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Old 18th September 2009, 11:06 AM   #1
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Join Date: Nov 2002
Location: Bangalore, India
Question A few doubts about side woofer line array.

The side woofer line array at
http://www.audioxpress.com/magsdirx/...a/bird2483.pdf
deviates quite a bit from the typical line array design:-


1) Its an array of just 4 mid drivers (if you call something with 4 drivers an array) .
2) Mid to high xover freq is 6 khz. Way too high for the CTC between the drivers.
3) Woofer to mid xover freq is 800hz. Way too high for a side woofer design, too close to the ground and that too firing sidewards.

How does it still measure good and sound good too?

Thanks,
Goldy
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Old 18th September 2009, 05:58 PM   #2
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To begin, something doesn't necessarily sound good even if it measures well. Measuring well is only the first step; there might be other variables such as dispersion hot spots, distortion, phase, and so on, which mean a speaker with flat on-axis response might sound different from another one.

First, side woofers work perfectly well at wavelengths long enough for the driver to look like a point source, but 800 Hz is starting to get into the range where beaming begins for 10", and since the crossover is first-order, that becomes a concern for the next couple of octaves.

Second, look at the graph: it looks seriously smoothed. I'm willing to bet a more precise curve would show all kinds of irregularities, especially below 800 Hz as the direct wave from the woofer mixes with the delayed wave bouncing off the wall.

Third, it looks as if the mid and tweeter crossovers overlap by a bit. Assuming the tweeters are about 6 ohms, a 5 uF capacitor means the -3 dB point is about 3.3 kHz, whereas the 0.2 mH inductor rolls of the mids around 5 kHz. Note there's no Zobel network, so the mid voice coil inductance means the mids roll off at less than -6dB/octave. I can see why the designer did that: there's a bit of a bump around 5 kHz where the mids and tweeter overlap, with a rolloff above that. Perhaps the series-parallel tweeter connection loses some efficiency.

Fourth, the article blithely says to just throw in any old drivers and the thng should work OK. Perhaps it's me, but I have my doubts about how well the duct would be tuned with another woofer in the box.

Long story short: not a terrible start, needs some work.

What would I do differently without too much expense?

1) The mids could be run without a crossover in the lows if the rear chamber is sized so the response has a Q of 0.5, so the mid system response has its own second order Linkwitz-Riley high pass filter, with the woofer on a second order LR lowpass filter. That means the crossover would be in the range of about 100 Hz, which works nicely with sidemounted woofers. You can actually mess a bit with the woofer LPF Q if the mid Q is above 0.5, but that's an advanced topic.

2) The mids need a Zobel network so the lowpass can do its work. Maybe the lowpass could be adjusted so it would cut off a bit lower, which would work in conjunction with:

3) Run the tweeters as three parallel strings of two series drivers instead of two parallel strings of three series drivers. That way the array would provide about 3 dB more output at the price of slightly lower impedance, which eliminates the need to overlap the mid crossover. The more efficient array also shows more output above 10 kHz, where this system is a bit weak. The tweeter cap would need to be increased to 11 uF; 10 uF should be fine

4) I'd stuff some damping behind the tweeters to keep the slot from resonating at inconvenient frequencies.

Some solder time, three more parts, and Bob's yer uncle.
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