Bill Fitzmaurice

A 102mm port has an area of 8167 square mm, two 50mm ports a combined area of 3925 square mm, so the answer is no. For the same Fs the 50mm ports would be much shorter. However, they also might chuff. Your speaker modeling software should show you port velocity at maximum excursion, which you want no more than 20 meters per second within the cab pass band. It will also show you what port area and length you can get away with to meet that requirement.

[quote name='alexclaber' timestamp='1505310789' post='3370935'] There's definitely a bigger issue in North America because they only have half as much voltage coming out of the wall. Yay for proper 220V+ power (we have the most here in the UK). [/quote]It's not an issue where bass amps or club size PA is concerned, nor for that matter with pro-touring sound, where the power amps typically run on 220v.

A fan would make the amp position irrelevant, whereas convection cooling can require specific orientation of the amp to function.

The possibility exists that it could reduce the effectiveness of the amp's cooling mechanism, especially if it's passive and not fan driven.

[quote name='Kevin Dean' timestamp='1504770251' post='3367123'] But He doesn't have this problem with my front ported cabs , To me it's obvious that the low end would also be louder to him with the rear port . [/quote]And again, the science is never wrong. As I already noted the main reason why some cabs are rear ported is that they're too small for the ports to fit on the front. Smaller cabs have higher Q than larger cabs, resulting in boomy response, and that's why what he hears with front versus rear ported cabs is different, not the port location. The difference will be less obvious to you, as the boom is masked by the mids and highs that you can hear because you're in front of the cab, and he can't, because he's behind it.

[quote name='Kevin Dean' timestamp='1504723659' post='3366923'] If I've had my rig in front of a drummer Ive had complaints when using rear ported cabs that the lows were too loud muddy sounding . [/quote]Look at the polar chart I posted. Of course it's muddy when the cab is in front of him, all he can hear is what's below at best 300Hz. To his side is better, but if he's going to hear the entire bandwidth you must either be in his monitor or you need two cabs, stacked vertically, with the lower cab aimed at him, the upper cab aimed at you. The science of audio always works, whether you believe in it or not. The problem when you don't learn the science is that when issues arise you won't know what causes them, so you can't know how to fix them.

[quote name='Ghost_Bass' timestamp='1504710670' post='3366821'] I stand by my words, having the port aimed to the front gives something more to the sound [/quote]Watch this, particularly from the 1:00 marker to the 5:10 marker, although almost every member here would benefit from watching all of it: [media]http://www.youtube.com/watch?v=BYTlN6wjcvQ[/media]

[quote name='Ghost_Bass' timestamp='1504690139' post='3366542'] Isn't it possible that those freqs might reflect inside the cab and leave through the port along with the bass and that may translate in to [i]something more[/i][/quote]It is possible, with a cab that's defective. One of the reasons why well designed and built cabs have internal damping is to prevent that. A few years back G-K brought out a line of cabs that were not damped, and poorly braced. They made the claim that they sounded better without damping. That claim was revealed as piffle when owners of them added bracing and damping and found they sounded much better after so doing. Not surprisingly Generation II of that line of G-K cabs were better braced and internally damped. By the same token there have been rear ported cabs that placed the ports where midrange cone radiation could make its way directly out the port. That as well is a design defect. [quote] Don't give me text book answers please, i'm not a sound engineer, but i'm a very practical person and this makes scense to me [/quote]Where the behavior of sound is concerned text book answers are a necessity, because what intuitively makes sense is usually wrong. Besides, you're a bass player. You're supposed to be the brains of the band. There is a different position for blokes who lack any clue as to how the gear they use to make a living with music works: guitar players.

[quote name='Ghost_Bass' timestamp='1504626354' post='3366161'] imagine a kick drum, stand behind it (next to the drummer) and listen to the sound. Now move to the other side, in front of the kick port, and listen to it. Isn't it different? [/quote]It is, but not in the lows. This is a polar pattern of a typical large speaker: The output pattern is 360 degrees to 80Hz. Port output is centered at the speaker tuning frequency, which generally runs between 40 and 50Hz. You will hear a difference with any source based on where you are, but it won't be in the lows.

There are no advantages or disadvantages. If you have am 8 ohm cab use the 8 ohm tap, if you have a 4 ohm cab use the 4 ohm tap. If you want to use two cabs then they'd have to both be 8 ohm, which when run together would go on the 4 ohm tap.

Mixed cab setups can sound fine. The question is whether or not a pair of mismatched cabs works better than a pair of matched cabs. The only way one can know that is to try A+B, A+A and B+B side by side. The sealed 4x8 may have better highs than the ported, but if it does it's not because it's sealed, it's because the drivers have better midrange response. Predicting if that's the case [i]should[/i] be a simple matter, you'd just look at the frequency response charts for the speakers. Unfortunately, manufacturers have a very low opinion of their customers, thinking them too daft to be able to make heads or tails of a frequency response chart, so they don't provide them.

What you'll end up with is a chain with a weak link. You may have seen it said that sealed cabs have more definition than ported. It's not true. Definition occurs in the midrange, and there's no difference in the response of sealed versus ported cabs in the mids. As for a 410 atop a 115, that's probably the worst possible cab configuration, nothing about it makes any sense.

[quote name='Lozz196' timestamp='1504387163' post='3364593'] On stage I don`t really notice the difference, but in a small rehearsal room I find rear-ported cabs seem a bit boomy [/quote]That has nothing to do with the port location. For the most part the reason cabs are rear ported is because there isn't room for them on the front. There isn't room because the cab is small. Small cabs tend to accentuate the midbass, which is the source of boom. Boom may be more noticeable in smaller rooms. [quote]What about side ported or bottom ported?[/quote]Radiation from ports is omni-directional. For that matter cone radiation in the lows and midbass is omni-directional as well. If you're able to stand behind behind your rig you'll notice that the highs are gone, and the midrange is muted, but the rest is unaffected.

[quote name='EBS_freak' timestamp='1504258093' post='3363682'] Bottom cab (assuming the impedance is the same) will get the same power as the top cab but its distributed between two speakers instead of 4. [/quote]If that's the case you want the 210 on top, so that you'll hear if it's distorting before the drivers are endangered. For equal power distribution the 410 impedance should be half that of the 210 impedance. However, in most cases they're both loaded with 8 ohm drivers, with the 410 impedance 8 ohms, the 210 impedance 4 ohms. The correct arrangement would be to have the 210 drivers series wired for 16 ohms, but that's seldom the case.

The symptoms are those of overheating.

The PJB wiring should be found in the owners manual.

What does the PJB show for connections?

Speakons can be wired more than one way. You have to make sure yours is wired correctly on both the amp and the speaker ends to function with the PJB.

On a technical note, the main reason why large PA systems use active (electronic) crossovers and multi-amping is they give much better protection to midrange and high frequency drivers than passive crossovers that use one amp to drive the entire audio range. Bass rigs don't need that kind of complexity. Dual-amping does make sense. The first time I heard it done was by Chris Squire on the first [i]Yes [/i]album. It took me 20 years to figure out how he got his tone, and then only because I read about it in an interview. You could never do it with a bass amp and pedal, because that affected the entire signal, not just above 100Hz or so with the guitar combo while leaving what was coming from the bass amp unaffected.

Bi-amping can work very well when done right, but where most bass rigs are concerned it's done wrong. The wrong way is using full range bass cabs for both amps. The right way is with an electronic crossover that sends the lows to a dedicated low frequency cab, say a 1x18 or 2x15, the mids and highs to a dedicated midrange/HF cab, loaded with six or eight inch midrange drivers. A variation is using a bass amp and bass cab for the lows and clean mids, a guitar combo for overdriven tones, but that's not bi-amping, it's dual amping.

[quote name='markstuk' timestamp='1503057017' post='3355356'] I feel a QI moment coming on [/quote]You have to bait a hook before you can set it. [quote]The issue is you cannot tell by just keeping the volume knob down.[/quote]+1. Besides, it's not power that trips protection circuits, it's current. You simply can't tell if you're pushing the current envelope until your amp shuts down. Having a big amp with lots of head room helps, but with today's typical puny power supplies you probably don't have as much headroom as the amp power spec would lead you to believe. Even if you really do have 10dB of headroom a hard transient can eat up that 10dB in a heartbeat.

[quote name='Happy Jack' timestamp='1503053982' post='3355335'] But with regard to all this, assuming that you're not driving your rig overly hard then does it actually "matter"? [/quote]It does. You may get away with too high an impedance load on a valve amp or too low an impedance load on an SS amp for brief periods but you still risk triggering the protection circuits of SS, and worse, damaging the output transformer and/or valves with a valve amp. [quote]I...never get anywhere near using half the power I have on tap[/quote]How do you know?

[quote name='Kevin Dean' timestamp='1502458939' post='3351636'] Greg Lake 20 plus cabs [/quote]I saw him some years back with 'only' two SVT cabs, and you couldn't hear a note from them anyway, it was all PA. Giant rigs are OK for impressing the kiddies, but that's about all they're good for.

Clamps aren't required. I use an external wood caul temporarily screwed to one panel to perfectly align the parts, isolated from the glue line with polyethylene sheet cut from a trash bag, with a few screws holding the joint secure until the adhesive sets. After the adhesive has cured all the screws are pulled, cauls removed, the screw holes filled with adhesive. Like so:

I wouldn't use either, both use too much material where it doesn't do anything useful. This picture compares typical 'window' bracing on the left with the better method on the right: The method on the right is more effective while using less material. I see you also have battens on the corner joints. They serve no purpose, other than adding dead weight, as that's the strongest part of the cabinet, requiring no reinforcement.