A couple of big experimental bass cabs

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Duke

A couple of big experimental bass cabs
« on: 1 Mar 2015, 05:53 am »
Over the past few weeks I got a couple of experimental bass cabs put together and sent off to their respective beta-testers.

Here's a head-on shot of the first one:



Dimensions are 36" tall by 18" wide by 22" deep, as I recall.  Weight clocked in at 66 pounds.   That's probably not too bad for a 215 cab, but as you can see, this isn't a 215:



Yes, there's a fourth 15" woofer on the other side.  It's a compact wide-dispersion lightweight midless tweetless 415.  I think that's a first.  The idea of course is to get 215-class tone across a wide listening area. 

In this shot, through that top port, you can see the motors of the top three woofers clustered together.  It's pretty crowded in there!:



The ports are simply holes cut in the back of the cabinet, they are adequate for airflow given the specifics.

You can see a single switch below the upper port.  The woofer I'm using has a substantial upper-mid peak, which often sounds great on electric bass, but not necessarily all the time.  So you can flip the switch down and that smoothes out the peak.

Fifteen inch woofers aren't known for having much top end and this one is no exception, but I use some sneaky circuitry to get a bit more top-end extension than the woofer normally has right out of the box.   And of course fifteens beam badly up high, but getting wider and more uniform coverage is what the Birdhaus configuration is all about. 

The cabinet is undersized for four 15" woofers, and here is how we get away with it:  The lows from those side-firing woofers wrap around and reinforce out front, but the mids and highs do not.  So we get, in effect, +6 dB from the two side woofers down low because they are adding in-phase, but only +3 higher up because their contribution to the reverberant field adds in random phase.   We do trade off some efficiency on paper, but we actually get more sound out into the room in the mids and highs than with a normal 4x15 cluster because we have less cancellation up there. 

The woofer is the Eminence Basslite C2515.  It's a 4-ohm woofer based on the Deltalite motor, but with a different cone that gives us that magnificent peak at about 1.5 kHz.  To my ears, this is the best-sounding off-the-shelf woofer for electric bass.   That peak is in EXACTLY the right spot.  It shares the drawback of all 15" woofers in a bass guitar application:  It beams.  And since it uses a Deltalite motor, it is outclassed in raw numbers by the new generation of "uberwoofers" that are out there.  But it sounds better as a stand-alone than any of these uberwoofers!   So I use four of them in series-parallel, keeping that 4-ohm load, and the Birdhaus configuration gives us far better dispersion than a pair of uber-fifteens in a vertical stack.

The build eats up a lot of time and a lot of expensive lightweight plywood, so if this cab goes into my line-up it will be special-order-only with a long lead time, and it will not be cheap.  But I felt like the idea had to at least be tried.

edit:  I cut it too close on the enclosure size, and this cab doesn't deliver a strong enough low-B.  But it works well with low-E tuning. 



« Last Edit: 16 Jan 2016, 06:25 pm by Duke »

Duke

Re: A couple of big experimental bass cabs
« Reply #1 on: 1 Mar 2015, 07:31 am »
The second big experimental bass cab is in response to luthier Skip Fantry's "Quake" bass, manufactured and sold under his KnuckleHead brand.  Skip routinely ventures where many fear to tread, and has gone public with basses tuned as low as G#00... which has a fundamental of about 13 Hz. 

I think this type of instrument would be called a "sub-contra bass".  It is not meant to be used as, or in place of, a normally-tuned bass guitar.  It is a different class of instrument, and from what I understand its role is more atmospheric than rhythmic.  When brought into the mix, it changes the whole feel of the room... assuming it is played through a cab that can do its part.

Fortunately, for electric bass, we don't have to reproduce the fundamental at anything approaching full power.  In fact, we can leave it out altogether, and the ear/brain system can fill in the missing fundamental from the overtone structure.  Given how expensive it is to reproduce infrasonic tones at high power, our target is the first overtone.  In the case of G#00, that first overtone is at 26 Hz.

The cab you see here is roughly -8 dB at 26 Hz:



Now -8 dB is not very impressive, but this cab is designed to take EQ.  And lots of it.  The woofer is rated at 1500 watts AES and 3000 watts "music program", and in this application, I think we can look to that 3 kilowatt figure as an indication of what its thermal power handling is.

To keep the thermal power handling up as high as we can reasonably get it, the cabinet is designed with ports both above and below the woofer's magnet, to facilitate chimney-effect cooling.  There are four ports, two in each side, staggered because they reach almost all the way across to the far side.  This geometry also has the beneficial side-effect of maintaining a symmetrical airload on the back of the cone, and was inspired by Eminence engineer Jerry McNutt:



Thermal power handling is only part of the story; mechanical power handling is almost always the limiting factor.  And that's a good thing!  We want our bass cabs to fartout and warn us that they are in trouble before the voice coil gets too hot and the magic smoke escapes!

The woofer used in this cab is a high-end prosound subwoofer, with a claimed linear excursion (based on voice coil overhang according to the manufacturer, which is a rather conservative yardstick) of twenty-two millimeters.  That's a lot for a bass cab.  The bad news is, we do have to be careful because the voice coil can overheat and the magic smoke escape before we get fartout.

With suitable EQ (which includes a protective high-pass filter to prevent over-excursion from the low fundamental, which is well below the tuning frequency), and a three kilowatt amp, this cab can theoretically deliver flat to 26 Hz at 122 dB, and would be 3 dB down at 20 Hz.   That's not bad for a seventy-pound bass cab. 

Above the region covered by the woofer, we have four little 3" cone mids in a cross-firing array.  The mids are in an open-format Hathor-style chamber, which gives a less boxy sound, improves the ability to hear the overtones from normally shadowed locations like right next to or virtually on top of the cab, and cooling is improved relative to shoehorning the little mids into a small sealed sub-enclosure. 

On the back of the cab are two switches.  The top switch up = mids at their loudest; top switch down = mids matching up better with the woofer; and bottom switch down = mids off altogether. 

Once again, if this cab becomes part of my line-up, it will be a special-order item and will not come cheap.  The woofer is a special-order-only unit from Europe that has a three-month lead time. 

Russell Dawkins

Re: A couple of big experimental bass cabs
« Reply #2 on: 1 Mar 2015, 08:54 am »
Pretty exciting, Duke. I'm guessing the driver in the second cab is an 18? You didn't say. I'd love to hear these in action.

Have you ever tried 4 15s all at the same height, so that all forces would be cancelled? Like the old EPI micro towers with 2 X 4.5" drivers at the top, with two dummy grills to look like 4? Like this (only with vastly different proportions!):



Duke

Re: A couple of big experimental bass cabs
« Reply #3 on: 1 Mar 2015, 11:55 pm »
Pretty exciting, Duke. I'm guessing the driver in the second cab is an 18? You didn't say. I'd love to hear these in action.

Have you ever tried 4 15s all at the same height, so that all forces would be cancelled? Like the old EPI micro towers with 2 X 4.5" drivers at the top, with two dummy grills to look like 4? Like this (only with vastly different proportions!)

Thanks, Russell!

Yes. it's an 18" woofers, I should have said so.

I used to own a pair of Epicure Model 400 speakers, with their 6" woofer + 1" tweeter module on all four sides (instead of just a dummy grills!).  Did some things well, and some things not so well. 

I think there might be a place for the bass cab you describe, but imo it would be for jam sessions (where your "audience" is your fellow musicians) and small acoustic gigs (where the quasi-omni behavior would approximate an unamplified acoustic instrument).  The Birdhaus configuration, as I call it, with two woofers forward and one to each side, does bias the output towards the audience area more than the omnibox would.  I considered both formats before doing my first little Birdhaus cab with four 6" woofers, and decided against the omnibox as too specialized.   

But I really didn't think about the force-cancellation aspect.   I can see that being a significant factor in a subwoofer system that has heavy cones.   Have you found it to matter in bass cabs or similar applications?