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Old 29th May 2005, 05:29 AM   #1
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Default Wanting to build a line array. Newbie troubles with impedance and efficiencies.

I'm wanting to start the very fun, and in this case cheap, project of constructing a line array. It might not be the best way to get my feet wet, but if I cant get excited about a project, I dont always finish them. And I dont find small and plain 2way systems or the similiar very titillating (not that there's anything wrong with plain 2ways!). Well here's my small problem. I'm having difficulties coming up with a workable impedance for my reciever. I'm trying to get an 8ohm load, or as close as I can to it. But i'd take 6-10ohms.

My plans are to use 8-10, 8ohm, 4" mids, and 4-6, 4ohm tweeters. With a 6db xover at 5000hz (my first xover, keeping it simple). How can I wire these to get my desired load? I havn't been able to.

Another question is how can I figure out the correct number of tweeters i'm going to need to keep up with the mids? The mids are 86db/1watt, the tweeters are 92db/1watt. With a 6db efficiency spread, AND the fact that they're 4ohm and the mids are 8ohm (extra 3db?), does that mean I will only need 1 tweeter for every 8 mids? Doesn't seem right to me. Or would that really hurt sound quality, and would it be better to to run like 6 tweeters in series to make a 24ohm load to attenuate them. That way I have a long tweeter line, but doesn't overpower the mids.
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Old 29th May 2005, 01:26 PM   #2
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Default Line Array Stuff

Guy,

If you haven't read my line array white paper, perhaps you should as it will help with a lot of your issues. Typically, it is best to have the tweeter line more sensitive than the woofer line--its is easy to pad down the tweeters. It is a bummer to have woofers more sensitive than the tweeters.

By the way, I lived in Colleyville for a number of years during my working days.

Jim

http://www.audiodiycentral.com/resource/pdf/nflawp.pdf
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Old 29th May 2005, 01:27 PM   #3
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Try this a a start to understanfing a line source.

www.audiodiycentral.com/resource/pdf/nflawp.pdf

Keep the number of tweeters you think will work and pad them down to match the woofers after all is assembled and tested. IF you don't have test equipment then at least get a Radio shack SPL meter. It will help alot.

Mark

P.S. Jim beat me to the punch!
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Old 29th May 2005, 04:54 PM   #4
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Great read. I read through it really quick. Late when I get time i'm going to really read through it.

Is it possible to change the impedance of my woofer or tweeter line. Not neccessarily to attenuate spl, but to change the load to one my amp will like pushing?
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Old 29th May 2005, 05:01 PM   #5
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Here's the line I currently have planned.

12 woofers. 8ohm. 85db. (Line = 10.79ohm, 96.43db)
6 tweeters. 6ohm. 92db. (Line = 7.78ohm, 99.27db)

total line impedance = 4.5ohms.

My reciever needs at least 6ohms.
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Old 29th May 2005, 05:34 PM   #6
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Default Theory is only good untill it gets smaked down by practice

You can tailor the impedance of your line. But what you are quoting are the lowest resitances. You either need to model them on a computer program or better yet wire them up and measure them. The computed spl and the actual SPL will be different!

Mark
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Old 29th May 2005, 05:58 PM   #7
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What's the best method of measuring the actual resistance? Simply wire them up and check with a multimeter? Once I find the actual resistance, how do you go about adjusting it?
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Old 29th May 2005, 06:02 PM   #8
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Guy,

You don't combine the impedance that way even though the tweeter line and woofer line are wired in parallel with each other.

The tweeters will need some type of XO to protect them from low frequencies. Even if it is just a cap, their effective impedance in the lower frequencies will be very high.

Even if you run the woofers full range, they have an impedance which increases in the higher frequencies, so your amp will never see the low combined impedance of two.

This is why a typical 2 way speaker can use an 8 ohm woofer and an 8ohm tweeter and still be an 8ohm speaker despite being wired in parallel.
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Old 29th May 2005, 06:24 PM   #9
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Of course! I understand now. Thanks guys. I'm sure i'll have more questions once I read more deeply into Dr. Griffins paper.
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Old 29th May 2005, 08:02 PM   #10
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Guy,

You said:


"12 woofers. 8ohm. 85db. (Line = 10.79ohm, 96.43db)
6 tweeters. 6ohm. 92db. (Line = 7.78ohm, 99.27db)"

I'll give you some thinking numbers in the discussion below but the actual sensitivity changes would best be determined by measuring these drivers mounted in the enclosure.

You will gain array efficiency with dome or circular tweeters if they are mounted within a wavelength center to center over their entire frequency band of coverage. That would be worst case within 0.67" c-t-c at 20 kHz and I doubt that you have tweeters that small and that close. If you do, count on all of that array efficiency improvement because of the spacing but likely you'll measure less than this value in reality.

With 6 tweeters you'll have an array efficiency gain of 10 log (6) = 7.78 db. For the tweeters I suggest that you feed them with equal power--perhaps three paralleled strings of two tweeters each or 2/2/2. That would be an array impedance of 4 ohms (for the 6 ohms nominal for each driver). You gain 1.76 dB in sensitivty with this feeding. which would yield a total tweeter array improvement of up to 7.78 + 1.76 = 9.54 dB.

If you don't like the 4 ohms arrayed tweeter impedance, you could use a 3/3 feed for an arrayed impedance of 9 ohms. A little less arrayed sensitivity (7.78 - 0.5 = 7.28 dB) for this connection.

On the 12 woofers (assume 8 ohms nominal for the individual driver) you can feed them with equal power such as 3/3/3/3 for a 4 ohms arrayed impedance. You would improve the efficiency by 10.79 dB and increase the sensitivity by 3 dB for a total improvement of 13.8 dB above the sensitivity of a single driver.

If you want a higher arrayed impedance you can do a 4/4/4 for an array impedance of 10.666 ohms but you give up -1.25 dB in sensitivity so that the total arrayed sensitivity improvement is 10.79 - 1.25 dB = 9.54 dB.

You will lose some of that woofer sensitivity, if you are are using a closed box woofer array, to account for baffle step compensation. That would be up to 6 dB in total.

Power tapering as explained in the white paper can yield benefits and would change the overall arrayed impedance and sensitivity and such.

Jim
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