User Manual

Townsend Labs Sphere L22 Precision Microphone System Guide Stereo Recording

Two Microphones

Using two mics lets you employ a wider range of stereo techniques. In general, you can use the same stereo

techniques you have used before, although there are some advanced features, such as

OFF-AXIS

CORRECTION

™, which open up new options.

With two Sphere mics you can even go beyond stereo to quad or 5.0 surround. A nice way to achieve this is to

use an XY mic configuration, with each mic positioned coincidently 90° off-axis from the other. From this you

can capture front left and rear right outputs from one mic and the front right and rear left outputs from the

other mic. Cardioid patterns are typically used for all mics in this application.

To create a center channel for 5.0 surround, the simplest option is to just sum the left and right virtual mic

outputs. This creates a center-panned virtual cardioid mic (assuming left and right outputs are cardioid). A

potentially better option is to sum the raw unprocessed outputs of both mics and send them to a stereo bus

with another Sphere plug-in. This lets the user set the center channel polar pattern independently. Setting the

pattern to super- or hypercardioid will produce better separation of the center from the left and right channels.

Since there are many possibilities, experiment to figure out what sounds best for your application.

Off-Axis Correction

Since cardioid mics lose directivity at low frequencies, they become omnidirectional and thus center panned in

the bass region, which blurs the stereo field.

Consider a stereo recording of a double bass, where the low frequency fundamentals will be perceived more

towards the center than the high frequency harmonics. Instead of occupying one point in the stereo field, this

single instrument will be spread out.

This change in polar pattern also depends on distance to the source, which is due to the well known proximity

effect. In fact, at low frequencies it is only possible to maintain a cardioid polar pattern at one specific distance

from the source (see Off-Axis Correction on page 24 for more information). Because of this, the

OFF-AXIS

CORRECTION feature has two distance controls, which tell the algorithm how far away the mic is from the

sources. The first control is

ON-AXIS DISTANCE, which specifies the distance of the on-axis source material

from the mic. The second control is

OFF-AXIS DISTANCE, which specifies the distance of the off-axis source

material from the mic.

Fortunately, these controls do not need to be set exactly, and can be dialed in by ear. If there are multiple

sources at multiple distances, then just use an average distance, or simply go with what sounds best.

OFF-AXIS CORRECTION can also be important for mid-side and other stereo and surround configurations that

use heterogeneous mic configurations. For example, with mid-side, the mid channel is either a cardioid or

omni mic and the side channel is a figure-8 mic. But practically speaking, the figure-8 mics always have a

different frequency response than omni and cardioid mics, so in effect the mid-side encoding will have

frequency dependent errors in the response. To some degree this is unavoidable, but with

OFF-AXIS

CORRECTION you can correct a lot of the differences in frequency and polar response resulting in a better mid-

side recording.

OFF-AXIS CORRECTION can be particularly important for surround applications where there are a lot of

overlapping polar patterns.