Aluminum/Titanium Diaphragms

[Steve Ty]

I've been wondering how the vintage aluminum diaphragms used in the old Altec drivers compare with the titanium diaphragms used in new drivers.

Or stated another way: How would the old 288's or 902's sound when compared to new JBL 2426 or 2446?

Has anyone stacked these up next to each other and compared them?

[Jim D]

There is a critical sound difference between the titanium and aluminum diaphragms. If we presume the motor and wave passage structure of two units is identical and we install an aluminum diaphragm in one and a titanium unit in another you will heare the following difference. In a word, the titanium unit will be zingyier. It will exhibit a silibance on the upper end the alumiun will not. Some people equate this to an improved HF. It is actually distortion. The alumiunum diaphragm is a softer metal and so has internal damping forces that do not allow (as many) spurious resonances to build in the diaphragm. Titaniun is quite hard, denser, tougher, and has almost double the weight for a given volume. This harder material allows distortion carrying resonances to sustain them selves in the diaphragm. Because of the higher weight manufacturers reduce the thicknes of titanium to get the weight closer to aluminum which in turn makes the diaphragm even more resonance prone. When Dr. Rex Sinclair was with Radian Audio he wrote a paper which described this phenomenia in depth. It will describe this tendency much better than this two bit tour. Other driver criteria also affects the performance, such as dome suspension. The tangential compliance of the ALTEC 802-806-902-909-288-299-old JBL 2210-2220-LE85-LE-175 does an excellent of decoupling the dome from the suspension. Titanium because it is harder does not do such a good job and you can get more resonance problems from this. The composite/symbiotic suspension as used in 807-808-908-291-Radians works fine but is heavier due to the weight of the Kapton/Mylar and the glue joint. Titanium works fine if you are not listening critically. Almost all musicians speakers made today use these diaphragms because they are tough. Another problem with titanium diaphragns is subharmonic distortion where the designers try to move the diaphragm closer to the phase plug to get more HF output. The titanium is tough enough that hitting the phase plug, something that will immediately destroy an allumiunm unit, is not as much concern and close order standing waves develop between the two surfaces. This often manifests itself as considerable ouput at 1/2 fundimental. The large format JBLs have a few dB less output at 5K (distortion?) than the original 10K fundimental! This is something a 288 will never do. If you think I'm joking, take a sine wave generator and hook it up to your big titanium JBL and feed it with a slow sweep from 7-11K. At 10K fundimental, the 5k is very noticable. Most of the low power aluminum diaphragms are made of good old pre WW2 aircraft soft duralumiun. The 909 and 299 diaphragms are made of a tougher aluminum alloy and is heat treated. In this authors opinion, this is the best compromise for higher power. It handles 2.5 times the power of the duralumiun units and retains most of the good qualities.
If you want good listening, stay with aluminum. Jim

[Todd W. White]

I was in OKC when Jerry Hubbard was developing the 299 for Altec Lansing - he tried all types of new materials, finally settling on the Pascalite alloy, which is a version of duraluminum.

When I asked him about Titanium (then the "newest and best" materials being used by JBL and others, he replied, "Titanium exhibits several undesirable acoustic anomalies that we don't wish our compression drivers to have."

Right on, Jim.

Todd W. White

[Steve Ty]

Thanks Jim -- Dynamite explanation -- as usual.

A couple questions:

I always assumed that the pascalite diaphragm used in the 909 was some sort of composition or plastic material -- guess that's wrong. Is pascalite the "tougher aluminum alloy" that you said is the best compromise?

And, if you stick a pascalite diaphragm in a 902 or a 908 -- do they basically become 909's, with the same power handling capacity and good sound?

Thanks, Steve

[Todd W. White]

The Altec Lansing 299/909 HF Compression Drivers utilize a diaphragm material that has the power handling capability of the Symbiotik diaphragms, and nearly the frequency response of the aluminium ones used in the 288/802/902 HF Compression Drivers.

Nearly.

There is a slight reduction of HF output in the extreme upper register. Though not terrible, it DOES exist, and in critical listening situations, the aluminium diaphragms ARE better!

It is my recollection that Pascalite IS a special alloy, not just heat treated duraluminium. I'll have to check with my friend Jerry Hubbard to check for sure (Jim - if you talk to him before I do, ask him, will you? Altec Lansing's (unofficial) Homepage