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Ramirez



Joined: 24/10/06
Posts: 465
Loc: Llithfaen, Cymru
Tried a Blumlein pair for the first time... questions.
      #1060312 - 01/08/13 10:52 PM
Evenin'

I've just tried a Blumlein pair for the first, just to see if I'd unerstood it correctly.

Whether or not I did it right, I was very taken by the stereo image I got (just strummed and sang a few bars with a 12-string)

I did it using two sE R1 ribbons - am I right in thinking that both mics' front and rear lobes should form a + or an X, and the source effectively positioned in the side null of both microphones (eg. 'north' of an X, or 'north-east' of a +) (which would explain the almighty gain I needed... right? The R1' are rather low output, but I've never encountered problems on quieter sources. This needed way more gain than usual.)

The sound also seemed to get noticeably duller when summed to mono. I checked polarity on both signals, so that wasn't it. It may well have been the effect of going from a nice (and quite obvious) stereo to mono - it didn't sound particularly phasey, just some loss of top end possibly. The R1 are quite dull mics at best, but the stereo signal was so much nicer the mono didn't even sound like a particularly good compromise.

Am I doing it wrong? I just tried again and I'm starting to think it's just the change to mono rather than anything frequency based. It's hard to tell.

--------------------
Bill Withers while Tom Waits, and Stan Getz


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Ramirez



Joined: 24/10/06
Posts: 465
Loc: Llithfaen, Cymru
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060313 - 01/08/13 10:59 PM
...just realised that the source is not in the null (ie 90 degrees off axis) of the mics, but rather 45 degrees off axis. Is this right? Is it normal that I should see need for a lot of gain?

--------------------
Bill Withers while Tom Waits, and Stan Getz


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 20846
Loc: Worcestershire
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060352 - 02/08/13 09:10 AM
Quote Ramirez:

am I right in thinking that both mics' front and rear lobes should form a + or an X




The standard Blumlein array is two fig-eight mics mounted coincidently -- meaning their ribbon elements are mounted one directly above the other -- such that their pickup axes form an X -- one mic faces 45 degrees left of centre and the other 45 degrees right of centre. This is the classic 'XY' arrangement.

If you arrange them in a + format, with one facing directly forward and the other sideways, you have an Mid-Side or MS array, which is a very useful configuration, but it requires 'matrix decoding' to produce a normal stereo image.

Quote:

and the source effectively positioned in the side null of both microphones (eg. 'north' of an X




That's not the side null! The null is quite narrow and centred around at 90/270 degrees off-axis. In the standard XY arrangement, a central source will be 45 degrees off axis -- where the fig-8 pickup pattern is still pretty sensitive.

Quote:

(which would explain the almighty gain I needed... right? The R1' are rather low output, but I've never encountered problems on quieter sources.




A stereo array is always used at a greater distance from the source than a spot mic, simply because the whole point is to capture a much wider viewpoint of the recording environment. That will mean that more gain is needed.

Quote:

The sound also seemed to get noticeably duller when summed to mono.




This would suggest that your mics are not sufficiently coincident, or they are too close to the source, and that their relative spacing is resulting in comb-filtering of higher frequencies due to different times of arrival. It's also possible that HF reflections from the back wall, picked up by the mics' rear lobes, is contributing to this effect. Try hanging a duvet (or similar) behind the array as a trial.

Quote:

Am I doing it wrong? I just tried again and I'm starting to think it's just the change to mono rather than anything frequency based. It's hard to tell.




A picture of how you've rigged the mics and where they are in relation to the source (and room boundaries) would help us to spot any rigging issues.

H

--------------------
Technical Editor, Sound On Sound


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Ramirez



Joined: 24/10/06
Posts: 465
Loc: Llithfaen, Cymru
Re: Tried a Blumlein pair for the first time... questions. new [Re: Hugh Robjohns]
      #1060357 - 02/08/13 09:35 AM
Many thanks Hugh.

Quote Hugh Robjohns:


The standard Blumlein array is two fig-eight mics mounted coincidently -- meaning their ribbon elements are mounted one directly above the other -- such that their pickup axes form an X -- one mic faces 45 degrees left of centre and the other 45 degrees right of centre. This is the classic 'XY' arrangement.




I had set it up correctly then - I only used the '+' symbol to indicate the symmetry, not orientation!

Quote:

That's not the side null! The null is quite narrow and centred around at 90/270 degrees off-axis. In the standard XY arrangement, a central source will be 45 degrees off axis -- where the fig-8 pickup pattern is still pretty sensitive.




Yes, I realised that after posting the inital post (see second post). The central source was indeed 45 degrees off axis for both mics.

Quote:

A stereo array is always used at a greater distance from the source than a spot mic, simply because the whole point is to capture a much wider viewpoint of the recording environment. That will mean that more gain is needed.




Explains that one nicely.

Quote:

This would suggest that your mics are not sufficiently coincident, or they are too close to the source, and that their relative spacing is resulting in comb-filtering of higher frequencies due to different times of arrival. It's also possible that HF reflections from the back wall, picked up by the mics' rear lobes, is contributing to this effect. Try hanging a duvet (or similar) behind the array as a trial.




Will take a look at this. Would "not sufficently coincident" be a positioning thing, or that the mics are not sufficently well matched? I'm pretty sure I had them as close as possible vertically. I''m going try try them on a drum kit this afternoon and will report back. Will also try a duvet behind.

Quote:


A picture of how you've rigged the mics and where they are in relation to the source (and room boundaries) would help us to spot any rigging issues.

H




Will try to get some up then, as well as a sound clip perhaps. It won't sound nice - it's just a small room - I'm just trying to get it technically right.

Thanks again,
Aled

--------------------
Bill Withers while Tom Waits, and Stan Getz


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 20846
Loc: Worcestershire
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060372 - 02/08/13 11:16 AM
Quote Ramirez:

Would "not sufficently coincident" be a positioning thing, or that the mics are not sufficently well matched?




If you have two identical signals and mix them together, the result is a 6dB increase in volume. You are saying that there is less HF which implies that at high frequencies the HF from one mic is cancelling out that from the other to some extent. That is most likely to happen if the HF is reaching one mic before the other, and that will be because the source is closer to one mic than the other.

Coincident mic placement means the two mic capsules should be occupying the same point in space... and that is obviously impossible... but we can place them directly one above the other so that they are aligned vertically and thus they are in the same place on the horizontal plane.
And since stereo is all about left-right, and not up-down, that's fine!

However, if the mics are displaced vertically relative to the source you're back in the problem of sounds arriving at one capsule before the other, and the difference needs only be centimetres for the HF to start cancelling because the wavelengths are very short at those frequencies.

So, make sure that the source is level with the gap between the two mics, and moving the mics further away minimises the time of arrival differences (because the angular error becomes smaller). It also makes the wanted source image width smaller, though!

Quote:

It won't sound nice - it's just a small room - I'm just trying to get it technically right.




Small room usually means strong early reflections. I suspect HF reflections into the rear of one or both mics is the problem here. The duvet absorber behind the mics might well make a useful improvement.

Good that you're checking the mono sum though -- so many people don't bother and live on unaware of the compromised mic set up.

H

--------------------
Technical Editor, Sound On Sound


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Ramirez



Joined: 24/10/06
Posts: 465
Loc: Llithfaen, Cymru
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060527 - 03/08/13 02:54 PM
Many thanks again Hugh. Here's how the mics were set up:


The camera was roughly placed where the source would be - the manufacturer logo on the mic indicates the front, so I'm hoping my positioning is correct in theory?

Here is how they were placed with regards to the source (drum kit)


Sound clip (MP3) can be found here (the Blumlein pair only with no processing apart from normalising. I haven't included the subkick in this clip):



There are some obvious resonances, but they were easily EQd out. I got a sound I quite liked with some plate reverb, slight HF boost, mild compression and a bit of the subkick mixed in (with a touch of exciter on the subkick)

Aled

--------------------
Bill Withers while Tom Waits, and Stan Getz


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Ramirez



Joined: 24/10/06
Posts: 465
Loc: Llithfaen, Cymru
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060666 - 04/08/13 08:11 PM
Another thing... is it OK that the mics are physically touching? That didn't occur to me until now, what with those big magnets 'n all.

--------------------
Bill Withers while Tom Waits, and Stan Getz


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 20846
Loc: Worcestershire
Re: Tried a Blumlein pair for the first time... questions. new [Re: Ramirez]
      #1060713 - 05/08/13 09:31 AM
It's not usually a problem, although most people keep them slightly apart if they can. There can potentially be issues with a ground loop forming through the two earthed metal cases touching, and there can be an issue with mechanical noise if vibration through the stands causes one mic to rub against the other.

Hugh

--------------------
Technical Editor, Sound On Sound


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