Fol. tutorial explains how the speaker design programs work, pros/cons of Sealed vs Vented Box....and the need for a "rumble filter" below 20 Hz or so, esp. if using Vented Box...and very important if still playing vinyl records:
Another aspect to consider is Transient Response....Bass Reflex frequently sounds "flabby" as the cone goes beyond and then somewhat short of a perfect "Step" Response as it oscillates toward the desired physical placement. Sealed Box speakers are much better....which is why I recommend it for the Woofer in full-range L/R Speakers. In either case, there should never be any Passive Crossover components on these speakers since it kills the Damping Ratio and ability to control the Transient Response....so a separate Amp and Active Crossover is recommended for the Woofers as well. You might want to design the box so you can try BOTH types of alignments (perhaps it's as simple as covering the vent).
As mentioned above, two Sub-woofers are better than one...esp. if you can locate them asymmetrically in the room, rather than equally spaced from each wall, so that they excite the room modes DIFFERENTLY:
[Additional Tutorial re Room Acoustics & Speakers]
As can be seen in the above article, the frequency response AT YOUR EARS will vary quite a bit depending on where each Sub-woofer is located and where your EARS are located. So if your ears are as acute as most musicians, you'll hear some Bass Guitar notes very loudly and others greatly diminished. The cure for THIS, as well as rolling off the Amplifiers low frequency response is either a One-Third Octave Active Equalizer (usually 31-Bands per Channel) or Parametric Equalizer placed just prior to the Sub-Woofer's (Stereo) Amplifier....any ol' 100 WPC Amp should work just fine since 15" Subs are bound to be very efficient....you'll be running way below max output power (average or peak) even after you've exceeded your ear damage level:
Note that Sub-woofer 1/3-octave controls are centered on 20, 24, 31.5, 40, 50, 80, 100, 125, 160, 200 Hz and you won't need to use the rest (set to 0 correction). You would think someone would make a lower cost, 8 to 10-Band 1/3-octave equalizer just for Sub-Woofers...let us know if you find any. BTW: My Pioneer Receiver has an Automatic (octave?) Room Equalizer...but it only has 40 & 125 Hz filters which affect ALL speakers, not just Sub-woofer....and not nearly enough filters.
Some people use a Parametric Equalizer, which has a LIMITED number of VARIABLE filters that can be set for a desired center frequency and "Q" (aka bandwidth). However, the fol. popular model only has NOTCH Filters (to suppress Acoustic Feedback using live microphones) BUT you might also need a BOOST Filter for Room Equalization:
[This is a high-end model with 20 Parametric Filters per Channel]
Note that Behringer DSP-1124P had an internal DELAY of 1 ms, which works out to ONLY about 1-foot you need to consider when setting speaker distance controls in your Surround Receiver/Amplifier. Of course, other equipments may have different Absolute Delay times.
Click on "Parametric Equalilzer" (and other topics) for a Tutorial/Demonstration:
Personally, I would avoid using a Parametric Equalizer without KNOWING the Absolute Delay and Group Delay (aka Envelope Delay) characteristics in those variable filters. In the Graphic Equalizer's Fixed Q Filters, they are "probably" using constant delay type filters...but again no specs and the delay might be higher on the lower freq filters....and a digital signal processor based system may or may not have better control over this issue.....Time Alignment is always something else I worry about...fortunately, human ears are fairly insensitive on the lowest frequencies....and on the mid/high freqs I use Planar Magnetic Mid-Range and Ribbon Tweeters for flat Group Delay and very fast Transient Response.
Most of these equipments are for Pro-Audio applications, with XLR and 1/4-in TRS jacks (Tip-Ring-Sleve, like old headphone and Guitar plugs), so you'll need to buy...or more likely fabricate a TRS or XLR to dual RCA plug cable.
I would recommend using Radio Shack DIGITAL Sound Pressure Level Meter (spot on calibration) and download the FREE True-RTA program to your laptop to measure 1/3-Octave Power Response:
Another alternative I just ran across (and haven't tried yet) is REW - Room Equalization Wizard which displays 1/3-Octave Power Response PLUS Step Respnse, Group Delay, etc:
http://www.hometheatershack.com/roomeq/Edited by holl_ands - 7/14/13 at 11:38am