DV Info Net - Physics of polar pattern interference

To round out this discussion, here is what Schoeps says in its marketing materials about the difference between their super cardioid and their shotguns. From page 129 of
Documents and downloads - Catalogues and brochures - SCHOEPS.de

Quote:

A ”shotgun” microphone has a pressure-gradient
transducer with an interference tube fitted in front. At
upper-midrange and high frequencies the tube suppresses
off-axis sound; this effect is more pronounced
at higher frequencies, and can generally be made
greater with a longer interference tube. At mid range
frequencies and below, however, a shotgun microphone
cannot have greater directivity than a supercardioid
unless its interference tube is so long that it
would be impractical for most purposes.
For the sake of high-quality sound, it is desirable for
a microphone to have similar frequency response at all
angles of sound incidence. However, this may become
a secondary consideration when very high directivity is
required. It is somewhat like the situation with windscreens:
Everyone knows that they affect the sound
quality, but sometimes they are necessary in order to
get a usable recording at all.
Here are some things which are worth knowing
about ”shotgun” microphones, including the SCHOEPS
CMIT 5:

1. For any given length of the interference tube, a shotgun
microphone's design can be optimized for maximum
directivity or for best sound quality, but unfortunately
not for both at the same time. The SCHOEPS
CMIT 5 has been optimized for best sound quality.

2. Room reflections and reverberance contribute enor-
mously to the character of any sound. With a shotgun
microphone, the pickup angle at high frequencies will
be narrower than at lower frequencies; as a result,
diffuse room sound will be picked up with a distinct
high-frequency rolloff, which can make the sound
rather dull. This tendency will be emphasized further if
a windscreen is used. For this reason a supercardioid is
often a better choice for indoor recording. Of course if
there is a special need to reduce the pickup of room
sound at high frequencies (for example, if there is
interfering high-frequency noise), a shotgun microphone
may still be preferable.
The tendency toward dull sound can be counteracted
with a high-frequency boost, which can also benefit
speech intelligibility. The SCHOEPS CMIT 5 offers a builtin,
switchable high-frequency boost of this kind.
The polar patterns of shotgun microphones often have
multiple narrow lobes of sensitivity. These can cause
disturbing comb-filter-like effects when the microphone
(or a sound source) is in motion, especially in indoor
recordings. Special care was taken to smooth out these
response lobes in the design of the SCHOEPS CMIT 5.
Since any off-axis sound will be picked up with
reduced high-frequency content, a shotgun microphone
must always ”track” (follow) a moving actor or other
sound source precisely – particularly if the microphone
is close to the person being followed and / or its interference
tube is a longer one. Precise tracking is not
always easy, and even if it can be done perfectly, other
sound sources in the room (including noise sources
and other nearby voices) will still be picked up from
off-axis.

3. Since shotgun microphones are frequently used outdoors
and / or on booms, they should have a low-cut
filter available to suppress wind and handling noise.
The SCHOEPS CMIT 5 has a switchable low-cut filter of
this type.
In addition, any directional microphone will increase
its pickup of low and lower-midrange frequencies
when it is positioned near a sound source. To avoid
false boominess, a shotgun microphone should also
have a filter to compensate for this ”proximity effect.”
These two requirements involve different frequency
ranges and different ideal filter slopes, however, so a
single low-cut filter is always a compromise at best.
The SCHOEPS CMIT 5 has a switchable low-cut filter
with two different characteristics available: a relatively
sharp cutoff for suppressing room rumble or wind and
handling noise, or a more gradual slope that reaches
some what higher in frequency to compensate for
proximity effect.

4. For high frequency sound, shotgun microphones
increase the distance at which a good proportion of
direct sound may be obtained. But their use still makes
clear sense only when they are already close enough
for direct sound to predominate in the result. For outdoor
recording this will not be an issue when there is
little or no reflected sound energy – but indoors, even
a shotgun microphone should generally be kept fairly
close to the intended sound source.