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hcirlub
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Joined: May 17 2007 - 9:57pm
Novice question

So this is my first post sorry if it is not the right place. I have just recently gotten the audio bug. I am in the early stages of trying to make the most out of the random odds and ends that I already own. I have been taking older beat up speakers and rebuilding them. Mostly just new enclosures and soldering heavier wire to the leads. I've had pretty good success so far.

So here is my question: I just opened up a seriously cheap pair of speakers and found two woofers and one tweeter. One of the woofers had no driver and was therefore not connected and basic just a show woofer. Is this normal? Is there any audio reason to do this or is it just to give the appearance of being a better speaker from the front view? Can I slap a random driver of reasonable size on the cone and call it good?

Alright that was more then one question. And most of you are shaking your head asking why bother. Well I have no budget and actually I just like to see what I can build out of free or nearly free parts.

Any input would be welcome thanks in advance.

cyclebrain
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Joined: Jun 16 2006 - 11:40pm
Re: Novice question


Quote:
So this is my first post sorry if it is not the right place. I have just recently gotten the audio bug. I am in the early stages of trying to make the most out of the random odds and ends that I already own. I have been taking older beat up speakers and rebuilding them. Mostly just new enclosures and soldering heavier wire to the leads. I've had pretty good success so far.

So here is my question: I just opened up a seriously cheap pair of speakers and found two woofers and one tweeter. One of the woofers had no driver and was therefore not connected and basic just a show woofer. Is this normal? Is there any audio reason to do this or is it just to give the appearance of being a better speaker from the front view? Can I slap a random driver of reasonable size on the cone and call it good?

Alright that was more then one question. And most of you are shaking your head asking why bother. Well I have no budget and actually I just like to see what I can build out of free or nearly free parts.

Any input would be welcome thanks in advance.

Oh Oh Oh I know the answer to this one.
The device with no active driver is a passive radiator.
This type of speaker system is closely related to a ported enclosure. The passive driver behaves like the column of air in the port of a ported bass reflex speaker.

absolutepitch
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Joined: Jul 9 2006 - 8:58pm
Re: Novice question

I think Cyclebrain got it right about the passive radiator. It's basically like a port on a "bass reflex" cabinet as he said, but the embodiment is a suspended cone that has a solid mass instead of a moving air mass of the port.

You could measure the internal volume of the box and try to install a different woofer after evaluating the "Thiel-Small" parameters that would match the right woofer to the box you already have. If you're not familiar with this, there are many websites that could assist you(www.madisound.com for example). One possibility is to seal the port with strong panel of wood or so and use a different woofer; the system would roll off in the low end more gradually (12 dB per octave) than a ported system (typically 24 dB per octave. But there are compromises, such as level matching to the tweeter, efficiency, frequency of where the lows start to roll-off, etc., that are the fine points of the Thiel-Small parameter for low frequency box design.

I got into this a long time ago and built a bass reflex speaker using the older Novak methods. The system had a low frequency roll-off at around 55 Hz. I then found a 12" woofer with particular Thiel-Small parameter that would theoretically give me 41 Hz in the same box, except that the port needed to be sealed. I did that modification and measured, getting around 39 Hz (not precise measurement), and I was happy enough to leave it that way. The efficiency was lowered so I had to turn down the mid-range speaker and the tweeter more than before to get the levels to match.

Good luck and let us know your results.

hcirlub
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Joined: May 17 2007 - 9:57pm
Re: Novice question

Thanks for the advice, I think actually instead of fitting a new woofer or closing the port I am looking into building a new enclosure entirely for the one woofer and tweeter. The current one is pretty beat up and cheap.

absolutepitch
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Joined: Jul 9 2006 - 8:58pm
Re: Novice question

If you want to fit the exising woofer into a new enclosure and keep the same crossover, take into account the "Thiel-Small" parameters if you have them to calculate the enclosure or else duplicate the existing enclosure if it's damaged. Be careful. A different efficiency caused by changing the enclosure volume can throw the crossover design out of alignment with the tweeter in both signal level and crossover frequency and unequal gain at the crossover point.

I built a one-way, then two-way, and then a three-way system, expanding on the prior work. The tweeter is front mounted on a baffle, and the midrange driver is in a separate enclosure in the same box as the tweeter, but isolated from the woofer box by a thick layer of foam; I don't feel any vibration of that upper box when the woofer is pumping air. A separate box for the tweeter makes sense to me.

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