bryonosos wrote:
> I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates.
With all due respect, this is why people who make recordings REALLY need to learn how to read polar diagrams. You would then know right away that this isn't so.
Perhaps when you say "a proper omni" you mean "a microphone that is fully omnidirectional at all audible frequencies." But essentially no professional-quality recording microphones are that way. And it's not because it's hard to build a microphone with this behavior; you can in fact buy suitable capsules ready-made for less than a dollar each.
The crux is that sound waves are reflected, either partially or completely, whenever they strike any rigid object whose dimensions are about 1/4 of a sound wavelength or larger. Sound wavelengths are inversely proportional to frequency, and in air, range from about 55 feet (for 20 Hz) to about 2/3 of an inch (for 20 kHz). One inch (the diameter of many large-diaphragm condenser capsules) corresponds to a sound wavelength at about 13 kHz, and 1/4 of that is just above 3 kHz. For a typical small-diaphragm capsule with about 1/2" diameter (the housing dimensions matter, too, but the capsule dimensions matter even more), the critical frequency is about twice as high. But in the top audible octave, even what we normally call "small" diaphragms aren't acoustically "small"--their size causes them to interact with the sound waves at those frequencies, in a way that alters their directional response (because the degree of reflection and diffraction depend on the angle of sound incidence).
Below I've posted a scan of the frequency response and polar response diagrams for an excellent, small omnidirectional microphone that I used a lot in the 1970s, and that also was used by WGBH-FM for live broadcasts of the Boston Symphony Orchestra back then. It has essentially flat high-frequency response for random-incident sound (i.e. when no particular direction of sound arrival predominates--an acoustically "diffuse" sound field). But as the graph shows, it has a pronounced high-frequency rise on axis, with the 3 dB point at around the 6 kHz that you would predict from the capsule's ~1/2" diameter (1/4 wavelength). So you damn well better aim it if you're going to use it at moderate miking distances! Sometimes you even need to aim it away from the sound sources (as WGBH did).
A capsule of about 1/4" diameter or smaller can avoid the bulk of these problems, and such capsules are favored for this reason in certain acoustical measuring applications. But the sensitivity of a condenser microphone is proportional to the capacitance of its capsule, and smaller capsules have lower capacitance (since capacitance depends on area, it's proportional to the square of the diameter). So sensitivity and noise become critical issues with very small capsules, and that's why they're not generally used for professional music recording.
--best regards