M-Audio Microphone, User Manual

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Choosing & Using Microphones
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separation and captures more room reflections. In general, the X-Y technique using cardioids yields an accurate
stereo image exhibiting minimal acoustic reflections, although the separation is not as significant as some other
stereo miking techniques.
Blumlein
Named after British stereo pioneer Alan Blumlein, the
Blumlein
technique takes advantage of the polar patterns
inherent in figure 8 (bidirectional) mics. Recall that figure 8
patterns pick up equally well on two sides while exhibiting
strong rejection at 90 degrees off axis to those sides. In the
Blumlein technique, two figure 8 patterns are oriented 90
degrees from each other with the positive sides facing the
left and right sides of the sound source. Due to the
inherent side rejection, the area of greatest sensitivity of
one mic is the area of least sensitivity of the companion
mic. While the patterns overlap in the center, the signal
from each is 3dB down and, when combined, pick up a
uniform center signal.
The Blumlein arrangement yields very good stereo separation. Due to the fact that figure 8 s are
equally sensitive on the back lobes, this configuration also picks up significant room reflections.
There are drawbacks to this technique, however.The fact that the back of the left mic is also picking
up reflections from the right rear of the room makes for poor mono compatibility. Further,
reverberant sounds coming from the sides of the acoustic space can enter the positive lobe of one
mic and the negative lobe of the other, thus causing the impression of poor localization and/or
hollow effects that can be disturbing. As a result, Blumlein is best used in situations where the
sound source, acoustic space and mic placement are optimal. Since this is a rarity, other stereo
techniques offering superior control are more frequently used.
ORTF
Developed by the French national broadcasting agency,
Office de Radio T l vision Fran aise, the
ORTF
technique is
intended to emulate the placement of ears in the average
adult human head. Two cardioid capsules are placed 17cm
(about 6 - 3/4 inches) apart at a 110 degree angle to one
another. ORTF can produce the wide imagery and depth
common to the Blumlein technique, however the use of
cardioids means that the configuration captures much less
reverberant reflection.
The specified distance for ORTF makes wavelengths below about 500 Hz effectively phase
coherent. The time delays or phase incoherence above that frequency typically contribute to a
sense of stereo separation, along with the perception of a pleasing open or airy quality. ORTF also
exhibits adequate monophonic compatibility. Similar experiments by the Dutch broadcasting
counterpart Nederlandsche Omroep Stichting yielded the NOS technique where a pair of
cardioids are placed 30cm apart at a 90 degree angle.
Mid-Side
The
Mid-Side
technique utilizes special processing to capture very precise stereo imagery with
excellent mono applicability. A mid microphone (typically a cardioid) faces the center of the
sound source and captures the primary sound. A figure 8 (the side ) is placed along the same
vertical axis with its lobes facing right and left, thereby picking up the extreme left and right
information due to the side rejection inherent in the figure 8 pattern.
This configuration does not constitute stereo until the signals are processed through an M-S
encoder matrix such as the M-Audio Octane Preamp. The encoder adds the mid and side signals
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Sound Source
The Blumlein arrangement relies on a matched pair
of coincident figure-8 patterns
Sound Source
17 cm
110 º
The ORTF technique positions a matched pair of
mics in a configuration similar to that of human ears