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Old 22nd January 2003, 06:49 PM   #41
Ilianh is offline Ilianh  Canada
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this isnt 1 watt, neither is at 1 meter...

i'll try to calibrate the mic, and to give it 1 watt
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Old 23rd January 2003, 12:16 AM   #42
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Was this chart measured close-range, with the mic an inch or so from the speaker cone?

Or was the woofer simply free air, measured a foot or more away?

Or did you mount this into some kind of big plywood or MDF piece, several feet across and high?

Close mike measuring is really good for unenclosed speakers or even those in a sealed box. They give results very similar results to measuring the speaker outdoors against a wall.

Measuring a woofer simply free air will give severely depressed response in the midbass and bass. The back waves all come around and cancel the low frequencies.
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Old 24th January 2003, 01:50 AM   #43
Ilianh is offline Ilianh  Canada
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Acoustical short circuits. yep, well what I was going to do is a 20hz quarter wavelenght panel and try to do it a free air response plot.

now im trying to figure out if simple el-cheapo stonger than normal cardboard would be fine...


any ideas?
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Old 24th January 2003, 02:02 AM   #44
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Quote:
Originally posted by Ilianh
Acoustical short circuits. yep, well what I was going to do is a 20hz quarter wavelenght panel and try to do it a free air response plot.

now im trying to figure out if simple el-cheapo stonger than normal cardboard would be fine...


any ideas?
Not sure what you mean. Are you abandoning the bandpass and going for a Quarter Wavelength Enclosure? That is a Transmission Line type enclosure.

Are you going bandpass or Transmission Line?
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Old 24th January 2003, 04:04 AM   #45
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i wanted to chirp in and say that once apon a time i tried doing such an enclosure with a really tiny chamber tuned to a high frequency. This was early on in my sound experience and I was using a pyramid brand 12" sub. One side was tuned with a 2 foot external PVC port, and the other side was tuned to somehting high via a circular hle cut nto the particle board. ( this was before I even heard about mfd.. back in 96 when i was still in highschool) anyways, to make a long story short, it sounded doodoo. I was kinda of amazed in that it did work and produce Bass, in quite usable amounts, but the high freq tuned side created terible port noise. Maybe this design could work if port windspeed was carefully acounted for, but i have a feeling that in order to manage it to usable levels the volume rises, or the freq goes lower, completely losing the huge peak the programs show over a typical sealed or ported box.

-edit out some port length infro that i realized was backwards
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Old 24th January 2003, 01:02 PM   #46
Ilianh is offline Ilianh  Canada
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Quote:
Not sure what you mean. Are you abandoning the bandpass and going for a Quarter Wavelength Enclosure? That is a Transmission Line type enclosure.
Hm, no, that was simply to avoid the acoustical short circuit you stated in low freqs when in free air. Just in order to measure the woofers frequency responce plot.
I was reffering to this sentence you wrote:

Quote:
Measuring a woofer simply free air will give severely depressed response in the midbass and bass. The back waves all come around and cancel the low frequencies.

anyway.




Quote:
Was this chart measured close-range, with the mic an inch or so from the speaker cone?
Yes, it was at the center of the cone, on the same level as the rubber surround.


I wanted to try to use 1 watt from 1 meter, but How can i test if the results will be anywhere near reality, no ideas if my mic is calibrated corectly. I'd need something like a real spl meter to do that correctly.. I guess...
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Old 24th January 2003, 01:56 PM   #47
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Quote:
Originally posted by Ilianh
[B] Yes, it was at the center of the cone, on the same level as the rubber surround.


I wanted to try to use 1 watt from 1 meter, but How can i test if the results will be anywhere near reality, no ideas if my mic is calibrated corectly. I'd need something like a real spl meter to do that correctly.. I guess...
Well, a rough calibration of sorts can occur in a two step operation.

First, use the close mike method to measure how far down the bass freqquency is from the midband. Pay special atention to where the free air resonance frequency is from the midband. If the midband-200 thru 1200 Hz fluctuates, try to take an average.

Second, take a commercial speaker whose specs you have reason to trust. Measure the sound 25 cm in front of it through the midrange, like from 200 through 1200 Hz. Run one watt through the speaker-pay attention to rated impedance!

Then, place you unenclosed woofer on a a few pillows on top of a table or stool, and measure the SPL at one watt, again 25 cm in front of the speaker. Try to have it so the speaker doesn't isn't near anything solid for several feet. The theory is, room reflections won't be a factor because any sound that bounces off a hard surface and comes back will have traveled so far, it's amplitude will be reduced to negligible compared to the direct sound hitting the mike from only 25 cm away.

Pay attention to the upper midrange, for those are the frequencies that won't "wrap around" the woofer to cause cancellation. How big is that woofer again? The higher the frequencies-assuming the woofer isn't in "cone breakup" mode-the more accurate the test.

Then compare the two readings. If your woofer is 2 dB down from a 88 dB rated speaker, then you are at 86 dB.

Incidentally, the reading you get for when the mike is placed 25 cm from the speaker? That will be 6 dB higher than the 1 meter reading. Double the distance = go down 6 dB. Except for tall, slender line arrays.

Let me know how you make out.
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Old 24th January 2003, 02:04 PM   #48
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Jared:

So the bandpass set up this way really does work. That is good to know. Since you seem to get more bass from a smaller box with this, both you and Ilianh could be onto something.

Seems to me the problem with the high frequency port could be gotten around. Either a DIY Passive Radiator, or sharp slope crossovers that cut the amount of sound coming out of the high frequency port so far down, it doesn't matter.
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Old 24th January 2003, 04:26 PM   #49
Ilianh is offline Ilianh  Canada
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What if the high freq ports were majorly flared?
That would reduce noise somehow, I guess...
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Old 25th January 2003, 01:09 AM   #50
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Assuming you are going to use 3/4", (18mm) MDF or plywood to plug the end of your pipe, then a 5.25 diameter hole, .75" wide-the width of your wood-will tune a .2 cu ft box to 250 Hz.

The wavelength of 250 Hz is 54 inches long, so I don't anticipate interference problems.

Another solution is to tune the thing real high-like up to 1000 Hz-with a Passive Radiator. The response spike at 1000 Hz can be taken care of by a 12 dB or 18 dB crossover.
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