All microphones have a frequency response that is dependent on the direction that sound arrives from. Audio engineers have made use of this fact to adjust the tonality of recordings probably for as long as microphones have existed.
For some mics, particularly cardioid condensers, about 20 degrees off-axis gives the flattest smoothest response. Moving farther to 45 degrees off-axis can result in a nicely warmer tone. Many ribbons mics actually get a little bit brighter when moving 45 or so degrees off-axis.
This effect primarily occurs at high frequencies (e.g. greater than 5kHz) where the wavelength of sound is comparable to the size of the microphone’s diaphragm. For directional, microphones the boost in bass frequencies due to proximity effect is also reduced when used off-axis. In fact, at 90 degrees off-axis there is essentially no proximity effect.
Here is a frequency response plot of a current production AKG 414 XLII set to the cardioid pattern, with levels normalized so it’s easier to make comparisons. The blue curve is on-axis, the green curve is 45 degrees off-axis, and the red curve is 90 degrees off-axis. As you can see by 45 degrees the response has dropped 2 to 3dB at high frequencies. At 90 degrees the overall response is considerably flatter, but the high frequencies roll-off drastically at about 10kHz.
Next, we have a plot of a U47 in cardioid mode. The change in response when moving off-axis is similar to the 414.
The results are even more dramatic for the U47 in omni mode.
But moving off-axis to shift the frequency response is not necessarily ideal. For directional mics this, of course, means that it will pick up a higher proportion of room reflections and potentially background noise. It’s as if the mic is becoming less directional.
Depending on the particular application, this increase in room pickup might be desirable, but often it’s not. In many cases, the goal of adjusting the microphone axis is solely to tame high frequencies, such as sibilance, and the increase in room pickup is just an undesirable side-effect.
Why Sphere Mic Modelling Is Different
Sphere mic modeling technology makes it possible to adjust these parameters independently, so that there never needs to be a compromise made between these different factors. And you can even make these adjustments after you’ve recorded the audio! The Sphere “Axis” control allows the user to virtually shift the microphone axis, so that the off-axis response of the microphone being modeled becomes the on-axis response. For example, if the user selects an “Axis” of 45 degrees then the on-axis frequency response of the Sphere microphone will match the frequency response of the microphone being modeled as if it was turned 45 degrees.
With a Sphere's Off-Axis Correction feature it is also possible to continuously adjust the directivity of the polar pattern, so if you want to add more or less room sound you can do that without significantly affecting the overall sound of the direct pickup. This means that if you’ve dialed in a vocal sound that you love, but you just want a little more reverberation, all you have to do is adjust the polar pattern from say cardioid to be more sub-cardioid or omni. The value of this feature might not be immediately obvious, but it can be incredibly powerful. See the Off-Axis Correction chapter in the Sphere User Guide for more information on this subject.
When moving off-axis of a directional microphone the level of the audio will drop, because the whole point of a directional mic is to accentuate what’s directly in front of the mic, reduce what’s not. To compensate for this, the Axis control normalizes the level so that the volume is as consistent as possible, which makes it easy to compare different settings.
I Like Your Backside
In theory, omni and figure-eight microphones should have exactly the same response from the front as the back. But in practice, this is never exactly the case, and with some mics the difference between the front and back is substantial enough that it’s like having two different mics in one. But the asymmetrical response means that these mics would not be a good choice in some circumstances, such as for a mid-side miking technique where you need symmetry, especially for the figure-eight mic, to produce a consistent stereo image.
In particular, some ribbon mics, such as the RCA 77DX and the Royer R121, have asymmetrical figure-eight patterns, which results in the backside of the mic sounding substantially different than the front. The 77DX has a variable internal acoustic labyrinth which allows the user to select various polar patterns, but this has the side effect of making the rear of the mic substantially darker. This is also true when the 77DX is set to omni mode.
For the Royer, the difference is entirely intentional due to the patented offset-ribbon design, which gives the mic two distinct and desirable sounds. The backside is actually quite a bit brighter than the front, and more appropriate for acoustic guitar or vocals where you want to keep the high-frequency detail.
As you can see here the 121 backside (green) is about 4dB brighter at 10kHz and 8dB brighter at 12kHz that the on-axis response (blue).
End address omni small diaphragm condensers are invariably darker on the rear side than the front, due to the body of the microphone blocking high frequencies. While using the rear of an omni SDC is a very atypical use case, who knows, it might work perfectly on overly bright source material.
To select the rear sound for a particular mic model set the “Axis” control to 180 degrees, and as always let your ears be your guide. Keep in mind that for cardioid mics, including most super, hyper and sub cardioids, the far off-axis response is very colored and generally doesn’t sound great. So don’t be surprised when using a cardioid model that setting the Axis control to 180 degrees sounds dreadful. Likewise, for a figure-eight model, an Axis of 90 degrees typically sounds very colored.
Of course, you can still physically move the Sphere microphone off-axis, just as you would with a conventional mic. When recording a point source (e.g. a vocalist, assuming minimal chest radiation and a relatively dry room), then the virtual axis control will produce results that are very close to physically rotating the mic.
On the other hand, when recording a source that is large compared to the size of the microphone, such as a guitar cabinet, physically moving a mic off-axis will focus on a different area of the source, which can result in a significantly different sound. Because the virtual Axis control does not rotate the polar pattern, it will not refocus the mic on a different area.
As discussed earlier, rotating the polar pattern is not necessarily the best approach in all cases, so experiment and do what works for your application. In other words, forget about all of this techno-babble, and try it out for yourself.