Despite the seemingly impenetrable set of numbers on
a mic’s data sheet, there is a wealth of information that
even casual users can understand and that will increase
their intuitive awareness of what the transducer can
offer. It all starts by spending a little time staring at the
frequency-response and polar-pattern charts.
Typically, the information from these two diagrams
is derived from sound captured at a distance of a
meter from the mic element. However, anyone who
has placed a directional mic—one with a cardioid or
figure-8 pattern, for example—closer than three feet
from a subject knows that you get a boost in the bass
frequencies as a result of the proximity effect. So even
though the frequency response chart only represents a
mic’s potential sound reproduction at a specific distance,
it nonetheless gives you a gestalt of that transducer’s
behavior that you will measure against your experience.
But there is another side to the frequency
response that you’ll want to pay attention to: how it
affects the pickup pattern.
Top of the Charts
Like the frequency response chart, a polar plot tells only
part of a microphone’s story. The directionality of the
majority of mics these days isn’t consistent throughout
the frequency spectrum: The polar pattern changes
based on the wavelength it’s picking up, as well as the
size and shape of the mic element and housing. Typically,
a mic’s pattern will become more directional when
it is capturing higher frequencies, which have shorter
wavelengths, and less directional with lower frequencies,
which have longer wavelengths.
This is clearly visible if a plot of the products’ performance
across the spectrum is provided with the
mic. Because Shure provides this information, I’ve
selected two charts from its new Beta 181 line—the
omni and cardioid plots. Keep in mind that the Beta
181 uses interchangeable capsules that are small and
side-address; a mic’s physical design plays a big role
in how the pickup pattern changes with frequency.
The nominal measurement you’ll see for most
microphones is based on a tone, or set of tones,
played at some distance from the mic—typically 1kHz
at a distance of 1 meter. As you can see in Figure 1,
the Beta 181 cardioid capsule gives you a near-perfect
cardioid pattern at 1kHz, with the strongest output
at 0 degrees (directly in front of the mic) and a
gradual decrease in level as you approach 90
degrees off-axis. By the time you get 180 degrees
off-axis, the output has decreased by more than 20dB, which is very significant. However, it’s worth
noticing that the rear pick-up is not completely zero,
and it rarely is with studio mics. Although, theoretically,
the null point at 180 degrees of a typical cardioid mic
has no output, there is indeed signal present, though it
is made up primarily of low frequencies at very low
levels. But for practical purposes, the drop of 20dB in
the rear results in a recognizable cardioid pattern.
Next, note how the pattern changes at two octaves
below 1kHz. The resulting increase in output at 180
degrees creates more of a subcardioid pattern, while
the levels at the front and side remain consistent.
The biggest surprise for me when I first saw this
plot was how the pattern transformed at higher frequencies.
At 2.5kHz, the shape begins to move
toward hypercardioid, then reverts back to cardioid at
6.5kHz before changing into a hybrid shape at 10kHz
that approaches a figure-8.
Now let’s examine the omnidirectional capsule. The
omni shape remains completely stable at frequencies
below 1kHz and very close to its intended shape at
2.5kHz. By the time it gets to 10kHz, however, the
output from the sides decreases well over 10dB, with
slightly less decrease in output at the rear, yielding a
shape similar to the cardioid at 10kHz.
Although these changes in shape look dramatic
when plotted, they aren’t inherently bad; a mic’s
response is part of what lends the transducer its character.
But by understanding, say, the polar response
of the omni capsule at all frequencies, you can judge
whether it’s the right mic for a given situation.