Here's a great post by Mark Eldridge over on CarSound concerning PLD.
Originally posted by Mark Eldridge:
Hmmm... path length differences... You might as well get that signal alignment device cause I'm not going to help you... [img]smile.gif[/img]
Actually, I'd recommend looking for past threads on the topic, in addition to what we discuss here. There have been a lot of threads on this subject in the last year or two, and you can find them in the archives and just by searching for threads on path lengths, imaging, staging, etc. Also, buy a copy of the Autosound 2000 Tech Briefs, and read tha articles in that collection. Between those two sources, you'll have more information than you can probably find form all other sources combined. [img]smile.gif[/img]
The real skivvy is that you want to minimize the differences, for a lot of reasons. Especially in the mid-bass through the midrange regions, the difference between left and right pathlengths is CRITICAL! In fact, between 100 and 400 Hz, the angle of the speaker won't even matter, as imaging cues are almost 100% determined by path length, not by intensity differences. Above 2000 Hz, imaging cues can be controlled by speaker angle and other intensity controlling techniques. Path lengths above 2000 Hz are not critical.
As far as the path length differences between the mid-bass driver, midrange, adn tweeter on a single side, you'll probably never get them exactly the same. The real problems with path length differences here will be int eh frequency and phase response in the crossover range. Signal alignment can help some here, as long as the speakers aren't too far from each other. It's more acceptable to have a mid-bass amd midrange separated than to separate the mid and tweeter. The mid-bass frequency range can make a difference in the perceived stage depth,a dn the center image stability. But, the upper midrange and high frequencies are responsible for the stage height, width, and depth, as well as image focus. Keeping these frequency range drivers close together will be a lot easier to control all the variables than separating them. If you need additional stage height because the mid and tweeter are in the kick panel area, then add a second set of tweeters high and wide, and crossed over pretty high as well (somewhere between 8000 and 20000 Hz). This configuration is what has been used in most all fo the best sounding cars that use conventional drivers. And in most of the HLCD systems, the additional tweeters are used for the same reason.
Anyway, locate the midrange drivers first, and work with them in their intended frequency range to achieve the best overall stage depth and center image. Don't worry too much about stage height or width yet.
Next, locat the mid-bass drivers so they blend well with the mids, have solid output, and do not detract from the center image. Doors will likely not be the best place for them. They can be mounted under the dash, in the floor, in the firewall towards the center of the car from the kick panel, or where ever else they work, adn can have a large enough enclosure.
The tweeters are the easiest. Mount them as close to the mids as possible, and make sure they give you the width and height you want, and help to focus the image performance. If you need the additional height, add the second set of tweeters.
Path lengths are a different animal. They're kind of like the impedance of a woofer. Industry wide, we call a speaker a "4 ohm" or "2 ohm" speaker, when in reality, it is only really that impedance at one or maybe two frequencies. That's the "nominal" impedance which means "in name only." The actual impedance varies widely accoring to frequency. A "4 ohm" speaker in a box may have impedances as high as 50 ohms at resonance, and as low as 3 ohms elsewhere.
Likewise, the sonic cues that affect what we percieve as staging and imaging are frequency dependent, and there is no one single number, technique, or what ever that can be used across the frequency board.