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Old 5th October 2012, 07:17 PM   #341
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Eighteen Sound have a dual 21 inch manifold design , interesting that they do not couple the two drivers. Worth a look. Best regards Moray James.

http://www.eighteensound.com/staticC...21_%20Prel.pdf
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Old 5th October 2012, 08:10 PM   #342
djk is offline djk
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"what tweeter this is?"

SEAS H253
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Old 5th October 2012, 08:23 PM   #343
kees52 is offline kees52  Netherlands
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Originally Posted by djk View Post
"what tweeter this is?"

SEAS H253
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it has no label anymore, but it sounds sweet, so why I did ask.
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Old 5th October 2012, 08:38 PM   #344
Scott L is offline Scott L  United States
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Default 18 sound dbl manifold

Quote:
Originally Posted by moray james View Post
Eighteen Sound have a dual 21 inch manifold design , interesting that they do not couple the two drivers. Worth a look. Best regards Moray James.

http://www.eighteensound.com/staticC...21_%20Prel.pdf
I don't care for the face-to-face mounting at all. If anything, a magnet to magnet coupling to cancel forces. Below 80hz for sub use only. Those huge 21 inchers have too high an Fs to be considered subs (in my opinion).

Here's how I did mine. The pic is BEFORE being installed in the Infinite baffle system.
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Old 6th October 2012, 05:30 AM   #345
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Originally Posted by Scott L View Post
I don't care for the face-to-face mounting at all. If anything, a magnet to magnet coupling to cancel forces. Below 80hz for sub use only. Those huge 21 inchers have too high an Fs to be considered subs (in my opinion).
Scott,

The 18 Sound 21" drivers have an Fs of 28 and 29 Hz, lower than the drivers elsewhere in this thread.
Sub means "below", the 35 Hz LF corner of the 18 Sound "Manifolded Double 21 Bandpass Subwoofer Kit" is well below most "full range" speakers.

As far as canceling vibration forces, face to face, magnet to magnet, or push pull (magnet to cone) are all equally effective. Your infinite baffle system could have reduced even order harmonic distortion had you used push- pull mounting.

18 Sound drivers have a fair amount of magnet structure heat vent noise when close to Xmax, magnet to magnet mounting would allow that noise to be heard, while the cabinet design with the cones both facing in to the manifold plenum dampens that noise.

Art Welter
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Old 6th October 2012, 05:36 AM   #346
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Quote:
Originally Posted by moray james View Post
Eighteen Sound have a dual 21 inch manifold design , interesting that they do not couple the two drivers. Worth a look. Best regards Moray James.

http://www.eighteensound.com/staticC...21_%20Prel.pdf
The drivers are coupled in a large manifold plenum, though not push pull mounted.
When the driver's heat vent noise is objectionable (as in 18 Sound speakers), it may be worse sounding to have the driver's vent "in the open" than the even order harmonic distortion reduction PP reduces.

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Old 6th October 2012, 06:54 AM   #347
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http://www.seas.no/images/stories/vi...253_1_dome.pdf Boston Acoustic used these in a variety of models for many years, they do sound pretty good.

Regards vent noise:

The original drivers I designed the PPSL around had no pole vents, they had a large felt dustcap and were dead quiet.

For hi-fi the driver under consideration should be evaluated for vent noise. Linkwitz discusses this on his PPSL design H-frame subwoofer, he found considerable differences in drivers that looked similar.

"Some drivers may buzz due to marginal attachment of the magnet assembly to the basket. This is difficult to determine before the dipole woofer is completed. To avoid this, apply a heavy bead of Silicone II Sealer & Glue to the gap between magnet and basket. "

"A driver with Xmax = 12 mm will generate quite a bit of aerodynamic noise, i.e. distortion, at such high excursions. The space behind the spider needs to be open to the outside to minimize noise. The pole piece should have a large vent, or the cone should have no dust cap, so that air trapped behind the dust cap does not contribute noise.
For a qualitative evaluation of a new woofer perform a simple test. Put a dot of white paint on the cone or dust cap to monitor excursion. Connect the driver to a power amplifier and drive it from a signal generator set to 35 Hz. Hold the unbaffled driver in your hands. Increase the generator output and observe how quiet the driver remains and how much excursion it can handle before sounding obnoxious. Remember that an open baffle cabinet will not attenuate this noise. Lower the frequency and repeat the test. Be careful not to overheat the voice coil or to destroy the driver mechanically!! The burst capability of the Agilent 33120A Arbitrary Waveform Generator is wonderful for this work. Unfortunately, the majority of drivers that I have evaluated required no further investigation after this test. "

"The major problem with increased excursion is the disproportionate increase in aero-dynamic noise. It is generated by the turbulent flow of air in and out of the magnet vent and by the air trapped behind the spider if it is not completely vented. The typical, low cost stamped basket design is not suited for venting the spider. The extraneous noise becomes especially noticeable in an open baffle speaker, because it contains many high frequency spectral components and is intermodulated with the desired signal."

For PA I did have a problem with some magnets making 'clanking' or 'clicking' noise as per above, I ran a bead of adhesive between the top-plate of the magnetic assembly and the frame and that fixed it.
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Last edited by djk; 6th October 2012 at 07:19 AM.
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Old 6th October 2012, 08:34 AM   #348
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Looking at these 18sound pics, I'm wondering why not mounting the drivers face to face but wired in counterphase, wouldn't that produce the same result as a magnet to face wired in phase with a benefit of having two faces radiating into a slot simultaneously?
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Old 6th October 2012, 09:06 AM   #349
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Uh, no.

The ones mounted magnet-to face have the driver with the exposed magnet wired out-of-phase (to produce in-phase cone output).
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Old 6th October 2012, 09:19 AM   #350
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OK, whatever-phase, what's the benefit over face to face mount?
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