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 11 
 on: April 09, 2019, 09:27:00 am 
Started by klaus - Last post by Jeffrey
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.

 12 
 on: April 08, 2019, 10:01:15 pm 
Started by klaus - Last post by Jeffrey
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.

 13 
 on: April 08, 2019, 06:40:57 pm 
Started by klaus - Last post by David Satz
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

 14 
 on: April 08, 2019, 03:55:41 pm 
Started by klaus - Last post by Jeffrey
The narrow stereophonic pickup angle of the traditional Blumlein can be altered with the 4 cartioid Blumlein by increasing the gain delta between the front and rear facing microphones, which would otherwise not be alterable with the traditional Blumlein. The sound field becomes more omni-like as the delta increases. Similar otherwise unalterable attributes are adjustable, e.g., one could compensate for the array placement located too close or too far from the reverberation radius, etc. - Jeff

 15 
 on: April 08, 2019, 02:06:18 pm 
Started by klaus - Last post by David Satz
I was trying to figure out how that photo could be a Blumlein pair! Thanks for explaining that it isn't.

While I was struggling to interpret the image, though, it occurred to me that if the two outer cardioids were 90į apart, and if the microphone in the middle were omnidirectional, with frequency response and sensitivity like that of the cardioids, one could matrix the three signals into a Blumlein-ish result: Take the signal from either cardioid, invert the signal from the omni, pad it down ca. 6 dB and sum the two together; do the same for the other cardioid as well, and there you have it. (The narrowing of the omni's pickup pattern in the top two octaves would unfortunately make that an inferior solution.)

All that being said, despite near-ideal behavior in certain respects, the Blumlein approach is highly problematic in practice. It has a rather narrow stereophonic pickup angle, which often requires placing the microphones at considerable distance from the sound sources in order to encompass them in the stereo image. But at the same time, being based on figure-8 microphones, it is as sensitive to rear-incident sound as front-incident sound. At the miking distances that it often requires, you pick up far too much reverberation relative to direct sound; the recording sounds "washed out". I haven't often found Blumlein usable in live concert recording, though for session recording you can sometimes arrange your performers to fit the mike technique, if that's how you want to work.

--best regards

 16 
 on: April 08, 2019, 01:24:25 pm 
Started by klaus - Last post by Jeffrey
Here is the more cogent explanation than the malformed sentence I mistakenly posted previously. I took a photo of an XY array that Klaus was kind enough to attach for me due to my shortcomings. I often use this XY array with a mutual angle if 110 degrees and an additional mic in the center of the sound field Ito fill in the hole that results from the wide mutual angle. The advantage is having this wide sound field captured with an additional on axis mic. Similarly, this is considered by many as an advantage of MS.

I posted it here to visualize 3 of the 4 mics in the 4 cartioid  Blumlein. One can see from the photo that two mics immediately one above the other will provide a much better mono compatibility than if the left and right mics were facing either forward or rearward. The diameter of the KM84 is 21mm, and the diameter of the diaphragm I’m estimating is approximately 19mm (not a published spec that I’m aware of).

Assuming this is a close estimate, a frequency of 17,895Hz has a wavelength of 19mm, equal to the diameter of the KM84 diaphragm. This represents the minimum wavelength that could be accurately captured. Frequencies with wavelengths above 1/2 this wavelength and less than one wavelength are captured less accurately than sound with equal to, or greater wavelengths due to diffraction at the diaphragm.

Accordingly, two mics that include the center mic and either the left mic or the right mic to capture the forward or rearward sound field have excellent mono compatibility in the upper octave of human hearing, as compared to captuing the forward or rearward sound field with the left and right mics separated by the middle mic. Sound arriving at both the left and right mics are not necessarily coincident. This is the reason I prefer alternative 1 that I described earlier for stacking the 4 cartioid Blumlein.

 17 
 on: April 08, 2019, 12:33:27 pm 
Started by klaus - Last post by klaus
Here is a photo of Jeff's XY arrangement.

 18 
 on: April 07, 2019, 11:05:56 am 
Started by klaus - Last post by Jeffrey
An analysis of the Blumleinís spaciousness is well done at this link:
http://taperssection.com/index.php?topic=110861.0

 19 
 on: April 07, 2019, 08:35:56 am 
Started by klaus - Last post by Jeffrey
I just noticed that the last sentence (...) is gibberish.

Did you fix it? Looks coherent to me (aside of mixing up 'effects' with 'affects'...).

 20 
 on: April 06, 2019, 07:56:16 pm 
Started by klaus - Last post by Jeffrey
My name is Jeff. I am the person messing around with Mr. Blumleinís array. Klaus was kind enough to post my Blumlein ideas in this forum. I didnít think anyone would be interested. I can say that it works for me, and Iím pleased to try to answer any questions if Iím able to.

To hopefully address Daveís concern, here are the two alternatives Iíve investigated for vertically stacking the 4 cartioid Blumlein array. It is easier to visualize by drawing a schematic:

1st Arrangement:
The two forward facing mics  are immediately one above one other. And the two rearward facing mics are immediately above one another. This arrangement necessitates the  mics participating in each of the two figure 8 components to be separated from one another by one of the microphones, a little less than an inch in the case of KM84 mics. i.e., this arrangement is optimized for coincident phase at the expense of a less symmetrical Blumlein sound field. I would like to put in perspective  Daveís concern vis-a-vis the Blumlein sound field created from four cartioid mics may be compromised, I would point out that the Blumlein composed of the 4 cartioid mics  Iíve described would be no less symmetrical than a traditional Blumlein array sound field composed from ribbon mics or multi pattern condenser mics that place the ribbons or diaphragms of the two mics a minimum of approximately 2 inches apart. This is often  necessitated by the minimum distance between the  ribbon or diaphragm to the cage of each microphone. This distance multiplied by 2 is the distance between the transducers of both mics in the traditional Blumlein array. Although the ribbon mics do not suffer from the asymmetry of two diagrams to create the figure 8 polar pattern, ribbon mics and side address multi pattern mics often necessitate greater separation between their transducers as compared to small diaphragm condenser cardioid mics of approximately  1 inch diameter.  The small effect it has on the sound field asymmetry and  phase relation consequences are regarded as negligible in these traditional Blumlein arrays. So to is the case with a Blumlein array composed from 4 cartioid mics with similar dimensions as the KM84 mic.. - jeff

2nd Arrangement:
The two mics participating in each figure 8 component are immediately one above the other. This arrangement is optimized for a more symmetrical Blumlein sound field at the expense of a less coincident relationship between the forward facing mics and the coincident relationship of the rearward facing mics respectively.
For what itís worth, I usually prefer the 1st alternative because the degree of coincidence in the 2nd alternative is audible but small in the top octave or so. A separation of 21 mm diameter mics by about a wavelength of 1 inch will effect frequencies above approximately 13KHz, although diffraction of frequencies above this are to some degree mitigated by diffraction at the diaphragm depending on the diameter of the diaphragm.

Regarding shadowing, although I have never experienced any audible shadowing effect using the KM84s, if anyone does hear shadowing with their mics, one could always use detachable miniature cardioid capsules or try a mic with a smaller foot print.

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