Cardioid Carrying Member
The March’03 Tech’s Files
2003 by Eddie Ciletti
Management is discussed is some detail elsewhere on this site, not
just for Surround Mixing, but a more global zoom-out of four low-frequency
perception variables. I encourage you to take advantage of the link
for the images it contains. Meanwhile, in summary, the issues under
scrutiny are the Equal Loudness Curve for humans (1),
the Loudness Button on consumer gear (2), Acoustics
(3) and the Proximity Effect of Directional Microphones (4). The latter
is this month’s topic, but first a quick review.
The Equal Loudness Curve (1) details
the ear’s unequal sensitivity to the audio spectrum at various levels and
insensitivity to Bass at low monitoring levels in particular. A digital
Sound Pressure Level (SPL) meter —about $50 at Radio Shack — is the first
step toward achieving consistent monitoring levels. With it, I learned
that my comfort level is around 70dB-SPL, 15dB lower than the 85dB-SPL
reference level specified for surround and other sound-for-picture facilities.
Depending on the version of Loudness Curve being interpreted, suffice to
say that for me, a Low Frequency (LF) boost is in order of not less than
6dB and possibly 9dB to 12dB. If not addressed in the monitoring system,
it might translate into that much more bottom in the mix.
NOTE: A $50 SPL meter is not
expected to be an Absolute reference to NIST standards — www.nist.gov
but it can provide the end user with a Relative indication of monitoring
level. Choosing and maintaining a target SPL is part of the ear training
process — how things sound at that level as well as what type of sounds
translate best to the consumer environment.
The Loudness Button (2) on consumer
gear boosts bottom. On some car stereos there is no button although the
curve is not always part of the equation — at the maximum "Loudness Control"
setting, for example, there is no curve. In my own private Utopia this
would be calibrate-able to the 85dB reference. Back in the real world where
there is no standard, the Loudness Curve is gradually applied somewhere
between 50% and 75% of the control’s range — more of the curve is added
as the control level is decreased. Now the cranial light bulb should pop
on and be white hot. Dig it! The curve is a moving target, so the relative
level of the sound source affects the amount of EQ applied to it. A super
hot, ultra-normalized CD gets more of the Loudness curve than one with
more dynamic range and less dynamics processing.
Acoustics (3) and the laws of physics
dictate that smaller control rooms will have more low frequency issues,
problems that can be tamed to a degree but not eliminated. My theory is
that the 1-2-3 combo — of Loudness Curve, inconsistent monitoring levels
and unresolved acoustic issues — conspire to reduce the perceived Low Frequency
Energy (LFE). Even before the listener becomes aware of this deficiency,
along comes the directional microphone. Used up close and personal (to
maximize isolation), Cardioid and Figure-of-8 Polar Patterns have a Proximity
Effect — a noticeable rise in LFE as the mic is moved closer to the source
— a bump that make things sound "right" and "warm" at least for the moment.
Furthermore, a multi-channel recording
— where many directional mics have been used at close range — may suffer
from a low-frequency "muck" build up. Better to fix the muck than compensate
by boosting in the 2kHz to 5kHz range where the ear is most sensitive.
Heightened awareness of these four variables— any one of which might be
your Achilles Heal — should make them easier to tame, hence this month’s
continuing zoom-zoom-zoom on the boom-boom-boom. This is My Personal Theory
currently under investigation. Care to join me?
MAPS and CHARTS
If you’ve never paid much attention
to the Frequency Response of a Microphone, now’s your chance. More than
any single piece of audio gear, mic and speaker (transducer) idiosyncrasies
are the most easily identifiable by ear. You should be intimately familiar
with at least one of the seven microphones represented in Fig-1
NOTE: Figures one thru six are included
within this article, but you can also link to
the "master" composite image where I have rescaled all of the response
charts for equality.
There are all sorts of bumps and
lumps — electronics by comparison are almost indistinguishably flat. The
charts were downloaded from the web, then resized for equal amplitude (vertical)
and frequency (horizontal). The editors were especially kind this month,
allowing all six pix to be published. If you need a closer look, they are
already posted at www.tangible-technology.com by clicking on "recent
To generate their published "flat"
response curves, microphones are typically measured in the lab at a distance
of 1-meter (39.37 inches) from the source. For directional mics, any distance
closer to the source yields increasingly more "bottom," hence the proximity
Effect. (Omni mics are not affected by Proximity.) Knowing that their products
will be used up close and personal, directional microphone designers may
incorporate a bass roll-off feature either by switch or by default, the
latter can be parsed from the 1-meter response although detailed Proximity
Curves are preferred. Three of the mics investigated here have published
proximity response, detailing that all the action is at 1-foot or less
—reinforcing an oft-suggested phrase to "move the mic first " rather than
touch the EQ.
Manufacturers are not consistent
in their presentation of data, both in graphic and text form, although
the Internet is much easier to update than reams of product literature.
I suggest gathering as much info on the mics you use the most — even going
so far as to request the proximity curves — because this knowledge will
assist your intuitive sense. On the net, I learned that the roll-off of
the AKG C-12VR is 6dB/octave at 100 Hz and 12 dB/octave at 130 Hz) while
the Neumann TLM-170 roll-off circuit "attenuates the frequency response
below 100 Hz to suppress undesired structure borne noise." Another example
is the Earthworks SR-77, which is "flat" at 15cm (6-inches) so obviously
the response at 1-meter shows a substantial roll-off. All are helpful bits
of information, but the addition of proximity curves can help identify
a problem and determine whether the best solution is the built-in filter
or external EQ.
I had originally planned to start
in familiar territory with the Shure SM-57 (Figure-1a) but instead
chose the Beta-57 because it provided proximity curves at 2-feet, 2-inches,
1-inch and 1/8-inch (Figure-1b).
THE LOW DOWN
Studying these charts made each mic
more tangible — mics I have used and abused ad infinitum. From this research
I learned that many directional mics are designed to be "flat" not at one
meter but at unique distances, predetermined by their "typical" application.
I hope this article shed some light on the power of Proximity — knowing
where "flat" is and from there, realizing the dramatic difference plus
or minus four inches can make.
It was equally interesting — surprising
in some cases — to discover each mic’s unique characteristics at the opposite
end of the spectrum. We all know how different models respond to similar
applications, we all have favorites and now the "why" is a little more
apparent. Like the proverbial squeaky wheel, it’s the odd track that gets
our scrutiny, so being familiar with microphone characteristics goes a
long way toward reinforcing an engineer’s intuition at the moment of capture.
In our business there are more Relatives
than Absolutes. Whether tracking, overdubbing or mixing, the primary job
is to make each channel relate to the whole almost regardless
of the monitoring system being used. If the process of making the puzzle
pieces fit becomes a struggle — particularly when the bass needs more room
— it’s the monitoring system that gets the scrutiny. Then we want absolute
confirmation of the truth, but in the end, it’s all relative. Isn’t that
something Stephen St. Croix would say? I meant to say that the final judgement
rests with how it plays in Peoria.
Eddie researches, repairs and consults
from the afternoon to well past midnight. From dawn till noon he can be
found "engineering" small wooden railroad systems, applying conflict management
techniques to toddlers as well as attending to their "Input / Output" detail.