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EQ spheres for Line Audio OM1

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EQ spheres for Line Audio OM1

Postby Ramirez » Mon Feb 19, 2018 11:42 am

Has anyone used these? - https://www.shapeways.com/product/5KY2M ... arketplace

Looks interesting... but also looks more textured than expected - would this have an effect?

Compared to the cost of the mic itself, it's a bit expensive, but I'm still interested.

I gather this is supposed to make them more suitable for distant micing by creating an on-axis boost (or making the mic more directional at higher frequencies? Same net effect?)

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Re: EQ spheres for Line Audio OM1

Postby Hugh Robjohns » Mon Feb 19, 2018 12:02 pm

Ramirez wrote:Has anyone used these?

No. But I've used 'acoustic pressure equalisers' or APEs from DPA, Gefell and other manufacturers from time to time.

Looks interesting... but also looks more textured than expected - would this have an effect?

Nothing significant at audio frequencies.

Compared to the cost of the mic itself, it's a bit expensive, but I'm still interested.

It's a lot of 3D printed substrate material! ;-)

I gather this is supposed to make them more suitable for distant micing by creating an on-axis boost (or making the mic more directional at higher frequencies? Same net effect?)

If it was just an HF boost you could replicate the effect with EQ... but it does rather more than that. (Edited for accuracy and understanding).

The sphere acts to create a pressure build-up in front of the diaphragm for on-axis sound sources, and that results in an HF shelf boost between about 2 and 8kHz for a 40mm sphere. (Bigger spheres exhibit the same effect, but starting from a slightly lower frequency, and vice-versa. The classic Neumann M50 used a 40mm sphere.) The pressure build-up accentuates the detail of sources in front of the mic and makes them sound a little closer and more focused with crisper transients.

But the sphere also increases the front-back attenuation so affects the polar-response in a frequency-selective way, essentially turning the mic more towards a sub-cardioid or even cardioid at the higher frequencies. This also reduces the HF content from off-axis sources (obviously) and that makes the mic more spatially selective, increasing the stereo width and -- significantly -- altering the direct/reverb balance. It is this last aspect which lies at the heart of the claim that it makes the mic more suitable for distant miking -- but it's only a marginal benefit. You might get way with positioning the mic a foot or two further away than without the APE for a given perspective or direct/ambient ratio.

APEs are useful tools to have in the toy box, but not essential by any means.

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Re: EQ spheres for Line Audio OM1

Postby Ramirez » Mon Feb 19, 2018 12:26 pm

Excellent, thanks Hugh. I might order a couple.

Re. the cost - it's more that the OM1 are so cheap rather than these being too expensive!

Although I don't quite get:

Hugh Robjohns wrote:If it was just an on-axis boost you could replicate the effect with EQ... but it does rather more than that.

Surely a typical EQ boost does not differentiate between what was on or off axis?
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Re: EQ spheres for Line Audio OM1

Postby Hugh Robjohns » Mon Feb 19, 2018 3:13 pm

Ramirez wrote:Surely a typical EQ boost does not differentiate between what was on or off axis?

Yes, quite. I should have said, if it was just an HF boost it could be replicated with EQ -- it is the fact that on-axis get's brighter while off-axis gets duller is what gives it the useful characteristics.

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Re: EQ spheres for Line Audio OM1

Postby mjfe2 » Fri Feb 23, 2018 12:58 pm

Fascinating looking product - has anyone tried them yet? I'm wondering if they'll work on other mics e.g. KM183

Neumann's own spheres cost around £90 each...
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Re: EQ spheres for Line Audio OM1

Postby Sam Spoons » Fri Feb 23, 2018 3:27 pm

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Re: EQ spheres for Line Audio OM1

Postby Hugh Robjohns » Fri Feb 23, 2018 3:34 pm

It's not big enough! The Linear Audio OM1 has a 20mm diameter whereas the KM183 has a 22mm diameter. You could try boring the hole out a bit... but I don't know how brittle or fragile the 3D printed material is.

In any case, the KM183 is equalised for the diffuse field, giving it quite a hefty HF lift in the near field. Adding a sphere would emphasise that even more which might not be very helpful unless the mic was quite a long way from the sound source... which isn't normal practice when using an omni mic.

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Re: EQ spheres for Line Audio OM1

Postby Sam Spoons » Fri Feb 23, 2018 4:02 pm

They do a range though, including the one I linked to which fits 22mm ø mics like the KH183....
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Re: EQ spheres for Line Audio OM1

Postby Aural Reject » Fri Feb 23, 2018 4:09 pm

Sam Spoons wrote:They do a range though, including the one I linked to which fits 22mm ø mics like the KH183....

Potentially more use with some of the Neumann modulars that don't have the HF lift of the KM183.
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Re: EQ spheres for Line Audio OM1

Postby Hugh Robjohns » Fri Feb 23, 2018 4:13 pm

Sam Spoons wrote:They do a range though, including the one I linked to which fits 22mm ø mics like the KH183....

Oh... didn't spot that! :blush:
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Re: EQ spheres for Line Audio OM1

Postby mjfe2 » Fri Feb 23, 2018 4:23 pm

Yes I've just discovered there's a whole range!

I realise the KM mics are already diffuse-field equalised but I was hoping to use them in a Decca tree arrangement on an orchestra. I understand the original tree mic was the KM53, then they moved to M50s. Is the M50 capsule equalised in any way before the sphere is applied?

I note that Schoeps recommend their own spheres even on the MK 2XS omni capsule which is why I thought it was at least worth a try on the 183s!
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Re: EQ spheres for Line Audio OM1

Postby Hugh Robjohns » Fri Feb 23, 2018 4:57 pm

mjfe2 wrote:Is the M50 capsule equalised in any way before the sphere is applied?

You can't separate the sphere from the capsule in the M50, and the published plots show a 5dB HF boost which is consistent with the effect of the sphere alone. It is claimed to have a flat response in the diffuse field.

I agree it's certainly worth experimenting with APEs (spheres) in a Decca Tree configuration, but just be aware of the potential for a bright high-end in situations where the acoustic is dry and the Tree is close.

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Re: EQ spheres for Line Audio OM1

Postby forumuser840717 » Fri Feb 23, 2018 10:24 pm

Ramirez wrote:
Looks interesting... but also looks more textured than expected - would this have an effect?

The texturing and imperfectly spherical nature of the surface will probably have an effect but it's unlikely to be particularly audible. In an ideal world, though, it should have a smooth surface.

What's more important than relatively small imperfections in the surface is that the APE fits properly to the mic - no big air gaps and a smooth edge around the hole where the mic pokes through the APE. If the edge of the hole is nibbled/chipped where it's in close proximity to the capsule then you can get audible effects and an uneven/variable modification around the polar pattern. Whether or not that's a problem would depend upon how accurately calibrated is the mic in the first place and how sensitive you are to very small variations in sound. Chances are that without a reference point to compare with it might not be problematically audible anyway.
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