Turning a Sennheiser MKE10 into a PZM

[TromNek]

Thanks all
Yes, I can derive the measurements from both those figures.

I searched for the research paper you referenced but couldn't find anything.
Can you share a link to it please.
I'd like to understand more of what their analyses showed.

 

[k brown]

Here's the Neumann AES paper.

At one point I actually made my own SASS-B type mic using the spiral boundaries. Unfortunately it looked like a couple of huge pinnae.

 

[TromNek]

Thanks so much for the paper. It has some very pertinent information.
From reading it, I see now that a larger 1,2m x 1,2m plate is not good.

I think I'll start with the GFM triangle made out of MDF or Hardboard .
Later try a spiral version.

I would have liked to see some more discussion of how they 'calculated' the dimension needed for the triangle and spiral.
They discuss the minimum size needed for half wave length of the lowest frequency desired. But that is for regular shaped object.

Perhaps for an irregular shaped plate (GFM Triangle or Spiral), you would use the maximum distance across the plate passing through the center of the microphone to calculate the lowest frequency.

 

[k brown]

This'll give you all the deets on boundary sizes regarding shelf frequencies, and much more.

 

[TromNek]

Very nice. Thanks so much.

 

[ubxf]

How can you get the actual dimensions from <GFM was 213mm x 168mm x 13mm> and also the placement of the microphone
I was originally thinking of using the BLM6000 with my DPA4060
https://www.dpamicrophones.com/accessories/table-and-floorstands/blm6000b?variant=163
but then I thought of also trying the GFM plate

 

[k brown]

How can you get the actual dimensions from <GFM was 213mm x 168mm x 13mm> and also the placement of the microphone
I was originally thinking of using the BLM6000 with my DPA4060
https://www.dpamicrophones.com/accessories/table-and-floorstands/blm6000b?variant=163
but then I thought of also trying the GFM plate

Not quite sure what you're asking, but perhaps the Neumann GFM research paper will help.

 

[ubxf]

Not quite sure what you're asking, but perhaps the Neumann GFM research paper will help.

Thank you for your response. I read the paper, but I can't figure out the dimensions. I'm guessing 13 mm is the thickness, 213 mm would be the base but what is 168mm ? Do we need to know one of the angles ? What is the position of the microphone ?

 

[k brown]

The dimensions I gave are from the user manual, attached below.

(If you can make the effort, doing the spiral is the ideal shape. The whole idea is that every point along the edge is a different distance from the capsule, so edge diffraction effects don't 'add up'. The triangle is a compromise to manufacturing practicality; because of the straight sides, there will be two of each dimension from the capsule to that edge; not true of the spiral.

I made spirals from 1/4" hardboard using a coping saw, and filing/sanding the bevel on the edge; then gave them a paste wax finish. Not perfect, but better than the triangle.)

Safe to assume that 213mm is the longest side; Logical that 168mm is the distance from the long side to the 'point' opposite it; you can pretty accurately infer the other dimensions/angles from this illustration in the AES paper (note that none of the angles is 90 degrees):

 

[ubxf]

Thank you so much, I think I will attempt the spiral.

 

[k brown]

Thank you so much, I think I will attempt the spiral.

I should add that optimizing the plate's shape is only of real concern in two instances:

1. The unit utilizes a capsule of significant thickness and is placed within the plate, as with the Neumann, therefore requiring a plate that's a good 1/2" thick.

2. The unit will be used not on a floor or wall, but in free space, as on a stand or in a stereo configuration like the Crown SASS.

If placing a capsule on top of the plate, either as a PZM like the original Crown units, or with the capsule lying on the plate with the diaphragm at 90 degrees to it, the plate can be quite thin (as it was with the Crown PZMs). When this is the case, and it is placed on a large surface like a floor or wall, edge diffractions are minimal and of little consequence.

But if such a thin-plate unit is used in free space, then the shape (and the capsule's position on it) becomes very important, as diffreactions are now quite strong.

All of which is a long-winded way of saying, if you're going to be placing the little DPA on it's side on a plate, the plate can be very thin, and if only used on a floor or wall, the plate's shape is unimportant.

 

[ubxf]

Thank you for the heads up, I'm planning on using them like Crown SASS

 

[k brown]

Thank you for the heads up, I'm planning on using them like Crown SASS

I've found a properly-angled and spaced pair to be easier to build than something like the SASS, and has certain advantages, as well:

Combines features of SASS, PZM wedge and GFM 132.

- Like the SASS, it is a pair of head-spaced small boundary mics that transitions from directional to
non-directional at about the same frequency as a human head.

- Like the PZM wedge, the boundaries are aimed 110 to 120 degrees, with no barrier between, less ‘dead zone’ from behind, very close to human hearing (the SASS is much more forward-biased). Also doesn’t have the ‘dead center’ issue of the SASS with sources closer than 3’. This makes it slightly less mono-compatible than SASS, but has same compatibility as dummy head.

- The boundaries are the size and shape of the GFM 132, thus producing much smoother frequency
response than the SASS or ‘traditional’ PZM wedge using rectangular boundaries. Unlike GFM,
the outdoor model has PZM mics rather than BLM mics, so is much more wind resistant.

This is what I built, using the mic units from the Radio Shack PZM (but without it's 'electronics'); this one optimized for use outdoors for nature recording, covered with a wind sock:

 

[ubxf]

That's great thank you for sharing, my intended use is for outdoors nature recording also. I have a pair of Radio shack PZM so I might give that a try as well. I'm thinking of using them as L/R surround so I might want to keep the dead zone from behind. Your rig looks great, nice use of metal braces to accommodate the wind sock.