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In Reply to: RE: Does the Altec 604 design make sense? posted by PaulF70 on December 28, 2014 at 17:50:55
Mating a 15" cone to a comp driver at 1600 Hz XO causes an unbalance in the freq response off axis as the freq WL(wavelength) approaches the width of the 15" cone. In other words beaming is ocurring. This is why 15" drivers are usually crossed over at no more than 800Hz. But as long as you remain on axis its no problem and indeed may be good as it reduces indirect reflections which muddy up the sound. So for nearfield listening on axis as in studio monitors which is what they were designed for they are great. But for coverage outside close proximity this freq unbalance would present a difficult problem. The strong alnico motor driving a light VC/cone with cupper clad edge wound aluminum VC on a cardboard former makes for electrostatic like responsiveness with more dynamics but falling short of Horns as you noted. For PA work their limited power tolerance and beamy nature limits their usefulness. But you can actually modify them.....unbolt their 15" basket and change it to a 8-10" basket use the same 3" VC on an 8-10" cone and end up with a more balanced driver x over at 1600 Hzs but needing a sub!!
Best
Rafaro
Follow Ups:
Using an 8-10" would largely cancel-out the reasons I like the driver so much; the very large driver (of course a 15" has nearly 4x the area of an 8" driver) gives superb mid-bass/lower-mid impact and extremely low distortion given that excursion even at high SPLs is tiny (I can play the speakers at 100 dB in my huge room and I have to stare at the cone from a couple feet away to discern any movement at all).
I don't detect ill effects from beaming, though you're right, it must be occurring. I sit 10'-12' from the baffles which are 3' out from the wall and 2' from the side walls, with slight toe-in.
But your response is just the kind of thing I was looking for: you definitely highlighted a point of technical weakness with this design.
(As all of us with good experience have learned, there is no speaker at all without some weakness, no matter the price.)
As originally designed the 604s had a simple exponential tweeter horn which also narrows the off axis freq response with increasing freq so detecting an inbalance would not be obvious. The sound spectrum would seem balanced but wrong when off axis.
Rafaro
If the beaming is an issue depends on the dispersion of the tweeter horn.
I have no experience with Altecs but Tannoy crosses all theirs at the frequency where the woofer narrows to the 90deg tweeter dispersion.
One thing that I've always thought was interesting is that many people mention the beaming of the 15" driver but the fact that the horn doesn't provide loading down to 1600Hz is rarely mentioned. I think the back of the horn could provide some dispersion for the mids from the bass driver as well.
I've used 604E, 8G and 16X versions and all of them sound great in home audio applications. In rooms varying from tiny to moderately large, I've had remarkably good results. If they beam, don't go low enough, don't go high enough or what ever an individual's complaint about them is, they sound really good in a variety of box sizes and designs.
Altec used large diameter drivers to good effect in several of their designs, whether they were optimal or not, they sound good and are still popular. The model 19 is another example of a high crossover point for a large driver (1200Hz) that works well in room.
I'll take actually sounding good over theoretically crap any day.
Had about eight of the 604 at one time including a black one from the 1940s!! They all sound great and are built the way drivers should be built. The idea of mating large cones to comp drivers is a realistic short cut if you want high output in a two way design. In the 40s we only had low powered tube amps nobody knew or needed subwoofers and were happy with a LF 40-80. And yes "The ear is the final arbitrer in all Sound things" but we must realize that "hearing" is just an interpretation of sound by our brains. And our brains are very capable of adapting to our evolutional needs focusing on midrange vocal frequencies and disregarding most of the parameters which we use to measure sound and equipment performance. The beaming of the 604s is actually very useful in reducing secondary reflections in near field studio monitors and who listens to music way off axis....right. For prosound and Coverage of audiences we have better means nowdays. Sorry for the diatribe...Guess I am just a theoretical kind of guy....."Theoreticophilia Nerviosa"??
Rafaro
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