Since the last post I've been waiting for the compression drivers and waveguides to arrive so that I can get started measuring and modeling the speakers, but to pass the time I've been doing some research about constant directivity loudspeakers, and the kinds of tools I'll be using to help me design them.
The Pi Speakers Whitepaper was the first source I looked at to get a sense of what's going on in a constant directivity system. It starts off with some background on horns and waveguides, and what kind of advancements led them to their current form, then it gets into how to integrate them into a two-way loudspeaker system. There is actually a lot more to consider when designing the crossover for a constant directivity system than just passing the correct frequencies to each driver. Since the goal is, of course, constant directivity, that means matching the directivity of the woofer to that of the tweeter at the crossover frequency, which adds a considerable amount of complexity.
Thankfully there are lots of resources available when it comes to crossover design. Next up on my reading list is also from Pi Speakers, and it's referred to as the Crossover Document. At over 70 pages, it's a hefty read, but I'm hoping it will expand upon what I've already picked up from Leach's Introduction to Electroacoustics and Audio Amplifier Design.
In addition to reading up on theory, I've been scouting out some programs to help me measure the drivers, design the crossover, and model the final system before spending any money on crossover components. Here's what I've found so far:
Room EQ Wizard (REW): I've used this in the past for measuring the in-room frequency response of my subwoofers, but it also has the option to measure driver impedance and spit out parameters for use in modeling software.
WinISD: Using the driver parameters, this program models the transfer function of the driver in different enclosures.
Passive Crossover Designer 7 (PCD): This program takes in raw impedance and frequency response data from each driver, and allows you to design and model a crossover while providing graphs for frequency response (at any position in relation to the system) and directivity. This program will also model different enclosures and can use that data for the crossover design.
Finally, last but not least, my compression drivers and waveguides just arrived today. The waveguides are made out of extremely thick and heavy plastic, and feel incredibly solid, although next to the compression drivers, they're lightweights. The DNA-360 is a monster! These things are much larger and heavier than I expected, and I wouldn't be surprised if the CD/waveguide combo weighed more than the woofers do. For some size reference, here's a picture of me holding one of them.