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Old 4th October 2006, 04:44 AM   #11
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Is this the design you are mentioning?

http://homepage.mac.com/tlinespeaker...-PP-BR-map.gif

I'm a newbie here looking for a moderate costing design and this one looks interesting. I wonder if it fits my needs?
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Old 4th October 2006, 10:13 AM   #12
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That design shows back to back mounting but not push pull. If it were push pull, one of the magnets would be facing out and the polarity of wiring on that driver would be reversed.

The output of that design is probably roughly equivalent of a single vented 12" driver of similar excursion. If you can get the drivers cheap at your location, then it could be a good cost effective sub.

I'd expect better performance by push pull mounting, effectively achieving the same result as a shorting ring, which is included in more expensive drivers of better quality. Push pull gives you that effect for free. You can get the vibration reduction with push pull as well, although not as effectively.

I'd consider a single larger driver in a vented box if you want maximum bang for buck on a budget.

Also consider that larger woofers, say 10 or 12" with say 9mm xmax in a vented box can still get some decent output, although you typically give up 20 Hz extension, you can normally get down to 25 Hz.

Back to that design, I would say it is a competent design, but I would do it differently, but that comes down to differing priorities I would place in designing a subwoofer.
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Old 14th October 2006, 06:04 PM   #13
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I've owned a number of motorcycles over the years. My favorite all around was my BMW R65 650cc "boxer". As every bike has it's quirks, this had a few. But with its opposed cylinders there are areas in the rpm range where the engine pretty much disappears beneath you - it's almost magical. Frustratingly, there always seems to be one or two areas in the rpm range where vibration is impossible to "iron out". But it's really only by comparison to the otherwise glassy smoothness. The trick ends up requiring jetting and timing adjustments to get the smoothest running in the range you spend the most time in. Boy is it worth it.
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Old 15th October 2006, 03:13 AM   #14
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Bluebeard, I didn't imagine bikes coming into the discussion, but interesting nonetheless. Why is it that vibration is different at different rpm?

I was just cranking up Jamiroquai as it has some potent bass. I had drivers at about 1" p-p, which is quite a lot for 40 - 60 Hz bass. I measured close to 120 db nearfield, about 110 db @ 1m and 101 db @ listening position 3m away. As a sidenote, I wondered for a while if subwoofers conform to the 6db attenuation per double distance. In this case it worked exactly.

Now with back to back mounting, I find this crappy box is sufficient, even with almost no bracing. Even at high excursion, the box isn't vibrating much, but the room is another story. It complains loudly. Some parts of the room enclosure vibrate more than the box.
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Old 15th October 2006, 03:49 AM   #15
Volenti is offline Volenti  Australia
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Quote:
Originally posted by paulspencer
Why is it that vibration is different at different rpm?
I would imagine the exhaust is acting like a transmission line and the vibration is the engine reacting to the extra (or lack of) back pressure as the line hits tuning along different areas of the rev range.

But anyway I can attest to the effectiveness of dual driver vibration canellation, I've used it in a few setups with good results.
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Old 15th October 2006, 06:14 AM   #16
Hara is offline Hara  United States
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Hello,

I'm not sure why this is such a hard concept to understand. The sum of all forces (as vectors) of the system is 0. The system is in a state of equilibrium and thus no net force = no net acceleration.

See diagram below.

As puggie mentioned, the mass that is accelerating is not only the driver, but the air itself, which should contribute significantly to box vibration if there is no vibration cancellation incorporated into the design.

The reason the motorcycle vibrates at certain rpms is the same reason drivers have the most displacement at their Fs (for a given amount of energy): Reasonance. Just as buildings move the most at different frequencies, so do engines. Strings of a piano also provide a good example.

Push-pull mounting, if the drivers are on opposite sides of the box, should give similar cancellation results to back to back designs. Back to back designs, however, still provide the best form of vibration reduction without increasing the mass of the box.
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