how can sealed subwoofers produce lots of low fast bass? - Page 2 - diyAudio
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Old 7th December 2001, 06:17 AM   #11
pkgum is offline pkgum  Australia
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what is ELF? and how do the bag ends achieve such LOW frequency response?!!!(8Hz - 92Hz)
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Old 7th December 2001, 07:10 AM   #12
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As far as I know, the Bag End strategy is the same as what Carver is using. Quite simple, really--use a too-small box on purpose. This causes the sub to be well into its rolloff by the time you expect it to reproduce any sound. The trick is that the response is now totally predictable, a nice, even slope. At that point, you use a nice, even, matching boost to compensate and give the sub flat response. It takes a prodigious amount of power to keep this up for long, which is why Carver has a 2700W amp in his subs. If I recall correctly (it's late, and I'm way behind on sleep), the power requirements are x4 for every octave you drop with a normal sub. Now, on top of that, you've got to account for the amount of power needed to put muscle behind the boost.
It's really only an extension of the idea of taking a normal sub and boosting as it starts dropping out. The difference being that you allow the driver to have a normal-sized box, which allows it to be flat in the pass band until it begins a normal rolloff. The Bag End/Carver subs bypass flat response entirely, manipulating the frequency response curve from the word go.
I've done the second strategy before with stereo subs, each with four KEF B-139s. Boost kicked in as the KEFs rolled off, somewhere (help me here, Geoff) around 40-45Hz. Sounded great at lower volumes, but as you turned it up, you quickly exceeded the Xmax of the drivers and it got very, very muddy sounding. A buddy of mine has Kinergetics subs which, if I understand correctly, do the same thing. They, too, sound muddy as you turn up the volume. The quantity is there, but not the quality
It would be more practical now with Titanic/Shiva/NHT class drivers, since they have a much larger excursion. However, I've given up on the idea and intend to go with servo loops on my current subs, hopefully optical, if all works well. I'm having trouble finding the parts I want. There are other options, of course. Dedicated accelerometers (nifty, but expensive), various kinds of pressure sensors (not as flat in response as I'd like), and the old standby--piezo units (lotsa phase shift). If anyone is interested, I'll report later.

Grey

Oh, sorry, forgot to answer the other part--ELF stands for Extended Low Frequency.
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Old 8th December 2001, 03:13 AM   #13
Wizard of Kelts
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Quote GRollins: "There are other options, of course. Dedicated accelerometers (nifty, but expensive), various kinds of pressure sensors (not as flat in response as I'd like), and the old standby--piezo units (lotsa phase shift). If anyone is interested, I'll report later."

Please report. Also, I have come across mention of the capacitance method of measuring cone excursion, based on the principle that as distance changes between plates, so does capacitance. You might consider that as another option.



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Old 8th December 2001, 07:21 AM   #14
pkgum is offline pkgum  Australia
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hey, anyone know a website that specializes in demonstrating to BEGINNER ppl how to construct a servo feedback system in a subwoofer?

[Edited by pkgum on 12-08-2001 at 05:49 AM]
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Old 8th December 2001, 11:47 AM   #15
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Wizard,
The problem with capacitance is that it's inverse square law, i.e. real non-linear. Maybe there's some clever configuration that will get around that, but I wasn't planning on following that option.
A possibility that I didn't mention was to build a miniature voice coil by putting a couple of layers of windings on a drinking straw. Epoxy a small magnet to a standoff and the standoff to the cone of the driver (total mass no more than a gram or two--no worse than the accelerometer chips). Yes, you could put the coil on the woofer if you wanted; advantage--lower mass, disadvantage--then you'd have to get the signal off of the madly jumping cone. Flip a coin and follow whichever strategy makes you happy.
I've also got a few other things I've thought of now and then, but the optical one is still my goal.

Grey
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Old 8th December 2001, 12:27 PM   #16
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Just a clarification: While we are talking about picking up information on the excursion of the cone (for feedback purpose), it is not possible to measure the pressure changes within a (closed) box to obtain and use similar information. I thought I'd just mention this, since this question arose recently in response to which John Watkinson wrote a recent column in Electronics World.

It is also possible to have a second voice coil and pick up information to correct the response curve of a sub-woofer, or for that matter a condensor mic placed very close to the dust cap so that at maximum excursion, the cap does not touch the mic.
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Old 8th December 2001, 12:33 PM   #17
Geoff is offline Geoff  United Kingdom
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pkgum

I don't know about websites, but there have been a number of articles in Electronics World over the years covering motional feedback designs. One of the more recent ones that I can remember is Russel Bredon's 'Roaring Subwoofer' in the Feb '97 issue.

One common feature of all the articles I have read is the difficulty in obtaining adequate stability in a motional feedback system, which has resulted in the burning out of the amplifier, the speaker or both at least once during the development stage. The most reliable feedback method seems to be the use of a dual-coil speaker with the second winding being used to provide the feedback signal.

Geoff
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Old 8th December 2001, 03:33 PM   #18
pkgum is offline pkgum  Australia
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ok..theres only one last route since i have little electronic background


Does anyone know of someone or a company that would custom construct a servo feedback system for a subwoofer?


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Old 10th December 2001, 06:33 AM   #19
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Grey,

Sounds like you have thought about your servo system quite a bit. What sort of servo system are you thinking about?

Also, you keep mentioning optical feedback devices. Are you thinking of DIYing the sensor, or do you have a commercial device in mind?

What sort of Xmax are you dealing with?

Erik
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Old 10th December 2001, 03:52 PM   #20
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Erik,
The drivers I'm using are the Dayton Titanic 12" (twelve of them). At present, they're in ordinary T-S boxes, so they're good down to upper 20's or so--good enough to get by on, but not deep enough to suit me. I got addicted to subs that dig into the teens years ago; now anything higher than that sounds like sub-lite, if you catch my drift. I've got plenty of SPL capability--far more than I need. Each driver has a rated excursion of an inch or so, if I recall correctly. I don't keep stuff like that in my head, but the specs will be on the website: http://www.partsexpress.com.
As for the optical part...let me see if I can beat this thing into submission first. Then I'll start a thread like I did with water-cooled amps, and the SOZ w/current sources. If optical doesn't work, I've got several fallback positions. Trust me, I'll get a working loop or perish in the attempt. It's just that I've got so many irons on the fire and so little time.

Grey
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