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Multi-Way Conventional loudspeakers with crossovers

Should I build this TL? - SB29NRX75-6
Should I build this TL? - SB29NRX75-6
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Old 15th July 2017, 12:01 AM   #1
Tadeusz is offline Tadeusz  United Kingdom
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Default Should I build this TL? - SB29NRX75-6

I became interested in transmission lines and wanted to design and build my own classic style, after reading about I naturally ended up playing around in the really impressive Leonard Audio program.

I wanted to build quite big bass cabinets with 10" drivers and an active crossover that includes a shelving low pass so I can correct any baffle step, so I input the specs for tons of different drivers and saw that I was getting the flattest response with an SB Acoustics SB29NRX75-6. I ended up with 800cm^2, 400cm^2 and 0.5m. this gives a line taper of 2:1. The terminus will be at ground level and forward facing:

Click the image to open in full size.

I read that if you aim for a drop-off of around 28Hz you will get a little bit of extra extension due to room standing waves between the drivers and the walls. In my current room the drivers will end up about 3.3m from the back wall, which should give a quarter-wavelength standing wave at 26Hz. I ended up with a 2.2m line length because the graph looked 'about right' for this.

I have also kept an eye on the displacement graph. The driver is rated with quite a large linear displacement of 11mm and a maximum power of 200 watts, I don't think I'll ever use that much but the displacement at 200 watts goes over 11mm at 26Hz and keeps shooting up, it looks like this can be sorted with a 24db/oct high pass on the active crossover set at 18Hz, I can adjust it lower just by swapping out some resistors depending on how much power I end up using:

Click the image to open in full size.

Due to the active crossover these cabinets will have their own separate amp. I've read that you should aim to have an amplifier capable of outputting double the watts the speaker is rated for, class D power is cheap so should I build a 400 watt amplifier or will there be no difference when using a 200 watt amp?

I don't know how an impedance graph should look so I will leave it up to someone on here to tell me how bad it is:

Click the image to open in full size.

So if anyone can tell me whether or not I should build something based on this design, or anything that needs changing, please let me know!
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Old 15th July 2017, 07:08 AM   #2
suzyj is offline suzyj  Australia
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Looks like heaps of fun - what sort of crossover are you going to use? I imagine with a bit of EQ you could push down well under 30Hz.
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Old 15th July 2017, 10:06 AM   #3
Tadeusz is offline Tadeusz  United Kingdom
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I already have a dual rail supply built into a case:
http://i.imgur.com/hb2KJ8C.jpg

For the crossover I'm going to build some of the filters on the LinkwitzLab page, mainly the 24dB/Oct Sallen-Key filters with OPA209 Op amps.

Active Filters
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Old 16th July 2017, 01:29 AM   #4
Tadeusz is offline Tadeusz  United Kingdom
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Also - a large cabinet like this will definitely need some bracing. For a transmission line is it safe to use the window pane braces that you see in ported cabinets? or is it critical that the line area be kept consistent and not get pinched?

When I do add bracing, should I adjust the cabinet to keep the total internal volume the same?
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Old 16th July 2017, 03:17 AM   #5
planet10 is online now planet10  Canada
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Should I build this TL? - SB29NRX75-6
Quote:
Originally Posted by Tadeusz View Post
or is it critical that the line area be kept consistent and not get pinched?

When I do add bracing, should I adjust the cabinet to keep the total internal volume the same?
Yes, yes.

Run the bracing in the same direction as the line.

dave
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Old 16th July 2017, 04:03 PM   #6
Tadeusz is offline Tadeusz  United Kingdom
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Alright, how's this for a cab? I think the sides will be the main resonance, I've increased the dimensions of certain areas to allow for bracing and some 10mm felt. The most comfortable internal width using this woofer and to get everything aligned was 300mm (12"):

Click the image to open in full size.
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Old 16th July 2017, 11:44 PM   #7
suzyj is offline suzyj  Australia
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I find that the interior bits can be quite thin, especially away from the driver, but the outsides of the box still need to be properly stiff. I'd be concerned about your top, as there's not terribly much in the way of bracing there.
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Old 17th July 2017, 12:41 AM   #8
hollowboy is offline hollowboy  Australia
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Quote:
Originally Posted by Tadeusz View Post
I read that if you aim for a drop-off of around 28Hz you will get a little bit of extra extension due to room standing waves between the drivers and the walls. In my current room the drivers will end up about 3.3m from the back wall, which should give a quarter-wavelength standing wave at 26Hz.
This link gives the best info on room gain I have ever seen. It is much messier than a rule of thumb might indicate.

Data-Bass

I think your 3D model looks great. You seem to have thought of everything, but in case it helps, some alternatives / additions to timber bracing are:

a weight on top (e.g. a big pot plant).

tying opposing faces together with threaded rods (really long bolts)
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Old 17th July 2017, 01:40 AM   #9
GM is offline GM  United States
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Quote:
Originally Posted by Tadeusz View Post
I read that if you aim for a drop-off of around 28Hz you will get a little bit of extra extension due to room standing waves between the drivers and the walls. In my current room the drivers will end up about 3.3m from the back wall, which should give a quarter-wavelength standing wave at 26Hz. I ended up with a 2.2m line length because the graph looked 'about right' for this.

Due to the active crossover these cabinets will have their own separate amp. I've read that you should aim to have an amplifier capable of outputting double the watts the speaker is rated for, class D power is cheap so should I build a 400 watt amplifier or will there be no difference when using a 200 watt amp?

I don't know how an impedance graph should look so I will leave it up to someone on here to tell me how bad it is
Well, ideally we want the speakers, listening position at a room odd harmonic, though unless you have a basic cube or rectangular one, it may take some measuring to find 3rds, 5ths, etc., but wondering where 3.3 m might be.

Anyway, normally folks have speakers somewhere near enough to a wall to be in the 1st harmonic, so a somewhat rolled off bass often works well, but at 3.3 m, guessing the higher tuning will be preferred and if not, then partially blocking the vent to lower it is an option.

Note that reflections are 1/2 WL 'round trip', so ~344/2/3.3 = ~52 Hz. Ditto room eigenmodes.

BTW, with vented alignments, it's considered a good plan to limit the vent area [Av] to driver effective piston area [Sd] and yours is nearly 30% larger, though due to liberal stuffing it hopefully won't allow the driver to unload if high power is used.

There's different rules-of-thumb depending on the app with up to a 6 dB [4x] crest factor, but for HIFI/HT apps, folks tend to set SPL mostly by ear based on how much audible distortion one can tolerate, which is usually well below a woofer's rating, so 200 W should be sufficient in your app as long as they can handle a sustained LF note without exceeding 200 W, if not, then driver heat rise will increase the system impedance, requiring more reserve power.

If they were playing full range though, then transients in the 250-500 Hz BW can be quite high if you listen to classical, and especially organ, symphonies, so your mains could be the power suckers and combined with the woofer's required reserve power you can easily hit the 6 dB crest factor and why I run wide BW prosound [sub] woofers up to 500 Hz to handle them.

Impedance looks fine, just shows you tuned it above Fs.

GM
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Old 18th July 2017, 11:45 AM   #10
Tadeusz is offline Tadeusz  United Kingdom
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Thanks for the help everyone, I'll let you know how I get on (:
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