Bill Fitzmaurice

The difference in sensitivity with 4 ohms versus 8 ohms is only 2dB. The difference in maximum output between the two is 0dB. There are a lot of factors that determine how well a cab will work. Impedance isn't one of them.

If we go back to 1965 that scenario was more or less the case. Fender, for instance, used the Jensen C10, C12 and C15, similar in all aspects save cone size and Fs, so the fifteen went lower than the twelve, which went lower than the ten. By 1975 driver options had expanded to the point that one could no longer make any judgement on potential response based on driver size.

+1, and where low frequency response goes it doesn't matter very much. This charts three speakers, one loaded with a ten, one a twelve, one a fifteen: If you were to believe the size matters crowd you'd have to think that the red line is a fifteen, the blue line a twelve, and the green line a ten. You'd be 33% correct. The blue line is a twelve, but the red line is a ten, the green line a fifteen.

Insufficient data. Only by seeing a measured SPL chart can you know what's really going on. If only manufacturers posted them, but none do.

The combination might work fine. It might not. There's no way anyone can tell you what the result will be. The only valid method of comparison would be to have the 210 plus 15/6 and the 2x210 side by side. Otherwise it's all pure speculation.

Since we're still on this subject, no, you shouldn't presume that. For every manufacturer that does the required research another just tosses drivers into a box. The only factor determined by cone size in and of itself is the dispersion angle. It goes smaller as frequency goes higher, based on the size of the cone relative to the wavelength produced. To keep dispersion as uniform as possible as the wavelengths get shorter the cone size must get smaller. That, and that alone, is why midrange drivers are smaller than woofers, and tweeters are smaller than midranges. Of course the driver T/S specs and other pertinent features are optimized for the driver pass band, but those are secondary concerns, as no matter how well optimized they're of no use if the cone size is too large for the pass band to be heard other than directly in front of the driver.

I wouldn't be too concerned, the stand would have to be closer to 60cm to have a major effect.

That's correct. A single 12cm diameter round port is usually sufficient for a 15 inch driver. Split into four 6cm ports you might get some chuffing. The added area of four 10cm ports prevents it.

This calculates the effect of lifting the cab: http://www.tonestack.net/software/floor-reflection-simulator.html The higher it's lifted the lower the frequency of the floor bounce induced cancellation.

If you lift the cab more than 20cm or so off the floor you'll introduce floor bounce phase sourced cancellation in the mid bass. That can be beneficial on a boomy stage, so in that instance it's a good idea. But otherwise it's best to leave the cab on the floor, tilted back to aim the drivers at your head, the better for you to hear your mids and highs.

Assuming the total area of the four ports was the same as the area of one port that result is to be expected. There is an area of high friction close to a port wall. That can cause chuffing noise with multiple small ports or narrow slot ports, which put the center of the port closer to port walls. It's easy enough to cure, by making the port area larger when those configurations are used.

You'll want to play around with the tuning frequency some more, trying at 55 and 60Hz as well. Keep in mind to calculate the volume taken up by the ports, which gos down as they're made shorter for higher tuning. That adds to to box volume, which changes the overall result. It takes a while, but it's worth it to find out what the various tuning options will result in.

The program models the port, you just enter the desired resonant frequency.

50Hz tuning would be better, but you can't change the driver specs with tuning, and that's why I would not use any of those drivers, other than the B&C I posted, in a stand alone bass cab. I'd only use them in a PA system where they don't operate below 100Hz, that range being handled by sub woofers.

No chart required. When you change the signal level or EQ by 1dB at any point in the signal chain, be it the bass itself, a pedal, in the amp pre or amp EQ or anything plugged into an effects loop, the change at the amp output will also be 1dB. A 2dB change in the signal chain is a 2dB change at the output, a 3dB change in the signal chain is a 3dB change at the output, and so forth. Turn up the mids. That's because the configuration of the JBass pickup give less low end than the PBass pickup, which translates into more mids compared to lows with the JBass.

That's mostly correct. Doubling the impedance load reduces the maximum power delivery by about 40%, not 50%, with a reduction in maximum output of perhaps 2dB, a negligible amount, assuming no change in speaker sensitivity and/or frequency response. That last bit is the key factor. What you're experiencing indicates that the sensitivity and/or frequency response of the 210 is not equal to that of the 115. It's all about sensitivity and frequency response.

You don't have 21L per driver as the port and drivers take up volume. You'd model it in your case with 4 drivers. A pair of 10cm ID ports would give acceptable port velocity. With a 70L net cab volume they'd be 20cm deep. You have to figure out what volume they occupy, plus a liter each for the drivers, to find the remaining net box volume. Then you'd do any further adjustment of that box volume in the model to see any required lengthening or shortening of the ports. Don't sweat getting it all perfect, a tolerance of 5% or so in either direction is OK.

No doubt it's too large to provide a low enough tuning to work well with bass, but again at gig levels. Even open back, a misnomer as they're not totally open, can sound good in the studio, they just lack the output for gigging. The '59 Bassman for instance, or one of Carol Kaye's preferred studio amps, the Fender Concert, with reverb, which she often used. You can hear it on many of her Beach Boys tracks.

That depends on the 210 and the 115. But mixing a 210 with a 115/6 opens up more cans of worms best left undisturbed.

This is what's possible with an eight that's well suited for electric bass, in this case the B&C 8BG51, in 18 liters net, ported, tuned to 50Hz:

All porting used to look like that, being a hole cut in the baffle, or as in this case in the back, with no duct. The common use of ducts dates to circa 1965, more or less coming into use at the same time as Theile/Small specs. What's usually referred to today as a port is more properly called a ducted port.

+1, and the pertinence goes back to where the OP stated his reasoning behind wanting to add a 115 to a 210 is to get better lows. That assumption isn't supported by the facts.

It's considerably better than the Celestion, which should never have been loaded in a sealed cab for bass to begin with, but still limited in low frequency capability. I wouldn't consider anything with a higher than 60Hz Fs, and of course the rest of the specs would have to work in the intended enclosure.

You don't have to know what it means, so long as I know what it means. 😎 In layman's terms that speaker probably can't go either low or loud enough for gigging at more than what we refer to as coffee house levels on my side of the pond. I'd say it's much better suited to low level studio work than gigging.

Obviously one would not expect a five to equal the output of an eighteen, but a grouping of fives would. What determines a drivers low frequency response is the Theile/Small specs. Cone area (Sd) is only one of some fourteen T/S specs, and of those specs Sd has no influence on how low a driver can go. That is primarily the result of the driver Fs, Vas and Qts.