Phase IV rebuild complete

Over the weekend I finished the rebuild of the Phase-IV baffle. Actually the baffle was done a while ago but since my wife was unhappy having something so ugly in the house, I decided to try to improve how it looked by giving it a coat or ten of paint. There's a I need to learn about finishing.

The mid range and tweeter are now properly located in close proximity so the interference fringing around the crossover frequency should be fixed. By way of comparison, here's what the speaker looked like before. Not great, but a lot less like a building site.

The bass cab is now filled with a fair amount fiberglass insulation - I know there are better materials but this was cheap ($15 and I've used less than a quarter of the roll). Since this changes the bass response, I'll need to rebuild the crossover. So next weekend the measurement equipment comes out again.

Volume estimation

I am still going back and forth over the size of the cabinet; is 5000 cubic inches (about 80 liters) enough for the sealed bass enclosure? In building the Phase IV, I had used the volume information from the DM6 which has in internal volume of "between 51 and 60 liters" according the manual. I decided to look at the original B&W 801; based on the published overall dimensions and some pictures, I estimated the internal volume to be somewhere between 108 and 122 liters depending on the thickness of the cabinet walls (according to B&W's literature, the Nautilus 801 walls are 1.5 inches thick - I used 1 inch and 1.5 inches). Big difference!

The actual air volume is likely considerably smaller since there is a fair amount of internal bracing (the 'matrix' used in the Nautilus could take 10%-15% off this figure). However, since the dimensioning calculation I was doing ignores the internal bracing, I decided to compare only the raw internal volume.

The 801 bass cab volume (~115 liters ) was over double that of my Phase IV (54 liters) and suggested a complete redesign of the cabinet was needed. It probably also contributes to the poor performance below 100 Hz, though the small linear excursion of the Silver Flute drivers is clearly another factor. The Phase V needs a wider baffle to accommodate the 10" AC-250F1 driver, but this alone wasn't nearly enough; several iterations of changes led to a design that maintained the height at 40 inches, increased the depth from 21 to 24 inches (slightly deeper than the 801 which was 22 inches back to front) and kept about the same width (17"). The new bass enclosure will weigh in at 110 liters before bracing. (It will also be fairly heavy)

Still baffling

To fix the destructive interference issue, I building a new baffle. It will also be almost twice as thick, partly to reduce resonance, and partly as a way of loosing all the ugly screws on the front but still allowing access to the inside. The star of the piece is the new circle cutting jig.

(It fits only some Craftsman routers - the plunge router has a larger base, but it does fit the fixed base model). Depending on which way you put in the center locating pin slider thing, you can cute really small circles (<2 inches in diameter) to fairly large ones.

The router jig enabled me to rebate the drivers which I hadn't done on the first baffle. Although I don't understand the physics, there is a strong school of thought that the tweeter in particular needs to be flush with the baffle.  Presumably something to do with diffraction from the sharp edges?

This shows the double layers and the neat rebate the router cuts. The lower section of the baffle will be screwed in close intervals to the cabinet, and the upper section will be attached to the lower plate with the driver mounting  bolts and possibly four screw, one at each corner. 

The AC 250F1's are now in so I want to get this (Phase IV) completed before building he new cabinets.Once the new baffle is built I will likely line the cabinet with SAE-13 or SAE-15 1/4" felt. I didn't do this first time round since I wasn't sure I wanted to blow $80on felt if the sound really sucked. (I also wanted to test the reflective diffusing properties of the cabinet design without any absorbing material). 

Phase V

Current plans are forming around the AC-250F1, an AC-130F1 or MkII, probably the latter (I like the idea of using carbon fiber somewhere) and Hiquphon OW1. The two driver idea will have to wait. Some testing of the implications of the phase change created with any second order or higher filters suggest using only first order. I am assuming the trade-off is with harmonic distortion when significant signal gets to the drivers outside of their ideal operating range. This remains, as they say, an empirical question.  What bothered me was the real mess second order filters seem to make of a square wave.

Above is the simulated result of passing a square wave though the second order crossover designed around the AC-250, AC130 and OW1. Speaker Workshop generates the phase shift data in the total frequency simulation. Using these, I simulated a square wave from odd harmonics, and then applied the phase shifts from the Speaker Workshop output which I had dumped to a file. This (above) was the result and it looks terrible - nothing like a square wave.

This, by way of comparison, is the simulated for the first order crossover I designed for the same drivers. While obviously not square, it seems much better than the signal passed by the second order network.

If the rendition of detail has a lot to do with the accurate reproduction of transients, then the first order network should do much better at this than the second order.

My current plan is to build the speakers (when the wretched drivers get here in March or April) and two pairs of cross overs, one first order, the other second order, and see how they sound.

Doh!

You may have noticed that the mid range and the tweeter are too far apart. I completely ignored interference fringing around the crossover frequency. The units need to be separated by less than the wavelength of the crossover frequency - and they aren't. So as you move vertically (perpendicular to the axis), the waves from the two units interfere constructively and then destructively. Doh! This is something I really should have thought about. So I will be building a new baffle with the two units mounted much nearer together. Good job it's only screwed (sic) rather than glued. Oh, and I need to get a new amp or repair my old SUV6 which is cutting in and out on the left channel. A cheap valve amp would be nice but probably out of reach. 

Winding the coils

Since the exact values for the inductors I needed weren't available, I decided to buy some 16 gauge magnet wire and wind my own. After looking at a number of U-tube videos, I built a simple winding jig:  It wasn't quite as easy as the video made it look but eventually I had four coils. One was about 20 turns light because I ran out of wire. I think I was short changed on the spool I bough on Amazon - I won't be  buying from TechFix again. The online calculator seemed way off for two of them. Instead of 2.25 mH, the large coil was 1.6mH; not even close. So it was back to the shed with a hack saw and some nails. The addition of a steel core (should be iron but steel was the best I could improvise) boosted the impedance to 6.8mH. That meant I could unwind more than half the turns on both coils. The two home made cores are on the right - on the 2.25mH coil you can just see the nails under the blue insulating tape.  On the right is the network schematic. 

While I stated with a set of theoretical values, must succumbed to trial-and-error fine tuning in Speaker Workshop as I matched the network to the particular measured characteristics of the mounted drivers.

My wife's comment when I showed her the finished crossover was "it looks like something a 5-year old would have made". I was pleased; that something that looks so rough produces such nice results evoked the same feeling I had when I first visited the 8th floor of the Blackett laboratory. In display cabinets in the elevator lobby were devices that had been instrumental in a number of major experimental discoveries in particle physics. They too looked like "something a 5-year old would have made". It doesn't need to look good to work well, something Prof. Ken Bignell also taught me

Phase IV - measurements

Here are the far field frequency response graphs for my Phase IV prototype and my old B&W DM6s. Both have a trough at 110 Hz which I am assuming is a function of reflection in the room. The Phase IV has a trough at 215 which I think is internal reflection in the cabinet which currently has no damping whatsoever - so I need to buy the SAE 13 felt but at $80 I had been holding off. I  don't imagine that it will make a huge difference when I put it in but I may have to design the crossovers. From 700 Hz and up the response is very flat and looks rather better than the DM6 which has a biggish hole from about 550 to 1.5k. I'm pretty much done with this experiment now and in March will be beginning a new design based on a similar approach to the cabinet but with better quality bass and high frequency drivers.

Three goals were accomplished with Phase IV: the first was to test the general approach to the design of the cabinet; the second was to test the design process from beginning to end, in particular the design of the crossover based on the measured 'in-situ' response of the drivers; the third was to build a speaker comparable in quality to my DM6.