Re: Re: Re: Re: Stephen D
I'm afraid is not the case with my 24 Euro woofer.
Anyway, if the "suck in" is fixed with the steel rings, I suppose the target is done. I'll report my experiments. Any suggestions about new dustcap? Nevermind, I'll search older threads (this is to save BW)
sreten said:
I'm afraid is not the case with my 24 Euro woofer.
Anyway, if the "suck in" is fixed with the steel rings, I suppose the target is done. I'll report my experiments. Any suggestions about new dustcap? Nevermind, I'll search older threads (this is to save BW)
We are talking cheap drivers here.
I understand this is caused by the coil leaving the magnetic gap,
as it leaves the gap its impedance rapidly falls (at fundamental
resonance impedance is very high) causing a huge increase in
current that drives / pops the coil right out of the gap.
It is a known problem, addressed by long voice coils,
compared to the gap and progressive suspensions.
More advanced drivers add Faraday rings fore and aft
of the voice coil, when they enter the magnetic gap
they act as "brakes" on the driver.
IMO symmetry of the magnetic field is a minor influence on this,
and extending the centre pole piece will not fix the problem.
🙂 sreten.
Re: Re: Re: Re: Stephen D
Sreten:
Unfortunately, a lot of them don't seem to be aware of this. For instance, DB Keele, reviewing Audio magazine, reviewed some systems where "suck-in" takes place. This is also called "oil canning".
Linkwitz mentioned a Madisound woofer that "walks out" of it's center postion near resonance. Granted, Madisound is a 'house" woofer, but the Madisound name is a good one and I would use a Madisound woofer as an example of a good woofer.
I have contacted two distributors of Vifa, (DST does not take Emails from consumers), to ask about extended pole pieces or symmetry in the Vifa line, and got no response from two distributors that generally do give good tech support.
Unless the manufacturer specifies something that says, "symmetrical magnetic field" or "drastically reduced second order distortion", I am assuming that the problem is not dealt with.
It used to be that only the most expensive woofers dealt with this. I believe the Peerless CC line was the first under $100 woofer that offered a symmetrical field. I remember calling up Madisound about their Eclipse 1242s-an older woofer-to ask about a symmetrical field. "It's a very good American woofer", he said, strongly implying that only high quality European makes dealt with such things. And back then, that was indeed the case.
I have just come across a Tangband 8" woofer that does not mention this, but was assured by one who used it that it has an extended pole piece. So maybe, just maybe, we are finally reaching the point that manufacturers do not feel that they can ignore this issue.
But if that is the case, then we have only recently arrived at this understanding.
sreten said:
Sreten:
Unfortunately, a lot of them don't seem to be aware of this. For instance, DB Keele, reviewing Audio magazine, reviewed some systems where "suck-in" takes place. This is also called "oil canning".
Linkwitz mentioned a Madisound woofer that "walks out" of it's center postion near resonance. Granted, Madisound is a 'house" woofer, but the Madisound name is a good one and I would use a Madisound woofer as an example of a good woofer.
I have contacted two distributors of Vifa, (DST does not take Emails from consumers), to ask about extended pole pieces or symmetry in the Vifa line, and got no response from two distributors that generally do give good tech support.
Unless the manufacturer specifies something that says, "symmetrical magnetic field" or "drastically reduced second order distortion", I am assuming that the problem is not dealt with.
It used to be that only the most expensive woofers dealt with this. I believe the Peerless CC line was the first under $100 woofer that offered a symmetrical field. I remember calling up Madisound about their Eclipse 1242s-an older woofer-to ask about a symmetrical field. "It's a very good American woofer", he said, strongly implying that only high quality European makes dealt with such things. And back then, that was indeed the case.
I have just come across a Tangband 8" woofer that does not mention this, but was assured by one who used it that it has an extended pole piece. So maybe, just maybe, we are finally reaching the point that manufacturers do not feel that they can ignore this issue.
But if that is the case, then we have only recently arrived at this understanding.
sreten said:
Well, I 've read some papers or reviews by Keele and others which seem to indicate symmetry is important.
None of them advocated adding steel washers to extend the pole piece, of course.
The "suck-out" seems to happen in vented boxes. I have read that it can happen in a sealed box, but I have not noticed it in the sealed boxes I have built. Linkwitz has pointed out that it can happen in an unboxed speaker, as have other posters here.
I have begun to formulate a theory that manufacturers ignored the issue because up until 1980 or so, very few people built vented boxes. As the vented box took over, manufacturers had to finally face the problem, and they did it from the top end down. But this is just a theory on my part.
At any rate, good luck to anyone who wants to experiment. If you do, please post your results. I am thinking of performing the experiment myself-this thread is acting as a spur. 🙂
K C :
Your certainly right about this. most older drivers were designed
to be used in sealed boxes with low Fs and high Vas.
Theoretically they could be used in big reflex boxes but subsonic
power handling was very poor, as many an amateur found out.
Most older audio used sealed boxes. People do not like to see
the wobbles you get running a turntable through reflex speakers
at high volume. For serious volume a subsonic filter is mandatory.
It has been the advent of CD, with its lack of sub-sonic garbage
that has now made reflex loading the norm, to such an extent
nearly all bass / mids are optimized for reflex loading.
They have moderate Vas which helps below port resonance to
reduce cone movement.
Not so sure about the top down, manufactures seized the reflex
with glee once they got rid of the turntable in the package
(and replaced it with a CD / video / DVD).
All they had to do was make sure it didn't overload on low bass
below port resonance - hence the relatively stiff cheaper drivers
in drive unit manufacturers ranges.
(Cheap drivers generally now have high Fs and low Vas)
🙂 sreten.
P.S. magnetic symmetry is important for low distortion, I meant
I didn't think minor assymetry affected the pop-out phenomenon.
Stephen D
Well at any rate this has been an interesting thread & served well to ferrite out some worthwhile thoughts to ponder. At this point I think the answer to the question of whether or not a slight DIY pole piece extension tweak (on some speakers) could render an improvement is at least uncertain enough to warrant further pondering & investigation. I'd say to anyone interested with the time & proper speakers to experiment on..... go for it. Knowing for sure whether or not the
speaker experimented on had already been compensated for any magnetic asymmetry would
be key to well controlled experiment though.
Well at any rate this has been an interesting thread & served well to ferrite out some worthwhile thoughts to ponder. At this point I think the answer to the question of whether or not a slight DIY pole piece extension tweak (on some speakers) could render an improvement is at least uncertain enough to warrant further pondering & investigation. I'd say to anyone interested with the time & proper speakers to experiment on..... go for it. Knowing for sure whether or not the
speaker experimented on had already been compensated for any magnetic asymmetry would
be key to well controlled experiment though.
Re: Re: Re: Re: Re: Stephen D
How do you know the rear edge of the pole plate is not faceted ?
🙂 sreten.
Raka said:
How do you know the rear edge of the pole plate is not faceted ?
🙂 sreten.
I am going to try it regardless. i have a crappy little driver here that there is a chance i could improve, i am going to give it a whack. I have a 4 inch "sub" woofer that was in a 4th order bandpass box in a cheapo system i will try this on. i will sort out the TS parameters and build a box. then i will experiment with it and if it works, hooray! everyone do it! if it doesn't, then no biggie. I could even probably comletely dissasemble it and be a mad scientist, take some of that sticky aluminum tape and wrap a single layer around the pole piece. could ruin the driver.... but WHO CARES!! 
Hi SG,
a) Be my guest, but it won't prove anything for decent drivers.
b) There won't be enough room for aluminium tape in the air gap,
(and you will not be able to disassemble the magnet parts)
and it is a totally pointless exercise regarding cheap drivers.
🙂 sreten.
a) Be my guest, but it won't prove anything for decent drivers.
b) There won't be enough room for aluminium tape in the air gap,
(and you will not be able to disassemble the magnet parts)
and it is a totally pointless exercise regarding cheap drivers.
🙂 sreten.
Here is some good info on motor systems and faraday rings which some might find interesting:
LSDG summary of different motors
LSDG summary of different motors
Re: Re: Re: Re: Re: Re: Stephen D
You are right, I don't know 🙁 And I won't know if I don't dismantle/break the driver...
What would be the effect if it's faceted? Will I have a suck in instead of a suck out? I'll have to try, I'll post the answer.
sreten said:
You are right, I don't know 🙁 And I won't know if I don't dismantle/break the driver...
What would be the effect if it's faceted? Will I have a suck in instead of a suck out? I'll have to try, I'll post the answer.
Re: Re: Re: Re: Re: Re: Re: Stephen D
good link ! 🙂
By applying a bevel on the bottom edge of the pole piece the flux
here is reduced. The idea is to make the flux ~ = to the leakage
flux at the other end of the pole, i.e. making it ~ symmetrical.
This aproach leaves more flux in the gap where you want it,
the extended pole piece approach theorectically needs a slightly
larger magnet.
🙂 sreten.
paulspencer said:
good link ! 🙂
Raka said:
By applying a bevel on the bottom edge of the pole piece the flux
here is reduced. The idea is to make the flux ~ = to the leakage
flux at the other end of the pole, i.e. making it ~ symmetrical.
This aproach leaves more flux in the gap where you want it,
the extended pole piece approach theorectically needs a slightly
larger magnet.
🙂 sreten.
Attachments
Re: Re: Re: Re: Re: Re: Stephen D
She doesn't.
My advice is to build a ported box-makeshift will do-and test the woofer.
You test the woofer by running a set of tones through the speaker near it's box resonance. Put some power behind it.
If the speaker has a non symmetrical field, the voice coild will displace. This process is very visible.
If the voice coil does not displace when tones near the box resonance are run under power, then there is no need to do anything.
How to generate tones? Put them on a CD, if you have a recorder.
Here is a nice tone freeware tone generator:
www.satsignal.net
Theroetically it is a sweep generator, but you just click "No Sweep" and write in the box with the frequency you want, and you get that frequency when you press "start". And it stops when you press "stop". Very simple and easy.
Put one tone on each track of a CD, so you know what tone is playing when you select your track.
Then just play your CD through your system.
sreten said:
She doesn't.
My advice is to build a ported box-makeshift will do-and test the woofer.
You test the woofer by running a set of tones through the speaker near it's box resonance. Put some power behind it.
If the speaker has a non symmetrical field, the voice coild will displace. This process is very visible.
If the voice coil does not displace when tones near the box resonance are run under power, then there is no need to do anything.
How to generate tones? Put them on a CD, if you have a recorder.
Here is a nice tone freeware tone generator:
www.satsignal.net
Theroetically it is a sweep generator, but you just click "No Sweep" and write in the box with the frequency you want, and you get that frequency when you press "start". And it stops when you press "stop". Very simple and easy.
Put one tone on each track of a CD, so you know what tone is playing when you select your track.
Then just play your CD through your system.
By the way, if you are going to make a CD, you might as check your sound card's frequency response. I did it:
http://www.diyaudio.com/forums/showthread.php?postid=91126#post91126
All you need is a 1/8" to 1/8" stereo plug, available for a few dollars at any place that sells audio accessories. It is also called a Walkman-to-Walkman plug.
http://www.diyaudio.com/forums/showthread.php?postid=91126#post91126
All you need is a 1/8" to 1/8" stereo plug, available for a few dollars at any place that sells audio accessories. It is also called a Walkman-to-Walkman plug.
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