Home Theater Subwoofer

God Bless America


Hey, Here's a treat for your subwoofer - Some test clips!

Welcome to my Home Theater subwoofer Page.  I will describe one of my greatest (or rather most enjoyable) speaker building projects to date.  The reason I say it's the greatest to date is not just because of its size or that it uses two 15" woofers .  The reason is because I got the chance to hand build my own drivers !  Well, one anyway.  A friend from my church works at a speaker building factory and invited me there to build my own subwoofer drivers.  He built one driver showing me what to do and I built the other.  I spent a number of weeks checking various driver "recipes" from his database.  Of course any proprietary drivers were out of the question so I chose from their "general stock" of recipes with the plan of adjusting them to suit my needs.  I primarily wanted a driver that could put out deep bass in a sealed enclosure, but with some leeway to work vented if I desired.

I eventually settled on a driver he was designing for an organ company that builds church organs.  The 15" driver had a single 8 ohm voice coil with over 14 mm of xmax.  The organ company had him apply an additional 75 grams of mass to lower Fs.  I added another 50 grams on top of that to get the resonance down even further :-)


Okay, now that I've lived with this bad boy for over a year, it's about time for my evaluation of it (them).  My main critique is...
...I LOVE IT!!!!!  Well, what did you expect?!!!  Seriously, it's awesome!  I have plenty of reserve power for anything I play through it.  Those digital warnings on Telarc CDs?  I ignore them.  I can play anything I want to through them.  They sounded fairly smooth before I added the parametric EQ and the Soundcraftsmen amplifier, but with those pieces it truly is smooth.  The low Qtc helped initially, but the parametric EQ really smoothed out the bass.  It is not bloated or boomy by any means.  If I had it to do over, would I do anything differently?  Possibly.  I would probably not go quite as low on the Qtc.  As it is, the Qtc is about 0.54.  It sounds great, but it was an awfully big box.  Of course, I thought it would be used vented sometimes and I didn't think I would own a parametric EQ.  Oh, well, no big deal.  I was hiding the sub behind a wall, anyway.  I would not have been happy with a Qtc of 0.707.  I feel like the ideal Qtc would be 0.57 to 0.60.  I think I would have obtained a Qtc of 0.57 in about a 10 cu ft box.  And with stuffing, I would have dropped down to a 7 or 8 cu ft box.  But this is a very, VERY minor criticism.  Actually, there's probably not much difference between 0.54 and 0.577, but I would think anything higher than a 0.6 would sound slightly boomy.  I definitely thought a Qtc of 0.7 sounded a bit boomy from initial tests with one speaker in that 5 cu ft test box.  This is of course due to room gain and the low F3 (~27 Hz).  If there is one main thing I've learned about subwoofers, bass and room interaction, it's that people don't need to design for ultra low F3's.  If you do, you WILL end up with bloated sounding bass.  I now believe, and I've stated this on various tech forums from time to time, that to design a good subwoofer, you must allow for room gain.  In large rooms, room gain kicks in around 30 to 40 Hz, small rooms a bit higher - say 40 to 50 Hz.  Therefore, if you have a driver that will go down to 30 Hz or lower in a sealed box, you don't need to attempt a vented alignment.  I wouldn't even use a Qtc of 0.707 - use a Qtc of 0.6 or so.  If you still want to go vented, at least design a very large extended bass shelf (EBS) box.  This way, the response will droop through the range where room gain starts kicking in.  This will flatten the response.  If your driver won't go below 40 Hz sealed, use a fairly flat vented alignment to extend the response.  Between about 30 and 40 Hz, a sealed box of Qtc=0.707 or an EBS vented box should do quite nicely.  Remember that those sealed boxes don't lose bass very rapidly when you take into account room gain




Basic Driver Manufacturing Procedure

My friend keeps a file of  "recipes" to guide the assembly procedure.  It lists things like the size and part number of magnets, frames, plates, spiders, voice coils, etc...  First, he had to make sure all parts were in stock.  He then had two stamped steel frames and top and bottom plates painted so they would be ready when I visited one weekend.  The top plate had already been attached with welds created from  the pressure of compressing the top plate to the frame.  This pressure literally melted some "bumps" on the frame to the top plate.  Top and bottom plates are stamped from huge (maybe 8 feet in diameter), 6 to 8 inch wide rolls of heavy gauge steel.

The first thing we did was glue two approximately 5/8" magnets together.  It's a funny thing, but the magnets aren't magnetized at all during the assembly phase.  They are just like any other piece of steel at this point.  We epoxied them together.  Waited a few minutes for it to set, then epoxied the top plate (and its attached frame) to it.  Then we used a guide to set (and epoxy) the bottom plate in place.  This is necessary to center the pole piece in the motor structure.

We then glued the spider to a "spacer" which keeps the spider from hitting the top plate during long excursions.  We then inserted a "jig" (or shim if you will) into the voice coil and set the voice coil to the proper depth within the motor structure.  With this done, we could glue the spider and its spacer to the top plate.  Then we glued the voice coil, surround and cone together and to the frame.  (I won't mention the details as these are trade secrets.)  We let everything set up overnight and came back the next day to magnetize them, test them and break in the surrounds.  But before we did, we removed the voice coil jig and we set the dust cap in place, marked it, then glued it to the cone.

I have to say, it was the thrill of a lifetime for someone interested in speaker building to build his own drivers, and I can't thank my friend enough for the opportunity.  Maybe someday if I have some money burning a hole in my pocket, I may just order up a batch of these drivers to sell.  They are excellent sealed box woofers.  But when does anyone have that much money burning a hole in their pocket?  :-)

Here are the averaged specs (well, the important ones anyway):

15" driver with a Kevlar impregnated paper cone, large foam surround, double stacked magnets and 2 1/2" voice coil,
Qts    =   0.394,
Qes   =   0.412,
Vas   =   338.7 liters,
Fs     =   14.5 Hz    :-)
xmax =   14.3 mm,
Sd    =   823.72 sq. cm,
Impedance = 8 ohms (single coil),
Power  =     ?   (We ran 300 watts through them at 20 Hz in free air to loosen the surrounds and they didn't bottom!),
SPL  =   86 dB (of course two in parallel would be 92 dB).

Modeled Systems
Alignment F3
Box Volume
Qtc or Fb
Sealed #1
26 Hz
5.365 Cu Ft
Qtc = 0.707
Sealed #2
27 Hz
13.635 Cu Ft
Qtc = 0.54
Optimum Vented
15.2 Hz
10.100 Cu Ft
Fb = 14.9 Hz
Quasi-EBS Vented
20.6 Hz
13.635 Cu Ft
Fb = 10.0 Hz

15" Drivers
Test box - My friend loaned me his 5 cu. ft. dual 12" Sub box which had two 15" passive radiators.  I covered all openings except for one 15" to which I isobarically mounted the drivers.  Sealed (isobaric) it was equivalent to one driver in a 10 cu ft box (approximately Qtc = 0.57).  Vented (isobaric) it was near optimum though I had to use an elbow in the vent.  I also tried it with a single driver.  A single vented driver was way too boomy (box was way too small for a single driver).  A single driver sealed was only slightly boomy (Qtc~ 0.71).  My main concerns were to keep from exciting the 32 Hz room resonance and to get the deepest bass possible.  Therefore I decided to go with a box built for a quasi-EBS vented design (true EBS would have been up to 16 to 20 cu. ft.), but I made covers for the ports in case the sealed was more than adequate.  It was MORE than adequate!  Therefore, I will always leave it sealed.  (See frequency response below).
Subwoofer Box - Divided internally into two 13.6 cu ft. halves.  It's sealed, but has covers over the ports (Fb = 10 Hz) at the top of driver opening in case I want to use it vented.  Ignore those Bose-ish looking boxes to the left ;-)
Box Dimensions - It actually works out to a little less than 13.635 cu. ft. due to bracing.  Since particle board comes in 4x8 sheets, I had to special order industrial grade particle board for countertops in a 5' x 12' sheet (~$35).  I knew I was in trouble when they brought it out on a forklift!  I had them cut 5' off one end in order to transport it.  The completed box weighs around 318 pounds.  Without the drivers!!!  The box is unstuffed except for some foam carpet padding and 1 1/2" fiberglass behind drivers.  If I notice any midrange sounds from the sub I will stuff the box more, but so far I haven't noticed any midrange sounds (90 Hz crossover at 18 dB/octave on a sub with Qtc = 0.54).
Initial Construction - This is the center which supports the two drivers in a push pull arrangement and divides the box.  The pencil lines show the locations of the 1x4 Oak bracing.  This assembly was sitting on top of the rear wall.  I drilled oversize screw holes in the outside piece the screw initially goes into so it would pull the two pieces I was gluing together for a tight bond.
MDF Brace ties - I was afraid the walls wouldn't be stiff enough because of the large box size, so I used some 1/2" MDF panels I had on hand to "tie" the front and rear walls together.  They were screwed and glued to the braces.  I also figured they may help break up any internal standing waves.
Vents - 4" diameter PVC with 1/4" radius ends to reduce port noise.  There is surprisingly little port noise considering these are small for a 15" driver.  In my initial test box I heard all kinds of whistles and chuffing due to the elbow and squared ends of the vent in the test box.  The rounding helped tremendously.   I only hear a little chuffing between 8 and 12.6 Hz (most noticeable at 10 Hz - the tuning frequency) with loud sine waves (no whistles).  Otherwise, they're silent.
Vent installation - I wrapped rubber pipe insulation around the vent pipe.  I mounted a scrap of wood (screwed and glued) to the center wall and attached a strap to it and used a piece of cardboard to help keep the pipe strap from cutting the foam rubber.  The piece of 2x4 and scrap plywood pieces in the picture helped align the vents while the glue dried.  They were removed later.  I used liquid nails and small brad nails (in pre-drilled holes at the pipe entrance) to mount the pipe.  I couldn't get to the back of the vent to seal it with caulk, so I eventually made a "dam" out of tape and poured wood glue all around the pipe base to make sure it was sealed.
Vent Covers - I installed vent covers to make the box sealed.  I used screws that had wood threads on one end and bolt threads on the other, and used wing nuts with washers to secure the covers in place.  The covers have foam rubber on one side for an airtight fit.
Close-up - The drivers are mounted push-pull into an opening which will be covered by a grille when I build basement walls.  It should look like a central air chase when walls are built around it.  I may even backlight it :-)
Another Close-up - This view looks into the right driver a little more.
Scale - My wife Linda (doing the Barker's Beauties impression) to show enclosure size.  All those screw lines show where I glued vertical (1x4 oak) bracing.  (The box is on its side.)  She is smiling because this thing is in the basement, NOT the living room.  ;-)

Frequency Response Graphs - The receiver's crossover is fixed at 90 Hz (18 dB Sub/12 dB Mains).  All measurements are taken at the listening position, 12.333' from the listening position to the center of the woofer opening.  Since the SPL drops off at a rate of 6 dB for every doubling of distance, you can add roughly 12 dB to all SPL measurements to get the 1 meter equivalent.  You know, after using pink noise and warbled sine waves, I think I prefer the warbled sine waves, so all my tests are based on warbled sine waves except for the very lowest frequencies where I don't believe warbled sine waves are as accurate.  All SPL measurements were taken through a SoundCraftsmen PCR-800 amp using one channel (~300 watts maximum into 4 ohms).  Previously I used the Adire AVA250 amp which had a mild boost (~ 1 1/2 dB at 18 Hz and dropped like a rock below that).  Using the Adire, the sub was flat to about 14.5 Hz with an F3 of around 13 Hz sealed.  Gee, that Adire amp knocked a whole 1 1/2 Hz off my extension!!!  Excuse me for being a little facetious, I couldn't resist!  The differences between the Adire and the SoundCraftsmen are very minor except that now I get measurable bass down to 5 Hz as opposed to "just" 16 or 14 Hz.  I can't really tell any difference between the two amps.  It's just neat that I can reproduce lower frequencies it if they're there :-)  It's not like there is anything useful below 16 Hz anyway.  Heck, there's really not much below 25 Hz that people are missing - down to 25 Hz, yes, below 25 or 23 Hz, some, but not much really.  Just a handful of organ CDs and DVDs at best.

Measurements with the Maggies and the sub were made using the receiver's crossover and the mains (Maggies) set to small.  Maximum subwoofer volume (with Maggies off) was obtained by turning up the volume until either the low frequency sine waves started sounding like "fwoop-fwoop" (spiders slapping the top plate or reaching the suspension limits) or I until I saw the clipping indicators light up on the amp which only happened once or twice with barely a flicker.  PS - Don't attempt test signals below 2 or 2.5 Hz.  when I originally used my Yamaha (before I got the SoundCraftsmen) I was greeted by a rush of noise when trying to test below 2 Hz, and instantly paused the test CD.  Oh well, I had to try it!  :-)  Luckily, the receiver (a Yamaha RX-V995) survived.  (I was acutely aware that I would be approaching possibly destructive levels so I kept my finger on the pause button as I backed down through the test frequencies I had recorded.)  It is...  interesting to watch the sub moving firmly at 2 or 2.5 cycles per second!  I could probably test the SoundCraftsmen to about 1 Hz, but during some high power testing below 10 Hz, the amp channel I was using quit.  I had a mild scare when the channel wouldn't work after turning it off and back on.  I switched channels and the other one worked fine.  Later I switched back to the other channel and it was working okay again.  So I like to live dangerously, okay!  Fun is what this hobby is all about.

This is a quick rough measurement in Audua SpeakerWorkshop.

More measurements with Parametric EQ - I have taken some new measurements with my Ashly model SC-63 mono parametric EQ I bought off Ebay.  I set the bass frequency to 32 Hz with about a 6 dB cut and the mid to about 160 Hz to a 15 dB cut because of some room resonances.  I was also able to purchase a SoundCraftsmen PCR-800 amp off Ebay; hence the new measurements.  Bass is extremely smooth sounding.  The sound rises some below 32 Hz and peaks at 16 Hz (not at 50 or 60 Hz like those wimpy store bought subs when they're tested).  Even with this rise, there is no noticeable "heaviness" in the bass because it occurs at such low frequencies.  Most music is above the 40 Hz range.  If anything, there could be a slight "lean-ness" (is that a word?) around the mid-30's.  I played around some with the center frequency and Q which smoothed this out a little.  One graph is using my correction factors, the other graph uses Home Theater Forum's corrections.    As can be seen, the sub is flat to about 13 Hz with an F3 of around 12.6 Hz sealed.  WoooHooo!!!!  YEAH!!!  :-D 

I have a deeper appreciation for certain movies with ultra low bass.  One example is chapter 28 of The Lord of the Rings, where the giant octopus attacks the group and forces them back into the cave where the entrance collapses.  Wow, this is the first movie that looked like it might stress my subwoofer some, but it came through it with flying colors (even if the drivers were moving about 1 1/2" p-p at THX home levels).  Another movie that has some good bass is the Matrix - the practice fight scene where Neo moves just as Morpheous' knee hits the floor and breaks through, the jump program scene when his feet land on the building and of course the helicopter crashing into the building - when the "wave" of glass comes across the screen, there is an ultra low frequency "kick" that will definitely make mediocre drivers "pop" at their limits.  And that's not including all the nice machine gun scenes loaded with bass!!!

I now have a new ultra bass DVD champion - Star Wars II - Attack of the Clones.  The THX intro has a mean thump to it.  Then the opening scene with the flying wing space ship - Boy does it rumble.  Finally, there's the explosion on the landing pad.  WOW!!!!!!!  This was more stressful than the Lord of the Rings, but again, the sub came through with flying colors.  However, a couple more CD's hit the floor.  And I was getting so good at keeping a closer eye on them and moving them back into place as they vibrate out.

Maximum SPL using the SoundCraftsmen amp - Originally, I tried to correct the meter readings by picking the meter corrections in the treble range from the Radio Shack (RS) meter graph, and using the C-weighting corrections for "normal" C-weighting in the bass.  This never quite worked and I finally had a chance to measure my center channel test baffle and compare it to an LMS frequency response taken at my friend's speaker building plant.  By comparing the two, I came up with my own correction factors for my RS meter which surprisingly, don't vary too much from the Home Theater Forum's corrections.  Note that the maximum SPL graphs are NOT frequency responses.  They are maximum outputs at various frequencies.

The Maximum SPL is 116 dB (113 dB uncorrected reading) at 32 Hz (and 300 watts).  Between 16 and about 40 Hz, it is greater than 111 dB.  At 10 Hz, it puts out 107 dB (93 dB uncorrected), and at 5 Hz it put out approximately 107 dB (84.5 dB uncorrected).  I had to estimate the correction at 5 Hz since I've misplaced my C-weighting curve formula.  This sub is more than enough to match the Maggies.  Heck, the amp is probably coasting along at 30 watts with the Maggies at maximum volume.  Of course, since the LFE channel is up to 10 dB louder, maybe I'm already at a perfect balance.  I want my next speaker project to be a few dB louder for true THX theater levels.  I just purchased over 50 of the PE Pioneer 4 1/2" buyouts.  Can you say line array???  Did I mention that all system responses are with the sub sealed?  Ahh likes it much better tha' way.  I did some measurements early on with the ports uncovered, but I'm never going back to using this as a vented subwoofer.  

Maximum SPL using HTF's Meter Corrections - In these curves, I corrected the meter readings by using the corrections according to Home Theater Forum's tests on an RS SPL meter for both frequency extremes.  The graphs are similar, but with more correction at the low end.  I think the HTF corrections may be a bit too much but, you never know :-)

SYSTEM Frequency Response using NO Corrections - Still an impressive 16 Hz flat response.

As you can see, it's relatively flat to 13 Hz in a sealed box (relative to mid-band level).  I actually didn't notice a difference in deepness of bass response between EBS and sealed with music and movies (although 12.6 and 16 Hz test tones were definitely louder with the vented version), but the sealed sounds so much more tight and clean.  13 Hz in a sealed (Qtc = 0.54) box?  - I'm not complaining!!!  If I talk when doing high power tests at 20 Hz, it sounds like I'm talking into a fan.  Early on, I dropped half a shelf of CDs that hit the basement floor while testing low frequencies even though I wrapped masking tape around them to hold them into place!  Addendum:  My last round of tests were much quicker this time so there was no time for CD's to move - very far, anyway!!!



Last Updated 12/15/03