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Offline connloyalist

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Shotgun mic test in a poor room
« on: September 18, 2015, 05:54:58 AM »
I had posted this topic on GS and Voltronic suggested I also post it here.

Something I had been wondering is whether a shotgun mic could be used in a large room. I had read that shotgun mics are pretty much for outdoor use only because the reflections you get indoors will cause nasty effects with the interference tube. However I seemed to remember reading somewhere about Tony Faulkner at some point having used shotgun mics indoors. So I decided to try this out.

Because connected to the above is the fact that most of my recording is done in a room with less than ideal acoustics. In addition it has a loud ventilation system that actually does have an "off" switch except it is currently broken (and always on). Also the compressor to the skating rink in the same building is behind one of the walls. So my thinking was that IF the room is big enough to avoid the effects from the interference tube, it might just work in cutting down the acoustics a bit.

Recently on eBay I found someone selling several Sennheiser MKH805's. This is a mid to late 1960's long (21") shotgun mic. T-power. I bought two (thinking that if I liked it, I probably wouldn't be able to find a second one), and bought a T-power adapter and a Rycote INV-BH shockmount (which turn out to be somewhat difficult to find around here).

The other day I went to the afore mentioned room (the place where my community band rehearses; I have a key to the place) in the afternoon so I would have plenty of time by myself to do some test recordings.

When I record a concert I usually have to be in the far corner of the room. The conductor doesn't really like a large stand behind him, and moreover there tends to be a lot of foot traffic there. So I need to be behind the audience, about 9 meters away. Another reason why a shotgun might be useful.

The test setup was a pair of Advanced Audio CM1084's with super cardioid capsules, 60cm spacing and angled 40 degrees. The Sennheiser MKH805 in the middle, straight ahead and for reference a CM1084 with an omni capsule.

Attached pics of the room and the test setup. The gray chair is where I was standing for the recordings.

Due to size limitations of attachments of 500KB on TS I had to cut the recordings down quite a bit. Here is the omni, the left hand supercardioid (the right hand one sounds pretty much identical to me, so I don't think it makes much difference) and the MKH805. I adjusted these to the same LUFS with no other processing.

As for the noise of the ventilation system, the omni is worst. The MKH805 is in the middle and the supercardioid is best. However, with respect to room acoustics, I personally like the MKH805.

As a result of the discussion on GS and an excellent suggestion by Voltronic I cleaned the files up with Izotope RX4. Results to follow in the next post.

Regards, Christine


Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #1 on: September 18, 2015, 06:02:50 AM »
As I mentioned in the previous post, I cleaned the files up in RX4 using Voltronic's method (see: https://www.gearslutz.com/board/11338967-post9.html). The makes a huge difference in the noise from the ventilation system and makes it easier to hear the acoustics of the place. The MKH805 has more audible noise left, but cutting it down further makes it sound strange.

To my ears and on my speakers, I can hear more room with the supercardioids. The MKH805 sounds drier to me, which was one of the objectives of the test.

I have decided to buy the components I need to activate that second MKH805 (a second T-power adapter and Rycote INV-BH) and then redo the test in stereo.

Regards, Christine

Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #2 on: September 18, 2015, 11:23:26 AM »
I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency.  That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong :)

Regards, Christine

Offline down2earthlandscaper

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Re: Shotgun mic test in a poor room
« Reply #3 on: September 19, 2015, 02:09:46 AM »
I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency.  That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong :)

Regards, Christine

I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16
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Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #4 on: September 19, 2015, 03:03:12 AM »
I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16

How did you have you Nak300/CP4's setup for that one? Sounds good to me :)

Regards, Christine

Offline boltman

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Re: Shotgun mic test in a poor room
« Reply #5 on: September 19, 2015, 10:27:06 AM »
I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency.  That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong :)

Regards, Christine

I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16

Just recall that it wasn't that long ago that this was how a lot of people recorded GD, myself included!  My first serious rig was Nak 300s and a D5.

Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #6 on: September 19, 2015, 11:31:37 AM »
How about stereo shotguns (without ever having tried it I would say 40-60cm seperation depending on distance, and 0 angle) and a side mic mounted on top of one of the two shotguns? Record each to a separate track. That way after the fact you can choose to do it one way or the other way depending on how it sounds.

My source won't be PA'd but a wind band that I need to mic from about 9 meters away.

Regards, Christine

Offline DSatz

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Re: Shotgun mic test in a poor room
« Reply #7 on: October 02, 2015, 12:36:32 AM »
The main problems with using shotguns for indoor stereo music recording are that:

(1) they have narrow pickup patterns only in the upper midrange and above (i.e. the part of the speech frequency range that controls intelligibility). Below their "crossover" frequency, they're generally supercardioids or the like. The longer the tube, the lower the crossover point--it's a wavelength thing.

(2) in the frequency range that the tube affects, the response for any given angle of arrival can be extremely irregular--peaks and valleys of 6 to 10 dB can readily occur. And the frequency response varies greatly by angle; those 6-to-10 dB peaks and valleys shift up or down in frequency in response to slight changes in the angle of sound arrival.

(3) the overall, summed pickup of reflected sound in a shotgun microphone (i.e. its "diffuse field" response) is very rolled off in the treble. This is only partly compensated (and in an uneven, unnatural sounding way) by the high-frequency peaks that many shotguns have built in to their overall response.

Consider point (2) carefully. It's why boom operators work so hard to keep shotgun mikes aimed precisely at the person speaking. It's not because the voice won't be heard due to the narrowness of the pattern; the pattern isn't all that narrow. Rather, it's because the voice will be heard, but will tend to sound like shit if it's (say) 45 or 60 degrees off-axis. At frontal angles where shotgun microphones don't suppress sound very strongly, they still only sound good within a narrow angular range. (That problem is why some shotgun microphones cost ten times what others cost--and why professionals use short shotguns rather than long shotguns whenever possible.)

When we record live music from a good miking position, direct sound reaches the microphones from a relatively wide angle. But if we're forced to record from near the back of a room, direct sound is coming from only a narrow range of angles, and so the main part of the sound is arriving at random angles (i.e. it is diffuse sound). That's when point (3) above applies. In that situation, a pair of good supercardioids (with a near-constant pickup pattern across the frequency spectrum) will beat the pants off of any shotgun microphones in the world.

Plus you can use supercardioids as a closely-spaced or coincident pair, which can't be done with shotguns; the shotguns' varying pickup patterns in different parts of the frequency range would logically require you to place them at different angles for different frequencies--which is physically impossible.
« Last Edit: January 10, 2018, 08:09:18 PM by DSatz »
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Offline John Willett

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Re: Shotgun mic test in a poor room
« Reply #8 on: October 02, 2015, 05:53:57 AM »
(2) in the frequency range that the tube affects, the response at any given angle of sound arrival can be extremely irregular--peaks and valleys of 6 to 10 dB can readily occur. And the frequency response varies greatly by angle; those 6-to-10 dB peaks and valleys shift up or down in frequency in response to slight changes in the angle of sound arrival.

This polar-pattern shows this very well:-




Offline Gutbucket

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Re: Shotgun mic test in a poor room
« Reply #9 on: October 02, 2015, 10:43:31 AM »
If interested, I've a suggestion on a configuration to experiment with-

As discussed above, shotguns do not work well as a stereo pair in standard stereo pair mic'ing configurations that rely on a smooth, well-behaved transition and "hand-off" between the two across the center region of the sound image due to their irregular off-axis response.  So instead of trying to use them as a substitute for well-behaved directional mics, consider using one for what they are best at - maximally isolating whatever they are pointed directly at as much as possible, without regards to the quality of pickup of everything else - and use that to supplement a stereo pair of microphones which provide what a pair of shotguns cannot - a smooth and even stereo image with a good sounding ambient pickup.   

I'm suggesting a three microphone configuration with a single shotgun in the center pointed directly at the source, similar to the image posted above of the test recording, with the stereo pair routed Left/Right and the shotgun panned to center.   

Consider it something of a more optimal division of labor, with each element contributing what it does best and covering for what the other element does not do as well.  The smooth spatial output of the stereo pair buries the off-axis issues of the shotgun, and the shotgun improves the reach and center solidity of the stereo pair. To work best, the stereo pair configuration needs to be to modified to accommodate the presence of the shotgun, so don't just add the single forward-pointing shotgun to a typical ORTF or DIN pair of cardioids.  The stereo pair should be spaced wider or angled farther apart, or both, to provide sufficient sonic space for the shotgun to solidify and fill the middle of the image.  It's something of a close-dance, both supporting and embracing the other, and bit more room is needed to keep the two partners from stepping on each other's toes than if they were simply out there dancing around on their own.  The stereo pair, if listened to in isolation, should be on the verge of "too far apart or too wide", and perhaps "too ambient" with a hole in the middle when the center channel with the shotgun is muted.

Record all three mics separately, and adjust the level of the center channel afterwards to dial in the optimal amount of up-front center information in a way somewhat similar to matrixing a SBD feed with a stereo AUD pair.   Think in terms of balancing the drier direct sound (primarily from the shotgun) with the wetter ambient stereo sound (the stereo pair).  Conceptually it's also sort of like M/S, but instead of a single figure-8 providing the side information you have a spaced pair instead.  I strongly suggest recording all three channels separately instead of mixing the three live and recording to 2-channel, because doing so allows for careful adjustment of the center level (and EQ if necessary) afterwards by ear.  That center level adjustment is critical, and the ability to dial it in perfectly by ear afterwards is one of the advantages a technique like this can provide.

The interesting question then becomes "what configuration is most appropriate for the stereo pair?", and that is going to depend on the situation.   I've not actually done this with a shotgun (I would, but I don't own one), yet I commonly use a single forward facing cardioid, supercardioid, or hypercard this way.  Outdoors I do this with a spaced omni pair, and it allows me to space omnis twice as wide as I would do without the single directional mic in the center (6' instead of 3').  Alternately, or in combination, I'll use a pair of supercardioids spaced about 2' apart (similar to the photo above) but pointing directly to the sides 180 degrees apart.  Remember that the presence of the microphone in the center requires the stereo pair to be more widely spaced and/or angled, than they would be alone.  So 180 degrees apart may seem ridiculous, but if all three mics were cardioids that configuration would be identical to two DIN pairs sharing one microphone along their common edge.  Inside this has worked very well for me from optimal locations, but from far back in the room you may not want to use omnis or supercards pointing directly at the side sidewalls.  In that case you might try using cards or supers for the stereo pair arranged at a smaller angle and spaced more widely to compensate for the reduced angle between them.  The logical extreme of that would be supers fairly wide-spaced but parallel to the shotgun, maximally excluding ambient room sound and focused on sound arriving from the front.  You'll probably will want some angle between them however, even if it's somewhat minimal, like in the photo above.

Worth some experimentation I think, and you already have all the functioning mics you'll need to try it, you just need a recorder which can record 3 separate channels.
« Last Edit: October 02, 2015, 10:46:10 AM by Gutbucket »
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Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #10 on: October 02, 2015, 11:08:52 AM »
I love these discussions. Very educational and excellent ideas :) I will definitely give that shotgun in the middle suggestion a try. Thanks all!

Here is a somewhat related question. If you have a non-coincident stereo pair. Let's say two supers 30 cm apart, whatever useful angle. If you then take the stereo file and run it through an MS plugin (such as Voxengo MSED) and mute the side signal, leaving only the mid signal. Would that result in the same mono file as what a super would have given if there had been one exactly been the two you originally recorded?

What I suppose I am driving at is this. Could you place for example a pair of supers with a slight angle at 60cm, and in between a pair of shotguns at 40 cm or so. If you could then take the mid signal from those two shotguns and add it to the widely spaced supers, would that give the same result as you describe, without actually having a center shotgun? If yes, you would be able to do both at the same time: the single shotgun as a center mic for the wide supers as a known-good, and a stereo shotgun recording for experimental purposes.

Regards, Christine

Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #11 on: October 02, 2015, 11:39:09 AM »
Nope.  Because of the spacing of the two shotguns, and also their varying frequency response off-axis, you'll get a very undesirable phase response if you sum them to a mono mid channel.

Hmm, too bad.

Regards, Christine

Offline Gutbucket

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Re: Shotgun mic test in a poor room
« Reply #12 on: October 02, 2015, 11:43:35 AM »
^ The classic "mono-incompatibility" issue- which results from mixing too many signals together which have similar, yet not identical phase, introduced from the close but not coincident spacing between the the mics.  In the case of the stereo incompatibility of a pair of spaced omnis, two signals is too many.  And that hints at why the stereo pair, used in combination with a single shotgun in the center, needs to be spaced and/or angled wider than it would be if were being used as a single pair alone.  In this case you are mixing three channels down to two 3>2 rather than 2>1.  The additional spacing and/or angle between the stereo pair reduces the "mono-incompatibility" issue which may occur on each side, between the summed L and C and the summed C and R.

FYI, the isolated Mid channel output from a MS plugin is the same as summing (mixing) the two channels together.
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Offline rocksuitcase

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Re: Shotgun mic test in a poor room
« Reply #13 on: October 02, 2015, 12:24:22 PM »
Christine,
I am adding to this discussion with  my direct experiences using my AKG CK8 short shotguns.
I have used them for many years in outdoor situations like us old skool GD tapers used to use the NAk guns. However, I typically use a technique taught me by one of those Old skool NAk tapers. He used to and I mostly point the mics at 0 degrees spread about 20 cm apart and make sure they were not higher than 7 feet, shoulder height was his optimal positioning from the TS at a GD show.
This seems to work great for outdoor recordings with large PA's which are somewhat linear arrayed. ( I am recording loud Rock n Roll or semi-loud jambands [most of which have high quality line arrays for PA's] in an open taping environment)
However, I have used them in indoor situations as a "stereo pair", attempting to aim them at 0 degrees spaced apart, but often they get aimed at about 110 degree angle in a quasi nortf configuration between them. Sometimes I do the 'PAS" configuration where I carefully aim the mic toward the middle of the line array taking care not to aim that at the bass stacks. I can second some statements from dsatz and gutbucket that angled away from 0 degrees they usually do not produce a great recording. Occasionally I have made some decent recordings with them indoors. one of note is the DBB|Kung Fu show from the Capital Theater Portchester NY which we attempted because previous experiences there resulted in poor cardiod recordings from the SBD area.

NOw, the AKG Ck8 is an upper-mid quality microphone capsule, but is still an interference tube design, so they are very directional, but somehow not as "bassy" as NAK 100/300s.
 
The gutbucket technique is one I would try given your available mics. kindms and I have been using his AKG 414's to do MS with a cardiod/subcardiod middle and figure 8 side. He has suggested we use a shotgun for the middle which we may do next time we get to do some recording.
Good luck with your experiments, and I must say I am liking that Sennheiser MKH805 shotgun mic you are using 
 8)
« Last Edit: October 02, 2015, 05:41:45 PM by rocksuitcase »
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Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #14 on: October 02, 2015, 01:00:39 PM »
I'm calling this one the "Gutbucket array". Pretty much as described above in Gutbucket's post, a pair of supercards, at 60cm with a 40 degree angle and an MKH805 in the middle. OK, it wasn't exactly in the middle but slightly to the left, but I don't think the difference is significant in this case. All three channels mixed equally and brought up to -16 LUFS.

The jet engine in background is the ventilation system.

I kinda like what the MKH805 adds, sound wise. To me it sounds much... "warmer" (an inaccurate term, I know) than just the supercards by themselves.

Regards, Christine

Edit: Found the actual angle of the supers in my notes.
« Last Edit: October 02, 2015, 01:16:24 PM by connloyalist »

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Re: Shotgun mic test in a poor room
« Reply #15 on: October 02, 2015, 03:36:50 PM »
I'm calling this one the "Gutbucket array". Pretty much as described above in Gutbucket's post, a pair of supercards, at 60cm with a 40 degree angle and an MKH805 in the middle.
Edit: Found the actual angle of the supers in my notes.

I kinda like that.  I preferred the super when it was just one or the other... 

As noted if you can mix them without significant cancellation issues it may give a little more dimension to the result than one or the other on their own.  Some of the added dimension/warmth may be that this is now a "stereo" result vs. the mono in the prior samples though. 


« Last Edit: October 02, 2015, 03:38:31 PM by bombdiggity »
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Offline Gutbucket

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Re: Shotgun mic test in a poor room
« Reply #16 on: October 02, 2015, 05:25:57 PM »
Not sure if anyone cares, but this is a good opportunity for me to further the explanation of my line of thinking about "stereo" in this way, and in particular these kinds of less-than-typical microphone configurations which spring from that line of thinking, designed with the intent of optimizing for the aspects of a recording which are most important and I want, given the constraints we are required to work around.  In other words- thinking more in terms of "Direct/Reverberant", "Front/Back", "Middle/Sides", and how some stereo requirements change by frequency range, rather than thinking in terms of "Left/Right".

Imagine we're recording outside, say in an amphitheater.  There is a PA, and we can setup pretty much were we'd like, but no SBD access, no on-stage access, and I'm going to record from a single location out in the audience.  My two channel starting point for that is a pair of spaced omnis, say 3' apart.  From there I add a center directional mic which directly addresses the two primary problems I hear with spaced omnis: It provides a far greater overall forward bias and "presence" (Front/Back, Direct/Reverberant stuff) because it is pointed so as to reduce pickup of everything but what is directly in front, and also provides a strong, solid center which allows for an even more optimal spacing of the omnis without getting a hole-in-the-middle.  To my ear that greatly improves upon the spaced omnis alone- sort of like teamwork, the addition of the forward facing center directional mic allows the omnis "do what they do best" and provides the ability of adjusting for the optimal balance between the omnis and center after I get home, which is a huge advantage.

I'm completely happy with those recordings, yet in search of sharper stereo imaging (which is important to me, but further down the hierarchical list of what is most important) I add the pair of supercardioids.  I space them ~2' or so apart and point them directly to the sides, forming a three-mic array with the center directional mic.  I choose that arrangement because I want to maximize the level differences between the signals of those three directional mics, while still keeping them in a viable stereo configuration.  That 180 degrees supercardioid angle is going to produce the greatest channel separation possible for sounds arriving from angles which approximately line up with the left and right speakers on playback.  The null angle of each supercardioid pattern points approximately 45 degrees off center to the opposite side, which more-or-less lines up with the opposite playback speaker as well as more-or-less with the opposite PA speaker in the venue (opposite edge of the SRA in Stereo Zoom terms).  Not exactly, but close enough.  Sound arriving from that direction will appear in the signals from near-side supercardioid and the center mic, but will be minimized as much as possible in the supercardioid channel on the opposite side.  (Farther off-axis than that, say fully to one side or the other, the inverse-polarity back lobe of the opposite supercard comes into play somewhat, introducing some anti-phase information in the channel opposite the sound source which is probably useful as well).

I retain the wide omnis, which are my fall-back starting point anyway.  As omnis, they compensate for the reduced low-frequency response of the supercards, and their wide spacing provides excellent stereo bass which a more narrow microphone spacing does not.  They also contribute to an open and "airy" stereo ambience and sense of space.  I now have 5 mics in the array, arranged to work optimally together, in a way which minimizes potential conflicts between them when their signals are mixed together, and provides even greater flexibility when I'm mixing to 2-channel stereo.

In a surround playback environment, the channels stay discrete instead of being mixed, but are still level balanced to taste afterwards for best effect.  The supercards are routed directly to the Left/Right speaker channels, and the center mic routed to the Center speaker channel.  The omnis get routed to the Surround channels (possibly with their bottom octaves mixed into the front Left/Right speaker channels to take full advantage of their deep stereo bass contribution).

Moving into a less ideal recording situation, say much further back or in a more reverberant space, I'd start pointing the Left/Right supercardioids more forward while increasing their spacing to compensate for the narrowing angle between them.  That's going to reduce the optimal channel separation somewhat, but increase the forward pick-up bias of the array and reduce sensitivity to reverb and ambience from the sides and back.  That's a trade off I'm happy to make because if I can't have both, I value getting a good direct/reverberant balance far more than super-sharp imaging.

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Offline Gutbucket

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Re: Shotgun mic test in a poor room
« Reply #17 on: January 10, 2018, 09:31:59 AM »
Hi Christine,

Old thread I know, but I came across it searching for something else the other day and realized I forgot to suggest a microphone technique which I think would be most appropriate to the this particular recording situation.  I'd let that slip in all the discussion about interference-tube microphones and 3-microphone arrays using them.

Anyway, if I was recording in this situation from that position in the back of the room I'd try first boundary mounted omnis spaced on the back wall.  This is an ideal situation for that technique.  The recording position is nearly at the back wall, the back wall is large and flat and the room is reverberant and somewhat boxy.  Boundary-mounting will increase the direct/reverberant ratio as much as possible from back there while greatly improving the apparent clarity, eliminating pickup of 1st reflections off the back wall entirely.  I think it could work quite nicely for this application.

You can try it by simply placing a pair of spaced omnis very close to the wall, perhaps even gaff-taped directly to the wall.  You might still record the shotgun to a third channel, which may or may not be useful or needed.  As stated previously, the other two mics should help bury the off-axis response irregularities of the shotgun used alone.  If you have three omnis, I'd try boundary mounting all three in a line across the back wall facing the band.
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Offline connloyalist

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Re: Shotgun mic test in a poor room
« Reply #18 on: January 10, 2018, 12:10:09 PM »
Hi Gutbucket,

Thank you for the suggestion! Yes, I think I have three omni's. Or more accurately, two pairs of different omni's. Strange question perhaps, but would the omni's be facing into the wall, creating a boundary mic effect, or into the room with the cable at the rear against the wall?

Can you suggest boundary mics that are appropriate to music recording (as opposed to speech as many appear to be) and don't cost an arm and a leg?

Regards, Christine

Offline Gutbucket

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Re: Shotgun mic test in a poor room
« Reply #19 on: January 10, 2018, 01:24:09 PM »
If you're okay with to attaching them directly to the wall with adhesive-tape, I'd point them toward the ceiling.  I think that should work well and at least be a good test.  Try whatever small diaphragm omnis you have on hand, but avoid electronically switched large-diaphram mics which have an omni-pattern.

If using mic stands you might point them directly at the wall, perpendicular and very close to it, like just a few millimeters away.  If you can monitor while setting up and hear any weird high frequency resonances with that arrangement you might play with angling them upwards somewhat so the diaphragm face will have a sightly varying distance from the wall across it's diameter. That should spread out any resonances occurring between the diaphragm and wall.

Ideally the mics would be mounted into the wall itself, with the capsule diaphragms flush with the wall surface, which obviously isn't practical as you'd have to modify the wall.  The old Crown PZMs have their capsule facing into a boundary-plate, and the Crown PZM guideline booklet talked about how closely they positioned the capsule to the boundary plate so that the boundary effect works cleanly up into the ultrasonic frequency range.  It goes on to talk about how the diameter of a typical mic body is big enough to cause the very high frequency response to suffer from reflection interaction at the points of the diaphragm furthest from the boundary when the diaphragm is perpendicular to the surface, but I'm remain to be convinced that really matters very much in terms of real world taper arrangements.  If it does its only affecting the very highest frequencies.

When I've done this I've used miniature DPA 4060 or 4061 omnis, and have both taped them directly to the wall facing up (easy to do since they are so small and light), and have used the intended DPA boundary-mount accessory for them, which is a little round rubber disk into which the microphone is inserted from the back.  The front has a little opening to the diaphragm and the surface of the disk sort of smoothly tapers so as to blend into the wall surface with no defractive edges.   Both methods worked well and the results seem to me about the same honestly, but these are very small microphones to begin with.  I use and highly recommend the 4060 omni (more sensitive) or 4061 omni (less sensitive), but they aren't inexpensive (not crazy expensive, but around $900 a pair new, probably $400-500 used).  Fortunately decent performing small omnis are easier to get right at lower cost than small directional mics.  Some around TS really like the Countryman B3, which is the same size and about half the cost last I checked.  The TS mic builders all offer inexpensive small omnis which will probably work well for this.  Any of these tiny lavalier-sized mics you can very easily tape to the wall.

I've never used either the Crown PZMs or the inexpensive RadioShack version which were popular years ago so I can't comment on them, but they are specifically intended for this kind of thing. Opinions I've read seem to vary widely.  More manufacturers make a dedicated boundary mic these days such as Audio Technica and others, but personally I lean towards small multipurpose omnis I can use for other situations as well.  Also many of those are intended for setting on a stage floor or conference table- and so are heavy, weighted, and/or armored against being stepped on.  Those types would be more difficult to mount on the vertical wall.

Its a great technique that really works well in situations when other things don't, but a somewhat special case thing which doesn't come up enough for me to have dedicated mics for doing it.
« Last Edit: January 10, 2018, 03:02:58 PM by Gutbucket »
volition > vibrations > voltages > numeric values | numeric values > voltages > vibrations> virtual teleportation time-machine experience
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Offline illconditioned

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Re: Shotgun mic test in a poor room
« Reply #20 on: January 10, 2018, 02:08:32 PM »
I've made some recordings with a pair of shotgun mics,
Spaced about 20-30cm, pointing almost straight ahead, at the back of the room.
I am using in a narrow room where I am trying to avoid reflections from the walls.
Not sure how good this method is, but I like the results to far.
I'm also considering this type of setup for permanent installed setup where I want the mics well away from the front of the room.

So far I've used various mics, including:
AKG C747 (very short shotgun, almost a hyper cardioid in practice)
Beyerdynamic CK706 (short shotgun)

I'm waiting on a pair of AKG CK8 next for my continued experiments (and continued gear acquisition!!).

 Richard

Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline EmRR

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Re: Shotgun mic test in a poor room
« Reply #21 on: January 10, 2018, 10:34:43 PM »
My only comment on shotguns is a bit of a tangent, I use them occasionally in recording studios, specifically to get the distant sound of an electric guitar amp, as part of an overall multi-mic blend of the total event.  The advantages have outweighed any negative consequences of using them within a small room with close reflections, FWIW.  Adding to it, many times a close mic on an electric amp sounds too clean, and nothing like the sound that congeals at listening distance, and the sound at distance as heard by a MICROPHONE sounds nothing like the way our ears reinterpret it.  I would not be afraid to experiment with a shotgun or two in a poor room, the balance may weigh in favor, but of course, throw up as many options as you can record! 

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Re: Shotgun mic test in a poor room
« Reply #22 on: January 11, 2018, 09:36:24 AM »
Here is a pretty decent boundary layer capture, done with DPA4060 pair, spaced ~6', and taped tight to the hard wall face of the theater balcony. Very much of a mid-room recordng.
https://archive.org/details/pso2017-03-19.pso017-03-19_dpa4060-ble_1644flac
https://archive.org/details/pso2017-03-19.pso201-03-19_dpa4060-ble-balcony_flac2448


Shotguns,....
Lets say that I prefer a more musical mic for music, and a more news gathering tool for news gathering.
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