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omnis and pressure sphere attachements
Old 28th November 2015 | Show parent
  #91
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John Willett's Avatar
 
🎧 10 years
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Quote:
Originally Posted by studer58 ➡️
Steering it all somewhat back on-topic, here's a nice paper from Neumann with some great photographs: http://www.neumann.com/download.php?...d=lect0043.PDF

It's a download link to the PDF paper
Interesting - thanks for this.
Old 3 weeks ago | Show parent
  #92
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A query of those who have made their own APE/pressure spheres (30-50mm diam) for omni mics via DIY drilling....which sphere material have you found easiest to drill through and work with, in terms of stability, appropriate hardness, freedom from melting (!) etc ?

A few possibilities: hardwood, delrin, acetal, acrylic, polycarbonate, nylon, phenolic resin etc
Old 3 weeks ago | Show parent
  #93
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🎧 10 years
Quote:
Originally Posted by studer58 ➡️
A query of those who have made their own APE/pressure spheres (30-50mm diam) for omni mics via DIY drilling....which sphere material have you found easiest to drill through and work with, in terms of stability, appropriate hardness, freedom from melting (!) etc ?

A few possibilities: hardwood, delrin, acetal, acrylic, polycarbonate, nylon, phenolic resin etc
Acrylic is VERY difficult to deal with.
I had success with 38mm billiard balls, i think phenolic resin is what they are made of.
Hardwood should be great, too.
Old 3 weeks ago | Show parent
  #94
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Quote:
Originally Posted by bremusound ➡️
Acrylic is VERY difficult to deal with.
I had success with 38mm billiard balls, i think phenolic resin is what they are made of.
Hardwood should be great, too.
Yes, you'd likely have a trade-off between drill speed and heat generation > possible melting and/or splitting, as acrylic is also quite brittle ?

I can imagine billiard balls might give the drilling bit some challenge, while hardwood is probably a good compromise material ?
Old 3 weeks ago | Show parent
  #95
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🎧 5 years
Tap Plastics sells clear acrylic spheres (just like in M50); I've had them drill the proper holes in them for me at reasonable price.

I've also made them myself from hardwood drawer/cabinet pulls from the craft store, using good Forstner bits. Some stores sell full wood spheres, but the drawer/cabinet pulls are easier for DIY because they already have a flatted area with a hole in the center, to guide the Forstner.

The only rub is that this being the US, these things are not made in Metric dimensions, so usually only available in 1.5" or 2". I've found the 1.5" (38mm) quite suitable; the shelf begins at only a bit higher frequency than the M50's 40mm - the 2" is very close to 50mm, which the Decca book claims sounds closer to the M50 (because it approximates the effects of the M50's head grille [?]), but I find that it's lower shelf produces a pushy upper mid-range.

I see no reason for them to be of anything more exotic than acrylic or hardwood. Josephson used to make hardwood ones, but no longer.
Old 3 weeks ago | Show parent
  #96
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🎧 10 years
Quote:
Originally Posted by studer58 ➡️
Yes, you'd likely have a trade-off between drill speed and heat generation > possible melting and/or splitting, as acrylic is also quite brittle ?

I can imagine billiard balls might give the drilling bit some challenge, while hardwood is probably a good compromise material ?
Never tried wood as i bought pre-drilled wooden spheres from an ebay dealer. I was surprised how easy it was to work on the billiard balls.
Old 3 weeks ago | Show parent
  #97
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Billiard balls (at 57mm) would produce a shelf at a much lower frequency than M50 (40mm); too close to the upper midrange for comfort.

Also, as you can see in the DPA graphics, use of a 40mm sphere requires a capsule with a fair amount of top lift (as the one in the M50 did), because the pressure boost falls back down to zero well below 20k. Not so with the 50mm - the boost extends all the way to the top, so one would want a flat capsule with that.
Old 3 weeks ago | Show parent
  #98
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🎧 10 years
Quote:
Originally Posted by M50k ➡️
Billiard balls (at 57mm) would produce a shelf at a much lower frequency than M50 (40mm); too close to the upper midrange for comfort.

Also, as you can see in the DPA graphics, use of a 40mm sphere requires a capsule with a fair amount of top lift (as the one in the M50 did), because the pressure boost falls back down to zero well below 20k. Not so with the 50mm - the boost extends all the way to the top, so one would want a flat capsule with that.
There are 38mm billiard balls available, that‘s what i used.
Had also success with (wooden) 50mm spheres on the 4006 (silver grid).
Old 3 weeks ago | Show parent
  #99
Gear Nut
 
Quote:
Originally Posted by M50k ➡️
Tap Plastics sells clear acrylic spheres (just like in M50); I've had them drill the proper holes in them for me at reasonable price.

I've also made them myself from hardwood drawer/cabinet pulls from the craft store, using good Forstner bits. Some stores sell full wood spheres, but the drawer/cabinet pulls are easier for DIY because they already have a flatted area with a hole in the center, to guide the Forstner.

The only rub is that this being the US, these things are not made in Metric dimensions, so usually only available in 1.5" or 2". I've found the 1.5" (38mm) quite suitable; the shelf begins at only a bit higher frequency than the M50's 40mm - the 2" is very close to 50mm, which the Decca book claims sounds closer to the M50 (because it approximates the effects of the M50's head grille [?]), but I find that it's lower shelf produces a pushy upper mid-range.

I see no reason for them to be of anything more exotic than acrylic or hardwood. Josephson used to make hardwood ones, but no longer.
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
Old 3 weeks ago | Show parent
  #100
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Non-standard pool/snooker/billiard balls seem like a good starting point eg:

https://www.amazon.com.au/Economy-Sp.../dp/B018XXTZC4
* https://www.amazon.com.au/Kids-Small.../dp/B01CGIM98E

* "We sell these balls in 38mm, 41mm, 44mm, 48mm sizes - (prices are different for other sizes, please buy from relevant listing if you require a different size)"

I could see them looking good with a final spray coat of Schoeps matte grey (or black or chrome/silver...to match the mic) ?

The intention regarding tighness is that they'd be an interference fit around the neck/collar of the mic (within drilling tolerances)...maybe some thin surgical tape layer-wrapped around the mic body could help with such a non-slip fit ?
Old 3 weeks ago | Show parent
  #101
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Quote:
Originally Posted by JGebauer ➡️
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
The spheres from Schoeps are attached via (that's Latin! :-) Plastazote foam.
Old 3 weeks ago | Show parent
  #102
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🎧 5 years
Quote:
Originally Posted by JGebauer ➡️
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
The bare wood balls, I gave a coat of sanding sealer, then painted them with artist's grey acrylic gesso (just because i had some on hand).
Old 3 weeks ago | Show parent
  #103
Before you take the effort to make your own acoustical pressure equalizers, take note of the following graph. Without proper testing methods you may well end up with something you do not want. Only a few millimeters misaligning the sphere can cause huge faults in the frequency domain.
Attached Thumbnails
omnis and pressure sphere attachements-mk221-40mm-sphere.jpg  
Old 3 weeks ago
  #104
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I home-built my own heart-lung machine . . .

D.
Old 3 weeks ago | Show parent
  #105
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Quote:
Originally Posted by Orgeltonmeister ➡️
Before you take the effort to make your own acoustical pressure equalizers, take note of the following graph. Without proper testing methods you may well end up with something you do not want. Only a few millimeters misaligning the sphere can cause huge faults in the frequency domain.
One only need look at CU pix of M50 to see how the actual diaphragm lines up with the sphere.

The original capsule, the diaphragm is right up to the surface of the sphere; in the two later versions it's recessed a mm or so.

Looking at the scale, and frequencies involved, I wouldn't call what's shown in that graphic 'huge'.

Meant to post this from Josephson earlier.
Attached Thumbnails
omnis and pressure sphere attachements-m50-cap_mnt.jpg   omnis and pressure sphere attachements-josephson-making-apes.jpg   omnis and pressure sphere attachements-m150-capsule.jpg  
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