The tweeters in Level Two are 28mm silk-dome units from Morel of Israel. They have the proper outer diameter to use BMWs OEM tweeter mounts, but their performance is far greater. Their upper extension is far more linear, allowing more “air” and “sparkle” without the brightness of “bite” which plague lesser tweeters in these cars. Their mechanical resonant frequency is low enough in the midrange that we can play the tweeter almost two octaves lower than most. (It helps that a 28mm dome has about 28% more area than a 25mm dome, which is the factory driver size). This means the tweeter plays all the treble – we don’t rely on the midrange cone driver to play any of the treble, and that’s a preferred approach.
The midrange driver is in the 4-inch/100mm class of speakers, but a close inspection reveals that it, too, has a greater diameter than most speakers in its class. Morel tooled their own frame, cone, and surround (no off-an-Asian-shelf parts here) and the result is 24% more cone area than most 4-inch-class drivers. The use of a 50mm voice-coil winding means that an amplifier with enough power can exert more control over the motion of the cone than with most speakers. The magnet exists inside that 50mm diameter, not outside it as with most speakers, and that allows the magnet to actually fit inside the door! The magnet is a neodymium hybrid, which allows it to be small enough to fit and strong enough to do the job we need. We modify it with our own mounting adapters and harnessing prep.
The woofers we import from Germany – 200mm woofers with neodymium magnets, paper-pulp cones, and our prep to both bolt in and plug in to replace the stock underseat woofers. (Many folks on the Internet call the underseat speakers “subwoofers”, but they are really playing a lot of middle bass and not that much sub-bass, so “woofer” is a much more accurate term.
The 6-channel amplifier powers the front, rear, and underseat woofers on 3 pair of channels. The front and rear speaker sets are high-passed (the bass is filtered out) and the underseat woofers are low-passed filtered (the mids and highs are filtered out). We set the crossover filters up so that the underseat woofers play regardless of fader position – we sum the front and rear bass notes together to create a “constant-bass” arrangement.