Handbook for Sound Engineers

Microphones 543

primarily by the difference in amplitude generated in
the two microphones by the sound source. A sound on
the right generates a larger signal in microphone B than
in microphone A. A sound directly in front produces an
equal signal in both microphones, and a sound on the
left produces a larger signal in microphone A than in
microphone B. The same process takes place with
spaced omnidirectional microphones, but because of the
spacing, there is also a time delay between two signals
(comb filter effect). It can also produce a loss in gain
and unpleasant sound if the two channels are combined
into a single monosignal. Since the coincident micro-
phone has both its transducers mounted on the same
vertical axis, the arrival time is identical in both chan-
nels, reducing this problem to a large degree.
Modern coincident microphones often use cardioid or
hypercardioid patterns. These patterns work as well as
the figure 8 pattern microphones in producing a stereo
image, but they pick up less of the ambient hall sound.
Probably the strongest virtue of the coincident
microphone technique is its simplicity under actual
working conditions. Just place the microphone in a
central location that gives a good balance between the
musicians and the acoustics of the hall. It is this
simplicity that makes coincident microphones a favorite
of broadcast engineers recording (or transmitting) live
symphonic concerts.

16.7.2 XY Stereo Technique

The XY technique uses two identical directional micro-
phones that, in relation to the recording axis, are
arranged at equal and opposed offset angles. The left-
ward pointing X microphone supplies the L signal
directly, and the rightward pointing Y microphone
supplies the R signal, Fig. 16-110. The stereophonic
properties depend on the directional characteristics of
the microphones and the offset angle.
One property specific to a microphone system is the
recording angle, which defines the angle between the
center axis (symmetry axis of the system) and the direc-
tion where the level differences between the L and R
define the angular range of sound incidence where
regular stereophonic reproduction is obtained. In most
cases there is another opening for backward sound
reception besides the recording angle for frontal sound
pick-up.
Another important aspect concerns the relationship
between the sound incidence angle and the stereophonic
reproduction angle. As both XY and M/S recording
techniques supply pure intensity cues, a relationship can
be applied that relates the reproduction angle to the

level difference of the L and R signals for the standard

listening configuration based on an equilateral triangle,

Fig. 16-111. This relationship is shown in Fig. 16-112

and is valid at frequencies between 330 and 7800 Hz

within ±3°. The level difference is plotted on the hori-

zontal axis and the reproduction angle can be read on

the vertical scale. A 0° reproduction angle means local-

ization at the center of the stereo base, and 30° means

localization at one of the loudspeakers.

Fig. 16-113 shows the XY properties of wide-angle

cardioids. The lower graph illustrates that the stereo

image does not cover the full base width but is rather

limited to some 20° at best. The recording angle can be

altered between 90° and 120°. In-phase reproduction

with correct side direction is maintained for all angles

of sound incidence. The downward bend of the curves

Figure 16-110. XY stereo technique patterns.

Figure 16-111. Standard listening configuration.

90 o

o

0 o

A. Cardioid microphones

90 o

o

B. Supercardioid microphones

3m

30 o

20 o

10 o

0 o

10 o

20 o

30 o