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In the Forum: Horn-Loaded Speakers
In the Thread: Driver Alignment, and Where to Draw the Line
Post Subject: time alignmentPosted by stuck.wilson on: 5/27/2007
hey jessie-
I've wondered the same thing-- and the more i end up twiddling around with it, the more I think it really either comes down to good measurements, or good listening! And in fact.. probably both in conjunction!
I have a small 2 way system (crossover's around 1400hz)-- and to ballpark it for starters, I looked on line for a wavelength calculator-- defining the offset of acoustic centers. Since I'm using an asymmetrical XO (bass box a 2nd order, horn a first)-- using what bad information I have available-- I extrapolated that there is a phase difference between the two of 135 degrees-- the first order being 45 degrees 'ahead', and the 2nd order being 90 degrees 'behind'.
SO-- that being the case-- I took the distance of a wavelength at 1400hz, multiplied that by 135/360, and got the offset from direct acoustic center of the driver..
After that-- I based the measurement to the approximate 'end of piston'- by eyeball.. I can't measure through the woofer, obviously.. but I've got vented dustcaps, which helps. Add that to the offset.. and then it's all by ear. but to get a good reference point-- i took a straightedge across my bass driver, and extrapolated the distance inset from the CABINET ( the straightedge lines up with the top lip of the cabinet, as it's front mounted) to the piston by eye. I'm a carpenter.. so I guess that crap a lot-- but you'd still be ballparking-- so a good guess is better than nothing. For the compression driver-- there's a silver band in the center of mine, which i believe holds the gap-- so i use the back of it as my 'guesstimation point'.
What I usually do after that is to keep a machinists rule nearby, and listen to things with a lot of ambient information, as well as things with big transients in the XO range... especially bombastic piano jazz.. which exposes bad phase problems like spotlighting deer in an empty field... big 'frrmmPPHHTTtt..'' every time theres a BIG transient.. you'd have a hard time missing it! the ambient information is more subtle.. but when you get it right... ('right'.. I'm no expert, I assure you...)- decay is MUCH better. Oh yeah.. the machinists rule is to keep track of the movement of the horn forward and back- best to keep track.. because sonic memory is usually pretty bad..
BUT.. I think you already said as much.. it's all trial and error, unless you've got a great RTA and mic setup, or bionic ears.. and even still.... Just keep track of your experiments and go with what you hear! I can't speak to the 'absolute correctness' of ANY of this data.. it's just my experimenting logic, if y' will.. but it's yielded decent results so far, best I can tell!
Good luck!
yrs
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