inspecting used metal compression diaphragms what to look for?

[moray james]

Have a chance to pick up some used metal diaphragms and I am wondering what things to look for? Are shch things as diaphragm slapping a phase plug always obvious? What about possible issues with voice coils? If things look clean and like new am I good to go? Any advice will be much appreciated. Best regards Moray James.

[djk]

They can be shot and the only way to tell is to test them in a driver.

Metal work-hardens and cannot be identified as such by any easy inspection.

A work-hardened diaphragm loses a lot of both high end and low end.

[PrestonTom]

They can be shot and the only way to tell is to test them in a driver.

Metal work-hardens and cannot be identified as such by any easy inspection.

A work-hardened diaphragm loses a lot of both high end and low end.

I am curious about the JBL spec on their diaphragms for 2445 and 2446 drivers for instance. They will give a spec or tolerance regarding the DCR of the driver (different number for the 8 and 16 Ohm versions, of course). What does a high or low number indicate?

[moray james]

Thank you that is very useful information I had no idea about the low end loss, very much appreciated. Best regards Moray James.

[djk]

"What does a high or low number indicate?"

The old method of winding the coils was to make a single big long stick voice coil and slice it into shorter pieces for the individual coils. The length could thus vary by ±1 turn. The current method is to wind individual coils, they have exactly the same number of turns. The old method made it worthwhile to match pairs for DCR, probably not need for newer ones.

When I get a set of drivers from a big install re-do I usually sort them by resistance, then swap out back covers to get serial number pair matches. I clean all gaps, re-install diaphragms, and then run the closest pairs out-of-phase with pink noise to make sure they match (the less sound the closer the match).

I have a whole bunch of large Altec drivers I am refurbishing at the moment. If you are looking for something specific, just ask. 288's, 291's, 299's, both Alnico and ceramic available.

Anyone wanting to buy or sell a single 805B or 1005B please contact me, I have one of each.

[John Warren]

They can be shot and the only way to tell is to test them in a driver.

Metal work-hardens and cannot be identified as such by any easy inspection.

A work-hardened diaphragm loses a lot of both high end and low end.

I'd propose a more specific phenomena...high cyclic fatigue.

In my little world, work hardening is a "macro" phenomena involving the entire volume element of material strained at stress magnitudes that exceed some yield criteria, a Tresca or von Mises distortion energy criteria as examples. Point is this is a bulk, macro plasticity phenomena. The strains involved are large, so-called permanent "set".

High cycle fatigue considers deformation (i.e. strain accommodation) at the micro level or, more specifically, at the level of grain-boundaries, occasional coarse grains, melt related inclusions, hard phases, etc that reside in the alloy comprising the diaphragm. The total macro-strains are elastic but, at the micro-level, intense and plastic. Enough to tear boundaries apart. These intense, locally "work hardened" volumes, to get back to djk nomenclature. exist at nodes on the surface of the diaphragm under the loads associated with operation.

Fatigue eventually leads to cracking but distress in the diaphragm is measured before crack nucleation as reduction of useful bandwidth. The fatigue process redistributes and relaxes residual stresses within the diaphragm developed at new make. This can lead to shape change. If large enough, will move the coil from the desired location in the gap.

I've attached a open source paper on this topic, published by a supplier with a bias to demonstrate superior performance of Be-diaphragms.