Help with understanding some body physics for a ribbon build

[midwayfair]

A second ribbon is on my list for next year. Last year's was an Austin build with a Cinemag and a 0.8um ribbon that I'm quite pleased with -- output is good (a little higher than a Sennheiser e609), hiss is workable, and the treble is actually better on it than some other ribbons I've liked and used in the past.

Since I don't like redundancy, I'd like to make the next one a little different.

I figured I'd start with learning how the body affects the sound. The Austin is a simple wire mesh -- I removed one layer I think (I tend to do this with most mics because I always use a pop screen if a lot of air is being moved anyway). The MXL 990 and 144 are obvious candidates for this. (The 990 in particular because they're dirt cheap.)

There is also the style of body like the Royer -- Arthur Fisher's RM-5 comes in that, or I could pick up a donor body from a company called Badaax, which is on Amazon and a really good price for including a shockmount and box (their MC410 looks like it might work instead of an MXL990 body, too).

Finally, there's the lollypop shape, like the Fathead and its clones -- ISK, Nady, GAP, etc. I really have no idea why these tend to sound darker than other body shapes, or maybe it's something to do with what's put inside them.

There's the really obvious consideration of the physical space for the motor. The 990 and 144 bodies only have about 60mm of grill (IIRC), of which less than 50mm is usable in the 144. A longer ribbon sounds different. So if I wanted a longer ribbon than what's in the Austin (which is 50mm I think), I would need to use the Royer-shaped mic body.

What I'm trying to figure out is how big of a useful effect those slats have on the sound, and what can be done about them. Just some brainstorming:

1) They ought to cut down on treble, but is the amount of treble lost comparable to a finer mesh?
2) If I simply cut off the slats on the Royer-shaped one, is there anything about the body shape that would change the sound appreciably? I wouldn't think so.
3) Would there be a compelling reason to affect the top end with the body shape instead of simply using a thicker ribbon or different transformer?

I'm not sure that I can construct my own mic body. I have extremely limited metalworking tools at my disposal.

Thanks for the help.

 

[leswatts]

I see no reply, so i'll answer if you're still around.

Grille/slats/etc equdistant front and back will create a pressure gradient antinode, which will create a high frequency PEAK.
This is often used on purpose to adjust the response of the microphone.


Les

 

[midwayfair]

I see no reply, so i'll answer if you're still around.

Grille/slats/etc equdistant front and back will create a pressure gradient antinode, which will create a high frequency PEAK.
This is often used on purpose to adjust the response of the microphone.


Les

Interesting. Thanks for the response.

This is the opposite of what I would have assumed, which is that excess grill would block treble frequencies. Though I've also seen graphs where pop filters reduced bass, so ...

Do all of the slats have to be symmetrical to get this effect? Like this:
a  a2
b  b2
c  c2
d d2

If I remove b2, b still has a node with a2 and c2, it's just longer, or it no longer has a node? Or and c also no longer have a node because they ought to have been equidistant from b as well?

I also found this, but the differences it talks about mostly relate to the ribbon length:
http://recordinghacks.com/2008/11/01/chinese-ribbon-microphone-designs/

 

[leswatts]

We call these on purpose frequency modifying grilles "wave plates". Think of them as semi transparent
walls. They set up standing waves  just live a room. The first one is a pressure null in the center of the volume in between them.
But just like a room a pressure null is a pressure gradient peak. And the ribbon responds to pressure gradient.

As an example our Polyribbon variable pattern ribbon has two plates  of 32% open perforated metal spaced
1cm in front of and behind the pressure gradient ribbon. This creates a +4 dB response at 12.5 kHz that compensates partially
for the null's effect, giving us dead flat response to 16kHz.

The same thing is done on the Coles 4038, the Shure 300, and others. In the shure the wave plate is also the outer grille.

This was patented about 60 years ago by Harry Olson at RCA.

If Grilles have a very great deal of acoustic resistance they can cause a reduction of bass response as well.

Les
L M Watts Technology