in the middle. I went ahaed and performed the three-hole mod on the remaining driver. I listened to the driver that I gave the dome ectomy to, and I was surprised how much treble information was still there. I played around with a couple of metal tool sockets and was thoroughly pleased. I put a round wooden drawer handle (roughly the size of a golf ball) at the end of the socket and walked around the room, off axis, and I gotta say, there will be phase plugs in my future.

So in short, those dust covers cause more trouble than they're worth. I have no measuring equipment, so listening is all I can do.
I'd say a hole in the dome, in my listening experience, works to aleviate some distortion in the upper mids, allowing the driver to sound a little more 'controlled' at louder volumes. All my drivers get the dammar treatment, but I'm now going to start tweaking the domes. I wished I could see if there was a measurable difference twix the dust cover with the three holes at the edge, and the dome with the hole in the middle. It seemed, that I heard a difference, but it wasn't a scientific test. I think I liked the domeless 1197 the best. It was pretty smooth compared to the ones with the domes.
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