Centre-Terminated Capsules

[Daniel Davis]

Just a quick question that I hope someone knows a little about.

Whilst some LDCs are edge-terminated, quite a lot of LDC have their capsules centre-terminated. You can compare both styles on the following page:

Centre or Edge Terminated

Now I understand that having a very thin diaphragm (some are only 6 microns) is a good thing for sensitivity, because the diaphragm has less mass and is less rigid - double win. However this surely must be wrecked by the heavy mounting and the wire attached - which must have thousands of times as much mass as the diaphragm. Or am I missing something?

If anyone has any technical knowledge and can explain, I'd be grateful

[SimonZ]

A centre terminated capsule has a plastic part at the centre of the backplate for the termination screw to fit into, this keeps the terminal screw insulated from the rest of the backplate.
There is then a spacer between the backplate and the diaphragm at the centre point where the screw goes through.
This spacer is the same thickness as the spacer around the edge of the capsule that keeps the diaphragm apart from the backplate.
(Not sure I'm explaining well )

Anyhow, at the centre point the diaphragm is not moving, just like with the harmonic at the twelfth fret on a guitar string.

So for a centre terminated diaphragm to have the same resonant frequency as an edge terminated one (where the centre does move) you have to stretch it to only half the tension.

For your visualization pleasure ........
http://wwwnew.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-standing-membrane.htm

So an edge terminated capsule can do all the modes seen there and a centre terminated can only do the ones where the centre is stationary.

[John Willett]

Just a quick question that I hope someone knows a little about.

Whilst some LDCs are edge-terminated, quite a lot of LDC have their capsules centre-terminated. You can compare both styles on the following page:

Centre or Edge Terminated

Now I understand that having a very thin diaphragm (some are only 6 microns) is a good thing for sensitivity, because the diaphragm has less mass and is less rigid - double win. However this surely must be wrecked by the heavy mounting and the wire attached - which must have thousands of times as much mass as the diaphragm. Or am I missing something?

If anyone has any technical knowledge and can explain, I'd be grateful

The centre termination does not add to the diaphragm mass at all.

The diaphragm acts as a ring (or do'nut) as it's held in the centre as well as at the edges.

[The Korff]

Interesting. Does that push the fundamental resonant frequency of a centre-terminated diaphragm up one octave compared with an otherwise identical edge-terminated diaphragm?

[SimonZ]

Interesting. Does that push the fundamental resonant frequency of a centre-terminated diaphragm up one octave compared with an otherwise identical edge-terminated diaphragm?

Yes it does, but it is compensated for by halving the tension to bring it down back again.