Recording - May 2014 - 51

creates the hypercardioid pattern, and turning off the
rear capsule entirely results in a cardioid pattern (as
you get from the single-diaphragm cardioid mic).
Dual-capsule condenser microphones tend to become
more omnidirectional at lower frequencies, and you can
see this in the two polar pattern diagrams shown in
Figure 4. Notice that in the C451B's polar plot (Figure
4a) the 125 Hz curve (in blue, on the left) isn't all that
different from the red curve that shows polar response at
250 Hz and above. But for the C414XLS, the curve up
to 250 Hz (the green line on the left in Figure 4b) is
much more omnidirectional than the red line that represents polar response above 500 Hz.
When your dual-diaphragm mic is close to the sound
source, this is not a problem because the source is so
much closer to the mic than the sound arriving from
behind the mic (wall reflections or other instruments). The
source is much louder at the mic position than the reflected sound, so you won't hear much of the low frequency
omnidirectionality of the mic. However, when the mic is
far from the source, the direct and reflected sound will
be closer to the same amplitude, so you may tend to
hear a muddiness in the sound as the low frequency
omnidirectionality becomes more audible.
When single-diaphragm mics are far from the source
they do not exhibit this increased low frequency omnidirectionality. However, they do start to pick up excess
low frequencies when they get close to the source due
to proximity effect, which tends to be more pronounced
on single-diaphragm mics than dual-diaphragm.
(Shure's Guy Torio and Jeff Segota published a paper
on this topic that's available on aes.org if you want to
read more.)

Put this all together, and you will have more accurate low-frequency
response if you choose dual-diaphragm mics for close placement and
single-diaphragm mics for distant placement.
Is the source wide or narrow? Use a large-diaphragm condenser
mic for a narrow source vs. a medium- or small-diaphragm mic for
a wide source.
Some sound sources are narrow, like a singer's mouth or a speaker
in a guitar amplifier. It is easy to place a mic for these sources; all you
have to do is point the mic at the source and you can be relatively confident that you are picking up all the sound. With wide sound sources-
a grand piano, a marimba, a choir, or an orchestra-it is important to
select a mic (or mics) that will pick up good sound from off to the sides
as well as from directly in front of the mic. This is called off-axis response
and it is strongly controlled by the size of the mic diaphragm.
We refer to condenser mics with diaphragms with a diameter of approximately 1 inch or larger as large-diaphragm mics. The Neumann M147 is
an example of a large-diaphragm condenser microphone. Microphone
diaphragms closer to 1/2" (or less, like the 1/4" diaphragms in Earthworks
mics) are called small-diaphragm mics. The diaphragm in an AKG C451
has a diameter close to 1/2". Microphones such as the Neumann KM184
and the Sennheiser MKH 40 have diaphragm diameters closer to 3/4";

http://www.aes.org http://www.isoacoustics.com http://www.isoacoustics.com

Recording - May 2014

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