Gear / Technical Help > Microphones & Setup
Naiant x-x as boundary
carpa:
I've read around that small omnis may be set up as boundary and I wonder if Naiant X-X's ( I think quite a few here own them) could be arranged that way.
I would be very interested in knowing what kind of mount ( even DiY) could serve at the purpose. Apart from the fact that it seems helping to increase the S/N ratio in the recording, what are other applications/benefits ( close/distant) ? Mostly recording piano here.
Thank you so much
Gutbucket:
No reason Naiant X-X should not work.
To clarify- It's the ratio of of direct-sound to reverberant-sound pickup which is increased via boundary mounting, not as well as S/N.
You want the microphone element as close to the boundary surface as possible, ideally flush-embedded into it. The Crown PZM implementation mounts the element a hair's width above the surface facing toward it. A microphone with a small enough diameter element facing sideways and taped directly to a flat surface is sufficiently close to the surface in my experience. Closer = smoother high frequency response.
The boundary surface should ideally be smooth and relatively hard. Size of the boundary surface (it's area and geometric shape) determine the low-frequency cutoff where the frequency and polar response returns to free-space-mounted (non boundary mounted) condition.
If recording piano with freedom to place the mics, it could be useful on the underside of the lid in a room with compromised acoustics. When recording from a considerable distance, it can be useful on a wall of the venue when one cannot otherwise get the microphones sufficiently close. Might be useful to try on a hardwood stage floor.
carpa:
@Gutbucket
Thank you very much for your response. The main think I'd like to understand, and which I didn't get from googling for pictures, is the position of the mics on a surface. Some pics show the head of the mic directly aiming at the surface ( as if the hard surface was the sound source ) with just a mm or two of air. That should be a bit difficult to achieve, while just taping the mic laying on the surface could be much easier; are you meaning this option by "facing sideways"?
In your example of application you have indicated taping underneath the piano lid or, on the much more distant option, the idea of taping the mics on a wall like ambience mics.
Does have a sense using them on a DiY thing having sort of two square pieces of hard surface with one mic each and using them at, let's say, AB or other configuration at 1,5/2 meters distance from the instrument or players? What will the difference be in respect of using them not boundary like?
Should it make some sense, what size of surface would you suggest? Something like a CD cover?
I hope I didn't mess up misunderstanding everything!
Thanks again
DSatz:
The microphone should definitely not be aimed at the backing surface. That was a big defect in the design of the old Crown (and Radio Shack) "PZMs", which caused a number of problems in the high frequency response.
Ideally the membrane should be parallel to the backing surface and as nearly flush with it as possible, but facing into the space that contains the sound sources. See for example the designs from manufacturers such as Schoeps and Neumann (sample photos shown below; the actual products have normal-length cables). The asymmetry of both designs is deliberate; the base of the microphone has a non-zero thickness so there are some minor diffraction effects at the edges, and the idea was to spread those around so that they wouldn't affect any one narrow band of frequencies as much.
Neumann no longer makes or sells this type of microphone, and Schoeps has discontinued its original, larger model (the BLM 3), but the BLM 03 C has the identical capsule. It's a damn nice-sounding microphone, as well as being my ready answer to any complaints about the phallic shape of most microphones. -- Sennheiser introduced the actual first commercial microphone of this type, which had its (electret) microphone element built into a cylindrical cage that sat above the flat base of the microphone. That's shown in the fourth photo. I don't know what the internal arrangement of the parts was inside the dome.
If you have to lay the capsule down on the floor or prop it up against a wall, with the membrane perpendicular to the sound sources, that's still better than having it face away from them and into an air gap. This type of setup can also be done with other types of microphone capsule; they don't have to be pressure transducers (omnis). I've recorded with cardioids or supercardioids on the little circular plate shown in the third photo below; of course I put something over them to protect the mikes from being stepped on.
--best regards
P.S.: Actually, placement on a boundary does increase the s/n ratio in addition to the similar increase in direct sound pickup relative to reflected sound, since the sensitivity increases while the self-noise does not. It's not an either/or situation; both improvements occur at the same time.
Gutbucket:
D-
Thanks for the clarification on S/N which I should have remembered but had forgotten about. I suppose that's because unless the noise floor is audible that aspect is not perceptually apparent, whereas the increase in direct sound pickup relative to reflected sound is immediately obvious at all levels.
And thanks for the warning.. I didn't mean to suggest trying to mount them so as to face the boundary in my attempt at providing some historical background of how it has been done and why. I imagine that unless otherwise compensated for, mounting a capsule in such a way could produce a significant HF peak relative to the gap spacing and capsule surface area. Might that be the source of the high frequency response problems in those designs?
C-
Yes, I mean the microphone diaphragm facing sideways in relation to the surface on which it is mounted. [edit- like the csm-BLC image 3rd from the top in the post above]
Assuming the use of omnis, if both microphones of a stereo pair are mounted on the same surface it acts as a spaced A-B configuration. If mounting each microphone on a separate boundary "plate", those plates can be faced in different directions and like directional microphones will begin to produce level-differences as well as time-of-arrival differences. That directionality will manifest only within the effective bandwidth of the boundary-effect region, determined by the area of the boundary at the low frequency end of that range and the spacing between diaphragm and boundary surface at the high frequency end of things.
A CD case cover is too small to be effective below the kHz region. It would act like the pressure modification attachments used on some omnis to change their presence range and higher frequency response and directionality. Those range from circular disks to spherical attachments. Some of the wooden circular disk ones I've seen are close to the diameter of a CD.
Generally an area of a few square feet at minimum are needed to be effective down through the lower midrange frequencies.
For stereo configurations, other than A-B mounting to a single surface, mounting to squares or rectangular pieces of polycarbonate (Plexiglas) is common, and the two can be arranged in a wedge shape, which achieves a similar angle/spacing relationship to a stereo pair of directional microphones.
Below are a couple examples of how Pierre Sprey of Mapleshade records small ensembles this way-
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