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Author Topic: Better Blumlein Technique?  (Read 1035 times)

David Satz

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Re: Better Blumlein Technique?
« Reply #15 on: April 08, 2019, 06:40:57 pm »

That's true, but when you change that front/back ratio, you are simply setting a different pickup pattern for the two "virtual microphones" whose signals you actually record. It's just like using the remote pattern control on a stereo microphone such as a Neumann SM 69[fet] or any of several past AKG models; their capsules were likewise made from pairs of front- and rear-facing cardioids, and the remote pattern control varied the relative sensitivities of those cardioids.

Since the angle between the main axes of the microphones remains fixed, along with their distance from the sound sources, the amount of reverberance in the recording and the width of the stereo image both depend on that same pattern setting at the same time. It's not a free choice--and in my experience, for any given physical microphone setup, only one setting of the pattern control sounds right at all.

It's the same predicament as in conventional M/S recording--again, a largely equivalent approach achieved by means of a matrix. There's usually just one "sweet(-ish) spot" setting, quickly found by ear; it's certainly not the free, wide-ranging aesthetic choice that might be imagined.

To have any greater freedom you would need Ambisonic ("Soundfield") recording or double M/S. And even with those approaches, if your mikes are too close or too far away or too high up or not high up enough, they're still too close or too far away or too high up or not high up enough.

--best regards
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Jeffrey

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Re: Better Blumlein Technique?
« Reply #16 on: April 08, 2019, 10:01:15 pm »

Altering the gain ratio of the front facing and rear facing mics is not tantamount to relocating the array further forward or backward in the recording space. However altering the gain ratio provides a means to either increase or decrease the relative ratio of reverberance as might be the case when the rear facing mics have more reverberant to direct sound than the front facing mics.

The compromises that ensue by altering the gain ratio is immediately apparent as the gain ratio is changed, not unlike mixing any other audio parameter. It is often possible for me to be very pleased with the trade offs, just as it is with any alteration of a recording in post. The 4 cardioid Blumlein provides a means that may be useful that would not be available with a traditional Blumlein array.
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Jeffrey

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Re: Better Blumlein Technique?
« Reply #17 on: April 09, 2019, 09:27:00 am »

Referring to the link at:
http://www.linkwitzlab.com/index.htm
Linkwitz Lab has an excellent analysis of rendering a recording for stereo playback. The SRA, stereo recording angle is discussed, and it is pointed out an SRA of approximately 70 or 75 degrees will render stereo playback for an idealized equal lateral triangular relationship between the listener and the left and right speakers.

Consistent with this, Dave correctly intimates there is a fixed relationship between the placement of the mic array to the sound source and the mutual angle between the mics to obtain the best sounding result that he readily recognizes. Additionally, I would submit that the polar pattern of the mics and their interaction to shape the sound field, i.e., the polar pattern of the array, which also determines  the acceptance angle of the array, and therefore the distance of the array to the performance to achieve the desired SRA.

Unique to the Blumlein’s among coincident and near coincident arrays is the minimization of monophonic reverberation in the sound field.  Dave also correctly points out that the fixed nature of the relationships he mentioned make the Blumlein difficult to obtain a good result unless the sound source is placed as necessary to conform to the Blumlein array  narrow acceptance angle.

And I agree this is not practical or even desirable, as many recording venues with great sounding acoustic characteristics would be excluded. The acceptance angle could be made wider with the 4 cartioid Blumlein by adjusting the gain ratio of the forward facing to rearward facing mics. If it were adjusted so the rearward facing mics were completely muted, then a forward facing XY coincident array with a wider acceptance angle than the Blumlein remains.

To the extent that the acceptance angle can be widened enough to capture the whole performance and allow for some contribution by the rearward facing mics, the less the reverberant aspect of the environment will be rendered monophonically.
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Jeffrey

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Re: Better Blumlein Technique?
« Reply #18 on: April 09, 2019, 05:02:04 pm »

Please refer to an SRA calculator found at:
http://www.sengpielaudio.com/Visualization-Blumlein-E.htm
Please observe what happens when the calculator is toggled using the radio buttons between bi-directional microphones showing the Blumlein array and the cartioid micrphones which will show an XY array,with both arrays defaulting to a mutual angle of 90 degrees. The resultant SRA will be shown in shadow when you toggle between these 2 microphone polar patterns and arrays. It is easy to see that as if a hybrid of the figure 8 and cartioid mic were available for selection the SRA would increase as the polar pattern shifts from bi-polar to cartioid. This is tantamount to modulating the relative gain of the forward and rearward facing microphones in a 4 cartioid Blumlein array. - Jeff

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