Bill Fitzmaurice

What software are you using to come up with the design? And what criteria to choose the drivers? For simple ported or sealed cabs I use WinISD 0.7. I consider not only frequency response but also sensitivity, maximum power (determined by Pe and Xmax, whichever is lower), and port velocity (no more than 20m/s at full power within the nominal passband). I choose drivers based on their cost effectiveness while producing the desired results. Also note that software is only effective within the region of pure pistonic function, which is roughly two octaves above Fs. Above that you must use the driver data sheet SPL chart. And that only gives you the raw numbers. Turning those into a cabinet design relies mainly on experience to know what works best. As for mixing drivers, it's like a box of chocolates: you never know what you're going to get. Sticking with one insures that you will know. I ignore when people say that 'this 115 and that 410 work a treat', because virtually any two speakers together will work better than either on their own. The only way to know if any given pair of mismatched cabs/drivers work better than two of each matched is to try all three combinations side by side. That's fine if you have two of each, but if you don't it's an experiment you can't undertake. Don't sweat tuning for the low B note. While the fundamental is 31 Hz the bulk of the content is in the 2nd and 3rd harmonics, 62 and 93 Hz. No one ever complained about an Ampeg Fridge handling low B, and its F3 is 58 Hz. I never tune bass cabs lower than 45 Hz. I only go lower than that for PA subs.

A bit of semi-interesting stuff is the design of the Altec A-7. The woofer horn had two purposes. One was increased sensitivity in the lower mids, but the other was to time align the acoustic centers of the drivers. The aligning procedure was to wire the HF driver reverse polarity, feed the speaker with a sine wave at the crossover frequency, then slide the HF horn on its sled atop the cab back and forth until the SPL reading was at a minimum. That placed the acoustic centers of the drivers correctly. Swap the polarity on the HF driver back and it was good to go.

The wider the source the narrower the horizontal dispersion. It's why midranges are smaller than woofers, and tweeters are smaller than midranges. Side by side placement reduces the horizontal dispersion by half compared to vertical at the lower end, and raises the potential for combing at the upper end.

Phase shifts like that are normal, and don't matter, as you can't hear them. The dip and shift in the crossover region could be polarity related, the only way to know is to reverse the tweeter polarity and test again. It could also be time align related, which can only be fixed by moving the tweeter to get the acoustic centers of the tweeter and woofer the same. The shift at 100 Hz is probably port related, possibly indicating port tuning at 100 Hz. The 200 Hz dip could be from floor bounce. The best way to take measurements is outdoors, well away from boundaries, taking two measurements. The first is ground plane, with the mic a few cm above the ground. The second is with the cab on its back, the mic suspended above. This measurement will have a dip where the wave reflected off the ground meets the front wave at 180 degrees, the result of the baffle being 1/4 wavelength from the ground. The two are spliced to get the final result. You can do it in one step by digging a hole in the ground, putting the speaker in facing up, back filled with the baffle flush to the ground. It's how Roger Russell of McIntosh used to take measurements. When you take measurements 1/6 octave smoothing is preferred, as that's the limit of what you can hear.

There's no advantage to that over a 410, as it's essentially the same thing. In fact the increased spacing is worse. If you must do it, though I can't imagine why, the bottom cab should be flipped upside down, placing the woofers closer together, but only if the tweeters are not used. If they are used the top cab should be flipped, placing the tweeters closer together.

Not ideal perhaps, but the lobing you get with a typical HF above the LF driver arrangement tilts the HF output downward too. I first saw/heard it done by Henry Kloss in his office at Advent. He used a pair of Advent speakers per side, the upper speaker flipped upside down, creating a W-T-T-W alignment. This was in '72, two decades before another colleague of mine, Joe D'Appolito, perfected the M-T-M speaker.

Running a second no horn cab below adds 6dB to the low frequency sensitivity. For it to sound right requires turning up the HF horn by 6dB, if it has that ability. Lobing isn't an issue if the two cabs are built mirror imaged, so you can set them with the HF horns adjacent rather than separated. I don't see manufacturers doing that, for inventory reasons, but it's simple enough if you build your own.

Of course. But one need not be an arborealist to know that maple and walnut will have higher density than poplar and spruce, while luthiers know which species give them what they, or their customers, want. Also of course. One disadvantage to high density wood is weight, so to keep the weight down you need to go with some measure of hollowing out the body. Mine I made of all rosewood, neck through, with the bouts made in two pieces, each hollowed out to about 6mm thick. This gives the high density advantages of bright tone, tremendous tuning stability and almost endless sustain without being too heavy. I like the looks of it too.

Wood species, and therefore density, makes a lot of difference in tone, as it directly impacts the resonant properties of an instrument. Speaker size alone doesn't. Interestingly I've noticed that a high percentage of those who think that wood density doesn't matter also think that the cone size does. 🤔

Myth. It would be true if voice coils reacted to electron waves within the realm of the speed of sound. They don't. They react within the realm of the speed of light. It's a good thing too, because the frequencies of the notes played are determined by how fast the cone moves. If larger cones moved more slowly you might play a C but it could be heard as a B. 😒

Compared to free-space, which is outdoors and elevated by at least a wavelength, on the ground is half-space, closer than 1/4 wavelength to a wall is quarter-space, closer than 1/4 wavelength to a corner is eighth-space. Each space reduction adds 6dB. This mainly applies to the lows, where speaker output is 360 degrees. 1/4 wavelength at 120 Hz is about 70cm, so ideally that's the furthest out you want the cab baffle from the wall. True. Where either the cab or you relative to each other or a boundary results in a 1/2 wavelength differential between the original source and a reflection there's a cancellation notch. Said boundaries include the floor and ceiling, so often the worst listening position is on the stage, where you can be getting multiple low frequency cancellations from walls, floor and ceiling. Further back in the room where the distance relationships are much longer the lows can be considerably louder than on the stage. This phenomenon was responsible for the myth of wave propagation, the notion that one must be a given distance from the source for a sound wave to fully develop and thus be fully heard. It's easily disproved by listening to the same speaker outdoors, where there are no boundaries that can cause cancellations.

Using more speakers outdoors is appropriate, as you're losing some 6dB or more of boundary reinforcement. That requires doubling the cab count to come out even, especially in the lows. The main issue with using dissimilar cabs is whether they'll integrate to help each other or whether they'll fight each other. You'll want to try it in a practice situation first.

When EQ'd for the same frequency response you cannot tell the driver size. You can have a dozen drivers of the same size with a dozen different frequency responses. When cabs have the same cone displacement, be it from one larger driver or more than one smaller drivers, they have the same maximum output. The only factor that's attributable to cone size alone is the dispersion angle, it grows narrower as the cone grows larger. Even that can be tweaked, and for that matter a pair of tens placed horizontal will have narrower dispersion than a single eighteen. In the end one factor, be it cone size or any of the more than a dozen factors that sum to give the end result, doesn't determine said result. It's that sum total.

While I'm at it, shelf style ports have two issues. The short height dimension can lead to chuffing, and they don't do double duty as braces unless located between the drivers. While this isn't a bass cab, it's a sub, I use the same design features in direct radiator bass and PA mains cabs: https://celestion.com/blog/build-this-21-inch-bass-reflex-enclosure-featuring-the-tsq2145/

Those dimensions give an approximate volume of 50L per driver with 55-60 Hz tuning, so the tuning is too high and the box is too big. A dividing shelf doesn't do anything other than to add bracing to the panels that it connects. You should have bracing, but as far as that's concerned a shelf is one of the least effective methods.