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Old 15th April 2009, 01:01 PM   #1
IG81 is offline IG81  Canada
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Default What I built, impressions and solutions?

Hello,

I bought a couple of 5¼" fullrange drivers from McBride, model Linear A 5.5, with the following T/S Parameters:

Fs: 70 Hz
Qms: 4.04
Qes: 0.49
Qts: 0.44
Vas: 8.72 liters

So I set out to build a ~54 liter sealed enclosure with the following inner dimensions:

h: 47.5"
w: 7"
d: 10"

I was going for infinite baffle with such a big enclosure, ~6 to 7 times Vas, that being my first build and this design looking pretty forgiving. I added series resistance of 4.7ohm to each driver to raise Qts and bring Qtc ~0.73. WinISD simulated an f3 ~72Hz.

I positionned the driver at a height of 36" to have it around ear height and stuffed the middle of the column, from just under the driver to 16" lower, with ½ a pound of polyester fiber to dampen the antinode area of the ~143Hz standing wave in the 47.5" dimension.

So that's what I build and here are my impressions now. This system is simply unlistenable. Not one frequency range sound right. Bass is muffled and boomy, mids are telephone-esque and the highs are kinda shrilly.

What could have happened here and what could be done about it?

I know my drivers are not high-end, but I was still expecting a bit better, something decent at least... I wouldn't think driver break-in would solve any of that, what do you think?

Could the stuffing be too dense? I have ½ a pound in the middle section, which translates to a density of ~0.75 pound per cubic feet, what do you think? Would too high a density fool the driver into seeing a much smaller Vb thus causing my boomy bass?

What about the telephony mids? I cannot EQ them out any way I tried. They just seem to be a "quality" of this driver, especially on the snare drum.

So I don't expect to make these sound like my Pierre Étienne Léon ML2's, but at least a bit better than my crappy Altec Lansing satellite+sub computer system, that would be a good start...

Thanks!
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Old 16th April 2009, 01:38 AM   #2
Nanook is offline Nanook  Canada
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Default where'd you get the specs?

I went to McBride's website and didn't find the speaker model you mention.

stew
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Old 16th April 2009, 02:35 AM   #3
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It took a bit of searching...
The Linear A 5.5 is on pg 42 of the Q-components catalog - download from McBride http://www.loudspeakers.ca/products.htm

Repeating the specs:
LINEAR A 5.5 $20.31

5-1/4” foam edge
dual cone fullrange
Outside Dim................134 x 134 mm
Impedance...............................8 ohm
Power ........................................35 W
Freq. Resp ................65 Hz - 18 KHz
Magnet Wt ................................10 oz
Fs .............................................70 Hz
Re .......................................6.7 ohms
Qts ..............................................0.71
Qes .............................................0.86
Qms............................................3.9 1
Vas .....................................8.82 litres
Voice Coil Dia .........................25 mm
SPL...........................................89 db
Depth ......................................61 mm
Baffle Opening......................123 mm
Mtg. Cntr...................139 mm BC x 4

Cheers
John
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Old 16th April 2009, 03:23 AM   #4
Nanook is offline Nanook  Canada
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Default thanx for the link..

I downloaded the McBride pages

stew
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Old 16th April 2009, 03:25 AM   #5
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there are others here are much more technically proficient on this stuff than I am. It sound like you have a good grasp of the concepts involved. Just curious, did you try the driver in other enclosures? I'm really fond of the 2 pairs of TQWTL speakers I made. I have also built sealed box as well as vented box single driver speakers. Are you perhaps expecting too much from a single driver of this size for this large an enclosure. I wonder if you wouldn't be much more impressed with these mounted in a simple tapered pipe speaker. I usually think of open baffle units being larger dia and perhaps multiple drivers. Could you or have you experimented with reducing the volume?
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Old 16th April 2009, 04:42 AM   #6
Nanook is offline Nanook  Canada
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Default plugging your number into WIN isdPro...

gives an Efficiency Bandwidth Product of 142.9, which indicates a 6th order bandbass would be best, (maybe I'm wrong , but I just added 4.7 Ω to the Re of 6.7 Ω , becoming 11.4 Ω ). This results in a Qtc of .475 with a 54 litre cabinet. Also your stuffing is making the box seem even larger, so the effected range will be greater than 135Hz-1000Hz

An EBP of 50 or less usually indicates a closed box, while one above 80 suggets a vented enclosure

F3 is at 135 Hz, but not smoothing out until 1000Hz. So any possible bass is too shallow, and everything up to 1000Hz is pulled back or "cupped"

now if making a BR or tuned enclosure, the results are much better, F3 of 59 Hz, , with 0 dB offset at 100Hz or so, and only an 11.56 litre box, tuned to 64.7 Hz. That's a 3.23" diameter X10" long port (or a rectangular one about 9.42" L X 4.04" X 1.85")

In this case the driver is so far from appropriate for any closed box , and certainly for one 54 litres. Hope that helps.

stew
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Old 16th April 2009, 12:40 PM   #7
IG81 is offline IG81  Canada
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The factory Qts of 0.71 was not very real, as I measured it at ~0.42 IIRC, using proven methods. This is why I added series resistance as to get Qts back up so Qtc doesnt get too low.

Bass is not the only problem, the mids are quite unlistenable, sound like they were routed through a telephone and EQ just cannot fix that, seems to be a built-in sound of the driver.

Win ISD modeled the bass response with f3 of ~72Hz, which would have been good. I did not measure, but it sounds more like a higher Qtc, at a higher Fb, boomy and not smooth. I tought of such a big enclosure to only contain the back wave and have minimal spring action from the trapped air.

I guess I could use T/S measurement methods with the driver in the box to see what kind of Fb and Qtc it's developping in there.

I'll do the above and try to play with stuffing, but I have a feeling that the drive units are just plain cheap ones and not much can be done in the way of hifi with them. Last resort I could also chop the tall boxes to smaller ones and add a port to try a BR alignement.

I'm bummed out, but I won't stop there, I have other build ideas, I'll just remember to spring for better drivers.

Thanks for the feedback!
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Old 16th April 2009, 01:54 PM   #8
Nanook is offline Nanook  Canada
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Default yup, I'd measure in the box...

seems most logical given various differences between "the answers" and the observations. Another argument against objective testing only...


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Old 16th April 2009, 02:41 PM   #9
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Tall, thin box = trouble. Model this with MJK's worksheets.

Bob
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Old 16th April 2009, 03:44 PM   #10
IG81 is offline IG81  Canada
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I did not get a chance to get those before access was restricted unfortunately.
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