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The birth of a 15 inch woofer
The birth of a 15 inch woofer
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Old 3rd September 2019, 12:29 PM   #51
Snickers-is is offline Snickers-is  Norway
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@ICG:

I believe all of the issues you mention in your post are well covered by previous answers. For the purpose of the thread, I see no point in discussing those details further.
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Old 5th September 2019, 02:07 PM   #52
Snickers-is is offline Snickers-is  Norway
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Things might appear to be quiet these days, but I can assure you they are not.

During the past few days I have made some important reviews of the suspension. The 204mm spider with a 3 inch coil gives a pretty hefty 56mm width of the suspension area. This is all good for the ability to move, but I am a bit worried that it might resonate, especially since it will be a very soft suspension for its size. Resonances originate from the mass and compliance relationship in the spider, so an idea that might work is to reduce the diameter a bit, and then make an adapter ring for the spider landing with an oval inner opening. That way, the width of the spider will be progressive, leading to an effective resonance distribution over a wide range of frequencies, where each resonance has extremely low Q. This approach should remove resonances and at the same time keep the Cms linear over a wide excursion range.

As I wrote before, the spider will handle the significant part of the Cms, while the surround will be far softer. There are several reasons for this.

First off all, the spider is easier to control than the surround, especially if we need some added coating on parts of the surround that has to be done manually. That means the variation between samples will be small.

It is also slightly easier to control the X-position of the coil if the spider is the dominant compliance. This might not always be true, but the selected assembly method means the coil will be fixed in the gap, and the spider will be mounted in the frame with only the coil in position. The spider will be glued to the voice coil former as it is in its resting position. Then the cone will be added in a similar way, but as the cone has its own mass, it will be a source for some error. We therefore need to support the cone mechanically during assembly to make sure we do not assemble the suspension under load. A 0,5mm error in the cone assembly will therefore translate to less than 0,15mm for the entire assembly.

As the spider is far wider than the surround (around 40mm vs around 20mm), it will keep its compliance over a broader excursion range. Since the spider is the dominant compliance, and also is the part that offers the longest excursion, we will get a more linear total suspension. This also reduces the drivers tendency to activate jumping resonances (as described in an earlier post).

The mass balance of the driver has been described before. However, the center of gravity has not been talked too much about. The center of gravity on the voice coil itself will be towards the back of the voice coil former, but not by much. It represents around 45g behind the spider. The rest of the mass will be around 60g as the spider and glue probably represents around 5g. The mass of the cone will increase towards the front. The length of the assembly behind the spider is around 110mm, while the length of the assembly in front of the spider is around 88mm. This means the balance of the mass in front of, and behind the spider, is almost perfectly in balance, leaving just a few grams resting on the surround. Having a more rugged spider makes sense as it will carry most of the drivers mass.

The oval spider ring will be mounted such that the driver is intended to have the oval standing vertically when the driver is mounted. The spiders horizontal width will be around 163mm, while its vertical width will be around 175mm. This gives a spider that is less prone to sag over time.
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Old 6th September 2019, 04:39 PM   #53
Snickers-is is offline Snickers-is  Norway
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I mave made some renderings to better show what is going on here.

Her is the oval fixing ring for the spider exposed:

The birth of a 15 inch woofer-spider-assembly-png


Here you can see the exposed motor parts:

The birth of a 15 inch woofer-exposed-motor-png


And a side view of the entire thing:

The birth of a 15 inch woofer-woofer-view-png
Attached Images
File Type: png Spider assembly.png (283.7 KB, 608 views)
File Type: png Exposed motor.png (134.2 KB, 623 views)
File Type: png Woofer side view.png (116.8 KB, 338 views)
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Old 9th September 2019, 12:13 PM   #54
Sabbelbacke is online now Sabbelbacke  Germany
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Nice
Not that itīs importan for sound quality - but is there any reason why your circles arenīt circles?
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Old 9th September 2019, 03:18 PM   #55
badman is offline badman  United States
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Maybe I'm not understanding the spider ring right, but wouldn't it create odd stretching and potentially mechanical noise?
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Old 9th September 2019, 06:57 PM   #56
Zvu is offline Zvu  Serbia
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It is a bad rendering. It is done to see if everything fits but utmost precision of the render is of no great importance.
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Old 9th September 2019, 07:09 PM   #57
Snickers-is is offline Snickers-is  Norway
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Quote:
Originally Posted by Sabbelbacke View Post
Nice
Not that itīs importan for sound quality - but is there any reason why your circles arenīt circles?
As Zvu points out, it is just a simplification the software does. I can draw it as a 192-edge shape, but by default, circles are made as 24 edge shapes, and for the purpose, I believe it is sufficient.
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Old 9th September 2019, 07:18 PM   #58
Snickers-is is offline Snickers-is  Norway
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Quote:
Originally Posted by badman View Post
Maybe I'm not understanding the spider ring right, but wouldn't it create odd stretching and potentially mechanical noise?
Yes, it does create some odd stretching, but the spider on this driver, who will normally work below 10mm of excursion, is actually wider than the one on the 18N862, which is intended for 19mm excursion. So the odd stretching will be extremely limited.

Also the material of a spider is made to be distorted without emitting any sound on its own.

A very small percentage of the total excursion goes to the outer roll of the spider. On the 0 and 180 degree points, there are 5 rolls, so 100% of the motion will be distributed around 20% on each roll. On the 90 and 270 degree points, there are only 4 rolls. So around 25% of the motion will be distributed on the 4 rolls. But as the motion in the 4.th roll is limited by the fixing ring at 0 and 180, the 5.th roll will hardly move at all. This means the suspension movement will be relatively even around the spider, and the odd stretching will be very limited.

The spider will, off course, be fully glued to the surface of the spider ring in order to make sure it does not flap around making any kind of noise.
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Old 10th September 2019, 02:11 PM   #59
Snickers-is is offline Snickers-is  Norway
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I believe I did not explain the resonance control ring on the spider properly.

The distance from the voice coil to the fixed outer edge of the spider, the spider compliance, its mass, and its inner loss, are the parameters that decide the resonant frequencies of the spider. By changing one of them (except the loss) you will change the resonance frequency of the spider. A normal round spider has the same resonance freqyency at all angles.

By adding this oval ring, and gluing the spider completely to it, the spider will have lower mass, shorter distance and lower compliance at some angles than others, all resulting in higher resonance frequencies.

Having several different resonance frequencies in one connected system, not being in phase, makes the system cancel out all resonances, leaving only tiny parts of energy for each resonance. The loss factor is more or less unchanged. So it can effectively remove the tiny remains of resonances left from the reactive parts of the spider.

This is especially important for this spider since it is the dominant part of the suspension, it is a bass + midrange driver, and it has very long excursion suspension, low moving mass, all leading to a large, light weight and highly compliant spider that would normally be prone to resonate.
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Old 10th September 2019, 08:12 PM   #60
manninen is offline manninen  Finland
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easily separable motor, for diyers who wants to mess around

MAG M1510 a Free-Air speaker size 15" or 38cm.
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