Bass Cabs: How Shallow Is Too Shallow?

[will33]

I'll pull the drivers tonight, but it feels like the slot port (just one across the full width at the bottom) is only about 1 1/2" deep or so, so there's not much room to cut anything. Per Astro's advice, I'm inclined to seal it off and cut in a couple of port tubes between the drivers for symmetry. The cab is light for its size, so there can't be much bracing at all. I'll add a few well-placed sticks for good measure.

Other points of reference: Two 1/4" speaker jack cups on the back and a "no tail" logo. Serial #F098998. I'll come back and post pics later.

You probably have enough room to lower the tuning (lengthen the shelf) as needed. I wish I could remember more details about mine but I just didn't know much about this stuff at the time and have been through/built/sold/swapped several cabs since then.

The symetrical port layout is a great idea, though probably not "mandatory" if you have ample port area and/or are not using high power amps and high excursion woofers, etc.

You can find your current (small signal) port tuning by laying the cab on its back, putting some kernels of rice (or salt, sand, dry peas or beans, etc.) on a cone and passing test tones (sinewaves) through it, fishing around between probably 40 to 70hz and finding which frequency makes the rice (or whatever) stand still and stop "dancing".

This works because the driver is held nearly still at the tuning frequency...just vibrating a little bit to "excite" the port tuning resonance.

That'll give you an idea of where you need to go with the port to suit the new drivers.

 

[will33]

Real nice 3012 build. If you routinely (or often) play at high levels you could avoid 'boom' by tuning somewhat lower - say around 47 Hz. The reason for the booming at high levels is in large part due to the increase in voice coil temperature and resulting higher voice coil resistance (and suspension stiffness, raising Fs). This increases Qts and changes your alignment to a more peaked response. Tuning lower just pre-compensates for this, but results in a slightly over damped alignment at lower spl's. You can boost the bass a little in those cases.

Man, that Mustang would be hard to pass up. Good luck with that.

IMO, this is one of those things some of us more laymen, backyard DIY'ers fail to account enough for. We'll spend too much time agonizing over a couple liters of box size here, or 1 or 2 hz of tuning there, failing to realize that our speakers don't perform the same with 100 watts as they do with 1.

It's one of those things that can turn a design that should be the cats pajamas in concept into a horses ass in practice.

 

[will33]

Ok...800 watt amp on this 215.

In that case, in that case I would opt for cutting/modding/building to either have a shelf port on both top and bottom of the cab...or at least four, 4" (or greater) round ports, or the equivelent (or greater) square, triangle, or other wood-built ports arranged symetrically to even out air pressure, prevent cone rock, and help hold things steady as the power gets poured on.

 

[FerruleCat]

Yeah; the 6cf box I spec'd out for a pair of Daytons has two slot ports - 1 each at top and bottom, I want to say 1.5" high by about 4" deep, but I'd have to check the WinISD file. IIRC, my maximum port velocity was going to be 60-some fps at 600 watts, which is where the system hit Xmax. (I didn't even check the SPL curve at 600w, which I'm sure is more than adequate.)

I'll measure the Fender cab and run the numbers in WinISD. I'm confident I can get in the ballpark with this box and not too much trouble.

EDIT: I'm reminded of that old Beatles lyric: "I-I-I dig a holesaw. You can perforate anything you like. Yes, you can perforate anything you like; it's out of sight."

 

[AstroSonic]

IMO, this is one of those things some of us more laymen, backyard DIY'ers fail to account enough for. We'll spend too much time agonizing over a couple liters of box size here, or 1 or 2 hz of tuning there, failing to realize that our speakers don't perform the same with 100 watts as they do with 1.

It's one of those things that can turn a design that should be the cats pajamas in concept into a horses ass in practice.

It's advice from someone who made a few 'horses asses' before figuring out what was going on. It's one of the reasons why the EBS alignment works so well for a bass cab: it's not an EBS at typical bass guitar (rock) power levels. It's much more 'flat' before rolloff once the driver is 'warmed up'. A person can also start out with an apparently over damped driver (say Qts around 0.20) and install it in a cab EBS aligned with the rolloff knee 4-6 db down, and end up with deep, solid low bass notes at 'rock' playing levels. While a person can cut the bass on a classically (low power) aligned cab, the alignment itself has excessive group delay and audible booming.

 

[FerruleCat]

Yep; there's the 2x4 brace. The internal dimensions are about 24.5 wide by 34.5 tall by 13 deep, for a total of 6.36 cf. That's right on the money for what I need.

The drivers were screwed in with coarse thread screws. I'll convert to T nuts and machine screws when I install the Daytons.

I haven't run the numbers in WinISP yet, but it will be easy to tune this cab to the low 50s.

I'm glad I went this route. It's kind of a friendly mutt of a cab, and I'll be able to dial it in without too much trouble.

 

[FerruleCat]

A pair of 6" diameter by 6" long port tubes would tune this box to 47hz with excursion-limited power handling of 500w, producing about 120dB with max port velocity of 67fps. I have enough room to mount the ports in the baffle, but since this would be a stage cab, I wonder if there's some wisdom in side mounting them to send some of the sound over to the drummer.

My luck is stellar on this project: The factory baffle cutout for the speaker ~357mm (measured) is a perfect fit with the Dayton frame of 355mm (per spec sheet). And I won't even have to cut a port tube; 6x6 is a stock dimension. The bulk of the effort will be installing T nuts, which is no big thing. (Just use an extra nut, machine screw and fender washer to pull them into the wood with a socket wrench.)

I'm about $150 in drivers and 1 hour of easy work away from having a banging 215.

 

[DukeLeJeune]

If you routinely (or often) play at high levels you could avoid 'boom' by tuning somewhat lower - say around 47 Hz. The reason for the booming at high levels is in large part due to the increase in voice coil temperature and resulting higher voice coil resistance (and suspension stiffness, raising Fs). This increases Qts and changes your alignment to a more peaked response. Tuning lower just pre-compensates for this, but results in a slightly over damped alignment at lower spl's. You can boost the bass a little in those cases..

Good point!

The great thing about the new generation of uberwoofers is, you can push them closer to their thermal limits without them farting out. And the bad thing about the new generation of uberwoofers is, people push them closer to their thermal limits because they don't fart out.

 

[Rockbobmel]

The Musicman RH115 is 13" deep with a black widow- it shoots through schools!

 

[Rockbobmel]

Like this? It is 16" deep though.

 

[FerruleCat]

Like this? It is 16" deep though.

Same slot port design, but the dimensions are apparently a bit different. Mine has silver grill cloth, too.

In other news, I've decided to go with rear porting using four adjustable tubes with a nominal ID of 4 5/16". I'll place them symmetrically to the drivers and fine tune as needed. This lets me keep the baffle and the grill frame stock. (The grill cloth is backed with a board, which would have to be cut for front-mounted ports.)

 

[will33]

Yep; there's the 2x4 brace. The internal dimensions are about 24.5 wide by 34.5 tall by 13 deep, for a total of 6.36 cf. That's right on the money for what I need.

The drivers were screwed in with coarse thread screws. I'll convert to T nuts and machine screws when I install the Daytons.

I haven't run the numbers in WinISP yet, but it will be easy to tune this cab to the low 50s.

I'm glad I went this route. It's kind of a friendly mutt of a cab, and I'll be able to dial it in without too much trouble.

Yup. Same cab I had. Good candidate for those Daytons.