those with a very
narrow pickup pattern,
are essential when trying
to capture distant
sound or pick out particular
sources in a noisy
environment. That’s why
you’ll see long, tubeshaped
mics mounted on cameras for news
or sports gathering or hanging from booms
on sound stages.
The tube shape is an integral element in
creating the directional characteristics of
the mic. A clue to its function can be ascertained
through another name for this type
of microphone—interference tube.
The tube is mounted in front of and over
the microphone capsule and is built with
an opening in the front and a series of side
openings or slots. The openings are covered
with acoustic material and sometimes a
grill, which allows sound to pass through.
The tube and slot structure is designed so
that sound waves are directed down the
tube towards the capsule rather than exiting
the opposite end.
Sound waves “hitting” the mic on-axis (in
the direction the mic is pointing) take the
shortest path down the tube to the capsule.
But sound pressure waves arriving off-axis
from the sides of the mic enter through the
various side openings. Once inside the tube,
they take more time to arrive at the capsule
than the direct sound since they have to
travel slightly greater distances.
Another way of describing these delays
is that off-axis sound waves experience
phase shifts, which are dependent on frequency,
angle of incidence and the design
of the tube. The result is that at the capsule,
the vector sum of the on- and off-axis sound
pressure favors the on-axis, while the offaxis
sound gets more or less cancelled.
Shure VP89 Shotgun mics
But the degree of cancellation isn’t total
at certain frequencies and angles of incidence.
If one were to go through the vector
arithmetic (here’s where phasor math
comes in handy for summing signals with
different amplitudes and phase angles), it
would show that there are dips and peaks
in the polar response and that the cancellation
isn’t total at some frequencies and
angles of incidence. There often are some
side and rear lobes in polar response. Also,
higher frequencies tend to have a narrower
pickup pattern than lower frequencies.
Yet the on-axis response could be 20 dB
or more than at the side or rear, with that
higher ratio allowing more mic preamp
gain to amplify the distant sounds, giving
the shotgun mic more apparent “range” in
direct sound pickup.
Low-frequency pattern control is one
factor to consider when choosing a shotgun
mic. At some lower frequency, the tube no
longer causes interference due to the longer
sound wavelengths. Longer tubes provide
more directionality at lower frequencies
than shorter ones.
The transition frequency where the tube
no longer has an effect is calculated by
dividing the speed of sound by twice the
length of the tube. Below this frequency, the
tube is effectively out of the picture as far
as directivity response is concerned. Directivity
now depends on the inherent polar
response of the microphone.
Table 1 illustrates this with some sample
calculations of the transition frequency for
some hypothetical shotgun tube lengths,
and also for a real product, the Shure VP89
shotgun mic. This Shure mic makes tube
length comparisons easy since a common
section can be outfitted
with one of three
depending on the application.
Table 1: Calculations for transition frequency for some hypothetical tube lengths, and for the Shure VP89 shotgun mic with the short (S), medium (M), and long (L) tubes.
The specifications indicate
that the long tube
has an acceptance angle
of 30 degrees with applications
sound pickup such as
for ENG, sports and field
recordings. (The spec
sheet does not indicate
which frequency or frequency
band for the acceptance
The medium length
tube is listed with a 50-degree
acceptance angle and is useful for “a
greater degree of ambience” for audience
response and live events, for example. Finally
the short tube has an acceptance angle
of 70 degrees and applications for near-field
sound pickup as on a camera mount.
Above the transition frequency the interference
effects start to kick in, and increases
with more frequency. At about two or three
times this transition frequency, the highly directional
really become apparent,
in actual performance
and as seen on the polar
patterns graphs in Fig 1.
In operation, the
shotgun mic should be
used out in the open,
and its slots shouldn’t be
blocked. Keep the mic
away from unwanted
loud sounds to the side
and rear. A loud sound
closer to the side or rear
of the mic can end up at
about the same level as
a more distant on-axis
sound, as “seen” from the
capsule. These will be
Study the polar patterns to help aim the
mic for optimum pickup and the greatest
rejection of unwanted sound, which may be
from the side rather than the rear.
Don’t try to be stealthy by placing only
part of the mic outside a window or door,
with the rest inside. That effectively blocks
the side slots and defeats the whole purpose
of having a shotgun mic in the first place.
Fig. 1: Polar responses of the Shure VP89 for the three different tube sizes (Click to Enlarge)
Even with good design and construction,
a shotgun mic will have bumps in frequency
and phase response and the polar patterns
due to the interference effects, with
the differences in the side pickup especially
prominent. Match the response to the application,
like voice (speech or song), music,
ambient sound, wildlife, etc.
Being so highly directional, shotgun mics
are sensitive to wind noise, not only from
nature, but from manmade sources. Use a
good windscreen, and if needed, add on a
furry cover. Just make sure the furry cover
doesn’t become wet or matted down.
Many shotgun mics provide a switchable
low-frequency filter to help reduce wind
noise. There’s a trade-off, of course, with decreased
low-frequency response, but it may
be the only choice in certain situations.
So experiment. Shotgun mics are a useful
addition to anyone’s microphone collection.
Mary C. Gruszka is a systems design
engineer, project manager, consultant and
writer based in the New York metro area.
She can be reached via TV Technology.