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Flaring Ports? (Read 2719 times)
hesster
Ex Member



Flaring Ports?
01/23/07 at 00:29:32
 
About to build a 15" HWK with quad Titanics to put the final touch on my THX 9.1 channel HT system.  Is it necessary to flare the ports, or buy flared port tubes, etc. since this thing will probably really move some air?  Also, is dampening material like fiberglass attached to the inside walls necessary?  Some "best practices" advice from any of you "pros" is very welcome to help me get it right - beyond the basic instructions.
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« Last Edit: 01/23/07 at 00:30:22 by hesster »  
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Adrian D.
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Re: Flaring Ports?
Reply #1 - 01/23/07 at 12:24:07
 
consiedring what drivers you have, i'd use an aeroport for the top port and round-over the lower port edges.
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60ndown
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Re: Flaring Ports?
Reply #2 - 01/23/07 at 15:07:30
 
build it exactly to plans first, if you experience any problems, go from there.
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Sean
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Re: Flaring Ports?
Reply #3 - 01/23/07 at 22:09:12
 
If you quad load, you will need to flare at least the inside end of the top port (although if you do this you may as well do both ends - it looks better and potentially sounds better).  The problem is that the magnet of the upper woofer gets so close to the port end that airflow in the region becomes turbulent, and this is definitely noticeable.  I have a HWK 15, built exactly to plans that at one time was quad loaded with 4 Dayton Series II 15" subs, and had this problem.  I have now reverted to a dual load configuration (one driver in each baffle, baskets in, cones out) and no longer have the problem, and if you dual load as such it may not be an issue.  I am planning to redo the lid and top port to include flares, though, just because I think it looks more finished.
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hesster
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Re: Flaring Ports?
Reply #4 - 01/23/07 at 23:47:38
 
Thanks much for the input.  So you reverted to 2 drivers due to the turbulance (noise?), but in doing so did you give up any performance?  It is hard to understand the principle of this setup being efficient in driving the soundwave pressure in outer chambers with the back of the Woofer cones in a Quad setup.  Also, what do you power them with?
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Sean
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Re: Flaring Ports?
Reply #5 - 01/24/07 at 00:10:14
 
That wasn't the only reason.  Although 4 woofers offers increased cone control (and tighter bass / better transient response as a result), quad loading reduces the effective volume of both the upper and lower chambers by twice the volume occupied by the driver, thereby increasing the tuned frequency of each chamber and limiting the low-end extension.
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Sean
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Re: Flaring Ports?
Reply #6 - 01/24/07 at 00:33:21
 
The principle is simple.  In a bandpass enclosure, the output comes from the port(s), not the woofer cone(s).  The woofers in this case act solely to vary the air pressure within each respective chamber in a manner that creates sound waves within the port(s) as their contained airmass resonates.  In that case, the shape of the woofer cone, and direct radiation pattern from it, is not critical as it would be in a direct radiator application.  (you couldn't mount a woofer backwards in a sealed enclosure with the woofer exposed and expect it to perform the same way, since the cone shape in that case governs the sound dispersion)  All that matters in a bandpass enclosure is the volume of air moved at what speed, which is identical regardless of which way the cone is travelling.

Clamshelling two woofers together as you would do (twice) in a quad load configuration increases accuracy, since the woofer surround presents a slightly different resistance to cone motion in the forward direction than it does in the reverse direction.  Using two woofers in this manner averages the non-linearity of this effect.

Having two woofers (or pairs of woofers) installed in separate baffles adds an airmass (and additional capacitance) between them which is driven in sympathy with the woofer cones, acting as a buffer which lowers the effective resonant frequency of the drivers.  By doing this, depending on the woofer specs, you might see increased performance by installing both woofers cone-up (hence increasing the center chamber volume), despite the fact that doing so lowers the volume of the lower chamber, increasing its tuning frequency.

The best thing to do if you want a rough idea of how your woofers will perform is to model them in WinISD or similar software as a 6th order bandpass enclosure, using 2 isobarically mounted drivers.  To model the quad load, just halve the Vas of the driver.  The response you will see will be a curve with two very pronounced peaks, which would not be a nice enclosure to listen to, but the HWK center chamber acts to smooth out that response a bit and increase the bandwidth.  By looking at the 6th order bandpass plots of your drivers in the various HWK configurations, you can get an idea of the best configuration for your driver, keeping in mind that the end result should be less peaked than the graph shows.  Once built, EQ can clean up the rest.

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« Last Edit: 01/24/07 at 02:50:31 by Sean »  
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hesster
Ex Member



Re: Flaring Ports?
Reply #7 - 01/24/07 at 23:00:51
 
Wow.  Great response.  I do understand what you are saying.  With a speaker depth of 200mm, I would only have 2-3/4" clearance from the port bottom to the magnet - not much so I can understand the turbulence factor.  You mentioned you reverted to (2) drivers with baskets in "I have now reverted to a dual load configuration (one driver in each baffle, baskets in, cones out".  This is a little confusing....does trhis mean both baskets pointing in the center chamber?  Did not see a load configuration in the reference material like that.  Bottom line, does it sound better?
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Sean
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Re: Flaring Ports?
Reply #8 - 01/24/07 at 23:37:44
 
The jury is still out on that, since I haven't actually performed any measurements to see what is going on.  To measure properly you really need to take the HWK outside to eliminate room reflections and standing waves, and you need to analyze the response using pink noise and a decent microphone.  

The reason I have my drivers mounted the way I do is because the reduced center chamber volume theoretically provides better coupling of the woofers, and because this provides the lowest possible tuning of both top and bottom chambers (without reverting to a simple 6th order bandpass box where one baffle is not used).  This doesn't necessarily mean that this is best - as I mentioned, I originally had a quad load, and I removed two drivers because they were superfluous and made the HWK tune too high.  I may still yet discover that it is better with both drivers cone up, I just haven't had a chance to play with that yet.

I am actually going to build a HWK 18 for a pair of Electro-Voice EVX-180B drivers.  If that works out, I'll probably clamshell the Daytons in one baffle only of the HWK 15 to get the lowest tuning for low frequency effects with that cabinet, with the HWK 18 covering the rest of the necessary bandwidth.

In the absence of accurate measuring equipment and an appropriate environment for measurement, you only have the best possible analysis tool available to you - your ears.  Try every configuration, and use what sounds best.
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Sean
Ex Member



Re: Flaring Ports?
Reply #9 - 01/24/07 at 23:56:13
 
Re: amps - I started with a generic plate amplifier, about 300W or so.  It was okay, but not great.  I only clued in well after the fact that mounting electronics to your subwoofer is probably not a great idea, so I removed the plate amp, patched the hole adding a Speakon connector, and put the plate amp in a box so I could control level and crossover point from my rack.  Eventually I replaced the plate amp with a proper pro sound amp - a QSC RMX 1450 - huge difference.  Lots of headroom, less distorsion, etc.  Now I am looking at replacing the RMX with one of QSC's new PLX2 series amps, but I'm waiting to find a deal before I purchase one.  I have since built a PA, so my signal path now goes:  Digital source --> Behringer SRC2496 (resampler / jitter correction) --> Behringer DEQ2496 (EQ) --> Behringer DCX2496 (active crossover) --> QSC RMX series amps, with an RMX 1450 specifically on the HWK.  When I can afford it, all of the RMX amps will be replaced with QSC's PLX2 amps.  If you can find a PLX2 at a decent price, you will not be disappointed with its performance.  Failing that, any reputable pro sound amp with sufficient power, low enough frequency response, and low distorsion will fit the bill.

If you are a hard core audiophile and your speakers are efficient enough you could also drive it with a tube amp, but I'm not a tubehead and have no advice to offer there.
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hesster
Ex Member



Re: Flaring Ports?
Reply #10 - 01/25/07 at 23:55:12
 
Thanks again for the info - you guys are "out there" with the extent of knowledge you possess.  I have a relatively simple HT setup: Yamaha 1000W 9.1 THX receiver, old rebuilt Pioneer HPM150 fronts with 16" woofers, dual 12" seperate woofers I built coupled with a modest amp off the LFE channel, 6 surround Bose speakers (presence, middle rear, and rear pairs), etc.  I have always wanted to build the HWK - just cause I can, and of course because one can never get enough bass. I will hook it up to the LFE channel, and drive the other woofers off the front channel outputs.  DVDs should send pictures flying I hope.
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