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Old 24th November 2004, 10:25 PM   #1
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Default Question about Qtc and sealed woofer box sizes

Designing some MTMs using these 6.5" woofers. I'm trying to build the cheapest 5 speaker setup I can with the speakers I have on-hand. Should be fun!

http://www.partsexpress.com/pe/pshow...number=299-493

Plugging the info into WinISD I get these figures for a sealed design. This info is for 1 woofer:

Qtc = 0.71 1.2 cubic ft (34.4 liters) F3= 56hz F6= 42.5hz F10= 32hz

Qtc = 0.8 0.75 cubic ft (21.2 liters) F3= 57hz F6= 45hz F10= 35hz

Qtc = 0.9 0.5 cubic ft (14.2 liters) F3= 59hz F6= 49hz F10= 39hz

Qtc = 0.98 0.4 cubic ft (11.3 liters) F3= 61hz F6= 51hz F10= 41hz

These speakers will be used 100% for Home Theater and will be using a sub with them. As you can see, these drivers require a lot of air space in a sealed box to achieve 0.71 Qtc. Seeing how these are pretty flat down to 60hz in a much smaller box, and knowing my sub can easily go to 60hz and integrate well, will the higher Qtc make the bass sound extremely "boomy"? At what Qtc should one stop? Note, these will probably never play music so they don't have to sound exceptional but I'd like for them to perform well in the HT environment. Also, I'd like to use them for the center and surrounds so making those cabinets small is pretty much a necessity. I got them for a good deal and would like to use them so please don't discourage me

Lastly, would you recommend doing an MTM or going with a TMM with the tweeter up top? Would that cause too much separation? I was leaning toward using either some cheap Dayton 3/4" tweeters I won or possibly getting the Dayton PT2 ribbon tweeters and seeing how I like them. Or could you guys recommend nice CHEAP tweeters that would integrate well with these? Here are the tweeters I won:

http://www.partsexpress.com/pe/showd...number=275-030

I will be powering the whole set off modified full-range Audiosource plate amps ($25 each) that were tested at 125w @4ohms 0.1%. Using dual Shiva sonotubes tuned to 20hz. each with 300w plate amps that I purchased for $110 with shipping. I'm trying to keep the whole project under $500. I'd like for people to come over, hear it, be blown away, and then collapse of a heart attack when I tell them I built the whole thing for under $500

Sorry for the long post and I'm truly grateful for any help you guys can provide. I ask a ton of questions here, I know, but I am learning. I've built only a few of the projects I've inquired about but you have my word I'll be doing this one!

THANKS!
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Old 24th November 2004, 11:47 PM   #2
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The Qtc in and of itself matters little, it's what the box does within the passband. If you want to use an 80 Hz crossover, for instance, then model the driver and see what the smallest box is that doesn't give a response bump near 80 Hz or higher and also gives the flattest possible response to 80 Hz. If you get a bump at 60 Hz it won't matter much if the crossover is adequate to filter it out anyway. With 6.5s I'd cross from 80 to 100Hz so that you don't have excursion issues. As for TMM versus MTM the MTM will almost always sound better. Decide on the tweeter by seeing at what frequency the midbass is down 6dB at 30 degrees off-axis, and don't cross over higher than that. Typically that's around 2.5-3.5 kHz with 6.5s, too low for the average 3/4 tweeter, but just as well as the 6.5s in an MTM wouldn't work well to the 5 kHz minimum that most 3/4 domes require.
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Old 24th November 2004, 11:56 PM   #3
Ron E is offline Ron E  United States
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I have these (infinity buyout 7" with the siver trim ring) woofers in a 24liter box and they do not sound boomy at all - actually they sound fairly lean to my tastes...I wish I had gone smaller I don't think that a Q of 0.9-1 will sound boomy, especially not if you are using a sub crossover. The lower box volume also increases power handling.
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Old 25th November 2004, 12:18 AM   #4
Bose(o) is offline Bose(o)  Canada
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The lower box volume also requires more power to reach maximum excursion or, say the same excursion in a larger box.

Depending on crossovers used, you may want a bump at the xo point. You can always make a test box of the largest volume then add pieces of wood in which to bring the volume down to the next level. Then, see if you like the sound. Measuring will also help you determine which one is 'best'. It may be tedious, but by the time you are done; you know that you have the best sound you can get within the requirements that you have set.
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Old 25th November 2004, 03:34 AM   #5
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It should interest you to know that back in the fifties, when sealed design was finally completely understood, loudspeaker designers aimed their designs for a final Qtc between 0.7 and 1.0.

Qtc = 1.0 was considered something of a compromise for bass freaks, but it was still considered an acceptable design. For a home theater setup, it is hard to imagine that it would be unsatisfactory.

Note that a Qtc = 1.0 is still going to have transient response MUCH cleaner than any ported design you will ever see. Yet ported designs are in common use in high fi systems, let alone home theater.

Bottom line: go with it.

Please note posts #31 and #33 here, and click on the thumbnails:
http://www.diyaudio.com/forums/showt...547#post366547

As you can see, the sealed box of Qtc = 1.0 is very much closer to Qtc = 0.7 than any practical ported alignment can get. Yet ported alignments sound good to most people. So the Qtc = 1.0 should serve you well.
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Old 25th November 2004, 07:27 PM   #6
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Thanks so much for all the replies! Really helps me a lot!

Quote:
Originally posted by Ron E
I have these (infinity buyout 7" with the siver trim ring) woofers in a 24liter box and they do not sound boomy at all - actually they sound fairly lean to my tastes...I wish I had gone smaller I don't think that a Q of 0.9-1 will sound boomy, especially not if you are using a sub crossover. The lower box volume also increases power handling.

Any chance you can let me know which tweeter you paired with these and where you crossed them over, and what order?

Thanks!
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Old 26th November 2004, 01:49 AM   #7
KBK is offline KBK  Canada
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Qtc=1: better midrange/upper bass.
Qtc=0.7 best compromise between midrange and bass transient response.

Qtc=0.5: best transient response in bass range.
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Old 26th November 2004, 10:21 AM   #8
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Default MTM vs MMT

MTM usually better for HT, because room interaction is less noticeable. Also both woofers are close to tweeter, which is nice... important to get the tweeter and woofers REAL close to each other, offset tweet to side, not completely, but a bit, or cut flanges if you are brave enough...

I dislike them for music... it seems as if the music gets letterboxed... but that may be because too high X-over frequency... But this is actually good for HT...
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Old 26th November 2004, 08:36 PM   #9
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Thanks for all the info! Looks like I'm going to be going with a 0.9-1.0 box, after testing them of course.

Now what I would like to know is if there are any good tweeters on clearance anywhere, 1" a must, that would mate well with two of these Vifas in an MTM? I'd like to keep the cost per tweeter under $15. I was considering trying the Dayton ribbon tweeter just because I've never really designed a speaker with such a tweeter and thought it would be interesting. Not sure if it would be wise because the CtC spacing for the woofers would be increased. One thing I did like about the ribbon is the higher sensitivity and higher output it has. I was even considering trying out an MMTMM 2-way sealed. Toughts?

http://www.partsexpress.com/pe/showd...number=275-085

As always, you guys rock! Thanks!
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Old 2nd December 2004, 10:03 PM   #10
Bose(o) is offline Bose(o)  Canada
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I'm not too sure about MTM design principles, but I know that the MTM design will control vertical dispersion so as to kill off reflections from the floor and ceiling. This may have been already mentioned.

Anyway, I think ribbon tweets have a rectangular dispersion pattern and so may not need the MTM design. You may be better off if you go with a TMM 2.5-way design.
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