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-   -   Symetrical field, is this? (http://www.diyaudio.com/forums/multi-way/20216-symetrical-field.html)

Raka 13th September 2003 07:22 AM

Symetrical field, is this?
 
Hi,

I've heard many times that in a vented box, the driver should have a simetrical something to avoid the efect of having the cone glued to one side if you move around the Fs. I'm planning to use one midbass driver that is told to have "cooper ring in the pole piece"
Is this something related?
The box I'll use is 15lts, with a driver of Fs=55Hz at free air. When does this infamous effect take place, at the 55Hz or at the resonance of the driver in the box? If so, and if my driver doesn't have the means to avoid the mentioned effect, maybe I use a closed box (not very low at F3=70Hz).

Raka 13th September 2003 07:25 AM

Distortion graph
 
In the frequency graph, I can see that the quoted distortion climbs fast to 60db (nominal sensivity is 90dB) when you drop to 70Hz. Does this answer my question?

roddyama 13th September 2003 01:38 PM

Hi Raka,

It is in the quality of the driver where the relationship between the shorting ring and the “suck-in” or “suck-out” phenomenon come into play. Suck-in can occur, usually in low quality drivers, when the driver is driven to high excursion under high power. In a closed box a woofer sees its max excursion at fs ~ f3. In a ported box, max excursion will occur, typically, just below fs, and worse, just above fs (this could account for the increased distortion figures at 70Hz in your driver).

The copper ring (Shorting Ring or Faraday Ring) is not directly related to the suck-in phenomenon you’re wondering about. The shorting ring is used to lower the distortion in the high frequency range of the driver. It will have only a small effect on the drivers operation at the lower frequency. It is usually the higher quality drivers that come fitted with a shorting ring.

So in a cheap driver, the distortion will rise dramatically as the excursion increases. While higher quality drivers, like those with shorting rings, will be designed to operate with lower distortion.

Raka 13th September 2003 02:08 PM

My drivers have cooper ring, so they suppose are not too bad, but I don't know if they behave well with the suckout thing, right?
Well, I'll fed a sinus at those low frequencies and see what says the mic.
So, what would be the technical name for the device that the manufacturer writes in the specs? "Symetrical field" maybe?
I read in another thread that some peerless have it, and that some Vifas don't. Vifas aren't supposed to be low quality, no?

roddyama 13th September 2003 02:51 PM

The aspects of the design of the driver that will help to prevent the suck-out effect will be a careful balancing of the speakers parameters. Parameters such as motor power, x-max, cone mass, suspension type, shorting rings, etc all interact to provide the peformance of the finished product.

The "symmetrical field" is a result of how the shorting ring is implemented. To an extent the magnetic flux can be routed in a more or less symmetric fashion through the voice-coil gap, in part, by the design of the shorting ring.

So is it real? Yes, but you'll have to take the manufacturers word on that. You're in sales, you know how it goes.

kelticwizard 14th September 2003 01:11 PM

Quote:

Originally posted by roddyama
The aspects of the design of the driver that will help to prevent the suck-out effect will be a careful balancing of the speakers parameters. Parameters such as motor power, x-max, cone mass, suspension type, shorting rings, etc all interact to provide the peformance of the finished product.

The "symmetrical field" is a result of how the shorting ring is implemented. To an extent the magnetic flux can be routed in a more or less symmetric fashion through the voice-coil gap, in part, by the design of the shorting ring.


Of course, I would emphasize that for the shorting ring, (Faraday ring) to be implemented properly, it first must be present in the design in the first place. Many speakers do not have them, therefore the "suck-in" phenomenon.

Rodd is an engineer, and I am not. However, I can tell you what I know and I have seen. I observed the "suck-in" phenomenon on my own, using a cheap speaker in a ported box. Nobody ever told me about this before I observed it. If you run a test tone near Fb, (box tuning frequency), the travel in the voice coil, (suck-in), is quite visible.

An article in Audio magazine by the well-known D.B. Keele dealt with the "suck-in" issue. To prevent it, one of 3 things is necessary.

A) A shorting ring, whether aluminum or copper;

B) An extended pole piece, so that there is an equal amount of magnetic material on both sides of the voice coil. There is a variation on this I will explain later;

C) A special progressive suspension designed to counterract the "suck-in" phenomeneon. Although Keele mentioned this a few times, I get the impression there might be only one or two hi-fi companies that use this approach. I have not seen it mentioned in any drivers available to the public.

The "suck-out" phenomenon occurs when the voice coil "sees" less magnetic material in one direction than in the other. So it gets drawn more to one side than the other. The extended core, (pole piece), simply puts more magnetic material on the outside end of the pole piece-the side that is usually truncated-to restore the balance. I have never totally understood the Faraday ring's action, but I gather from several explanations that it cuts short the magnetic field when in operation-hence "short circuits" the magnetic filed past a certain point-and restores symmetricality that way.

The variation on the "extended pole piece"? Apparently the well-respected McCauley and JBL lines of PA speakers cut a notch in the pole piece on the part that is near the magnet. Instead of extending the pole piece farther out to restore symmetricality, it shaves off a little of the pole piece that is on the other side. This restores balance.

I also gather that the Faraday ring reduces inductance, and therefore has benefits for the speaker at higher frequencies. Perhaps that was the original reason for putting them on, before the benefits of the "suck-in" phenomenon were observed.

Granted that any feature must be implemented correctly to work, I would think that any company that goes to the trouble of putting these features on will do so competently, considering that many companies don't even bother with them at all. Therefore, if you want to prevent "suck-in", go for a design that uses a shorting ring or extended pole piece, (or it's variation, the "notch" in the pole piece).

kelticwizard 14th September 2003 01:21 PM

PS: As for the Vifa line: the literature plays up the manesium basket,and hints that this is good for the magnetic system. but there is no mention of symmetricality. Vifa is part of Danish Sound Technology,l just like Peerless and Scan Speak, but Vifa makes no mention of symmetricality, and the other two do. Draw you own conclusions.

Vifa does have a good reputation, however.

Incidentally, ever since Danish Sound Technology took over these companies, they have been referring tech questions to the distributors via an Email link on the website. Peerless used to have an Email link right to the company-and you would get an answer. The Danish Sound Technology "distributor" link appears to yield nothing but frustration.

Two US Vifa distributors-Madisound and Parts Express-have excellent tech support on their own. I might call or Email them to get the lowdown on this issue.


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