Wanting to build a line array. Newbie troubles with impedance and efficiencies.

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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.
Line Array Stuff


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.


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?

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.

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.

After some more reading, and pondering, and figuring, here's what I have.

9 woofers, power tapered, 111.3mm CTC, 95.85dB theoretical sensitivity.
5 tweeters, power tapered(out of necessity), 38.2mm CTC, 99.44dB theoretical sensitivity.

One wavelength spacing on the woofers should be about 2950. With the xover (12dB) at 2500hz, that shouldn't be a problem I dont think. However with the tweeter spacing, effects will start to occur around 8950hz(1wavelength)

While the tweeters FR graph appears to do well from 2k+, it states 3.5-20k under it's description. So I hope i'm not asking too much out of these tweeters.

I'm also thinking about trying OB. Although my room isn't optimally layed out for it. If it fails i'm not out much and i'll try a ported cabinet tuned to maybe 95-100hz.

I think i'm on the right track now.

I think you are on the right track. Your design seems to get the most out of the drivers that you have in mind.

Yes, you can try the OB first but you'll have very little bass and would need likely stereo subwoofers crossed fairly high to mesh. As you say though, you can always port the cabinet.

Which configuration it is possible to compensate baffle step loss by adding more woofers to 0,5-way? One woofer parallel to midbass woofer gives 10 LOG2 + 10 log (8/4) = 6 dB gain to bass region (which is maximum that can be used, I think).

Is it possible at all to add more than one woofer to 0,5-way for one midbass (using same woofers)?
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