The Bidirectional Microphone: A Forgotten Patriarch *

Transcription

PAPERS
The Bidirectional Microphone:
A Forgotten Patriarch*
RON STREICHER, AES Fellow
Pacific Audio-Visual Enterprises, Pasadena, CA 91107, USA
AND
WES DOOLEY, AES Fellow
Audio Engineering Associates, Pasadena, CA 91104, USA
Despite being one of the progenitors of all modern microphones and recording techniques,
the bidirectional pattern is still not very well understood. Its proper and effective use remains
somewhat of a mystery to many recording and sound-reinforcement engineers. The bidirectional microphone is examined from historical, technical, and operational perspectives. It is
reviewed how it was developed and exists as a fundamental element of almost all other singleorder microphone patterns. In the course of describing how this unique pattern responds to
sound waves arriving from different angles of incidence, it is shown that very often it can be
employed successfully where other more commonly used microphones cannot.
0 INTRODUCTION
In the beginning of electrical recording there was the
omnidirectional microphone, and it was good, for it picked
up sound equally well from all directions, was relatively
easy and inexpensive to build, and, perhaps most important, it freed the performer from having to stare down the
mouth of the large black horn that made a direct acoustical
connection to the cutting head on the recording lathe.
Next came the bidirectional microphone, and it too was
good, for it offered the user directionality, that is, more
control over what it heard. By picking up sound equally
from the front and the back it enabled the performers to
work on either side, facing each other. At the same time it
rejected unwanted sound coming from the sides, such as
other performers and room noises. It also was relatively
easy and inexpensive to build.
Through the generations these two begot all the rest.
Their progeny has not always been as good, however, and
it certainly is neither easy nor inexpensive to build them.
Progress moves in mysterious ways.
The omnidirectional and bidirectional microphones
together can be considered as the matriarch and the patriarch of all other polar patterns. Why, then, is the bidirectional microphone the forgotten patriarch in the microphone locker?
* Presented at the 113th Convention of the Audio Engineering
Society, Los Angeles, CA, 2002 October 5–8. Specific facts for
this paper were derived from Harry F. Olson in [2, pp. 219–228].
J. Audio Eng. Soc., Vol. 51, No. 3, 2003 March
Every sound engineer knows intuitively that a cardioid
microphone picks up what it is aimed at, and this is good
enough for most sound-reinforcement and recording applications. The omnidirectional microphone, which gathers
sound from all directions equally, is most often used for
recording, but rarely is it used for sound reinforcement
because of its relative inability to reject feedback when
used at a distance from the sound subject. It is these two
microphone types, and their common variations, that
account for nearly 90% of all microphones sold. Among
the remainder are the shotgun and, yes, the bidirectional.
Although the bidirectional microphone has been with
us almost since the earliest days of electrical recording, it
remains the least appreciated and used of the polar patterns available in the modern microphone locker because
it is not well enough understood how to take advantage of
its unique polar pattern. How do you handle the rear-lobe
pickup? What can you do with the null plane? This is a
microphone that makes you think about how to use it!
1 MICROPHONE POLAR RESPONSE –– A BIT OF
HISTORY AND THE BASICS
1.1 Omnidirectional Microphone
All microphones respond to the motion of air particles
from which they generate analogous electric signals. Thus
they are transducers, converting one form of energy (air
motion) into another (electric). This may seem simple
enough, but how they go about this task is anything but
simple.
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STREICHER AND DOOLEY
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Like a barometer, the first practical microphones
responded to the changes in air pressure caused by the
compressions and rarefactions of a sound wave as it radiated outward from the source and impinged on the microphone diaphragm (Fig. 1). (Compressions exist where the
air particle density is greater than the average pressure;
rarefactions are where the density is less than the average
pressure.) These were called, naturally enough, pressure
microphones.
A microphone diaphragm moves only when there is a
difference in the air particle density between its front and
back. As a sound wave reaches the microphone, it causes
the diaphragm to move in direct response to these
changes in air pressure. With a pressure microphone, the
diaphragm covers a sealed chamber. The air within this
chamber has a fixed air particle density. Thus no matter
from what direction it approaches the microphone, the
sound wave will cause the diaphragm to move inward if
the pressure is greater, or outward if it is less than the
density inside the chamber. Because they respond equally
to sound coming from all directions, pressure microphones became known as omnidirectional. The polar
equation for the omnidirectional pattern is ρ 1. This is
a scalar function, because it indicates magnitude, irrespective of direction.
One of the earliest commercial microphones, the
Western Electric model 618A (developed in the mid1920s), was a moving-coil-type transducer. Fig. 2 shows a
simplified functional schematic. A lightweight coil of wire
was glued to the back of a very thin diaphragm and surrounded by a magnet. As the sound wave caused the
diaphragm to move, the coil was moved similarly within
the magnetic field. This is the essence of a small motor
generator which, in turn, creates a very small electric current corresponding to the original sound wave.
Also developed in the mid-1920s, the Western Electric
model 394 and the RCA model 11A were the first capacitortype microphones. These also were pressure transducers.
+
+ +
+
+
+ +
- - - - - - -
+ COMPRESSION
(positive pressure)
- RAREFACTION
(negative pressure)
Fig. 1. As sound waves radiate outward from the source, they produce alternating compression (positive pressure) and rarefaction (negative pressure) in the air.
Soundwaves can approach
the diaphragm from any
direction.
Motion of Diaphragm
when excited by the
changes in air pressure
caused by Soundwaves
2
1
3
Diaphragm
Voicecoil
Magnet Structure
Microphone Housing
Fig. 2. Simplified drawing of a typical moving-coil-type pressure sound wave omnidirectional microphone. The output is directly proportional to the motion of the diaphragm caused by the variations in air pressure as the sound wave passes by.
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J. Audio Eng. Soc., Vol. 51, No. 4, 2003 April
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BIDIRECTIONAL MICROPHONE
ribbon. This ribbon was suspended in a magnetic field and
thus generated a small electric current in direct response to
its movement. The ribbon microphone, like the movingcoil type, proved to be environmentally stable, easy to
maintain, and more reliable than capacitor microphones of
the period (Fig. 3).
Inherent to this design is the fact that sound waves coming toward the microphone directly from either the front
or the back will be picked up with equal sensitivity. The
only difference will be the absolute polarity of the electric
output: sounds arriving from the back will produce polarity opposite to those arriving from the front (Fig. 4). This
1.2 Bidirectional Microphone
In the early 1930s a fundamentally different type of
microphone was developed, the pressure-gradient microphone. Like the omnidirectional microphone, this also
moved in response to the difference in pressure between
the front and the back of the diaphragm as the sound wave
passed by. However, in this microphone the diaphragm
(which in these early versions was a very thin aluminum
ribbon) was exposed on both sides. Thus as the sound
wave moved past it, it created a very slight but nonetheless
distinct difference in the air pressure on either side of the
Simplified Side View
(shown without side
Pole Pieces for clarity)
Simplified Front View
Pole Pieces
Ribbon Diaphragm
Ribbon clamps
also serve as
contact terminals
Magnet Structure
S
N
(a)
As the Soundwave passes
the Ribbon Diaphragm the
Compressions and
Rarefactions result in a
difference in pressure on
the front and back of the
ribbon.
2
1
3
(b)
Fig. 3. (a) Simplified drawing of a typical ribbon-type pressure-gradient bidirectional microphone. (b) The output of the pressuregradient ribbon microphone is directly proportional to the differences in pressure induced on the front and back of the ribbon as the
sound wave (compressions and rarefactions) passes by.
Positive
Air
Pressure
2
1
3
2
Positive
Air
Pressure
Positive Voltage
Output
(a)
1
3
Negative Voltage
Output
(b)
Fig. 4. (a) As the sound wave approaches from the front of the diaphragm, positive pressure produces a positive voltage at the output
of the microphone. (b) As the sound wave approaches from the back of the diaphragm, positive pressure produces a negative voltage
at the output of the microphone.
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two-sided response led these to be termed bidirectional
microphones. (They also are commonly called figure-ofeight microphones because of the obvious shape of their
polar response.) The polar equation for the bidirectional
pattern is ρ cos θ, where θ signifies the angle of incidence of the sound as it approaches the microphone.
Because it indicates both magnitude and direction, this is
a vector function. The term velocity microphone also is
often applied to ribbon microphones because the current
in the ribbon is directly proportional to the velocity of its
motion in the magnetic field.
The significant operational difference between the bidirectional microphone and the omnidirectional one is that
while the omni responds to sounds arriving from any and
all directions with equal sensitivity, with a properly
designed single-diaphragm bidirectional microphone a
response null of almost 90 dB will occur at precisely 90
degrees from the principal pickup axis. Fig. 5 shows that
this null exists both vertically and horizontally because a
sound wave approaching the microphone along the plane
of the diaphragm will produce equal pressure on both
sides of the diaphragm. If there is no difference in pressure
on the front and the back of the diaphragm, there will be
no output signal. Because this null plane affects both sides
of the diaphragm equally, the figure-of-eight polar
response will be uniform with respect to frequency.
ribbon clamped in the middle. The lower half was exposed
on both sides, functioning as a conventional pressuregradient pickup, and the upper half was coupled at the rear
to an acoustically damped chamber so that it operated like
a pressure-response pickup.1 Thus the two halves of the
ribbon responded to both the pressure and the particle
velocity components of the sound wave, and because both
halves worked within a common magnetic field, their
combined output resulted in a cardioid pickup pattern.
The RCA model 77D shown in Fig. 6(b) was developed
in the early 1940s and employed a rotating shutter
between the ribbon and a damped chamber. This “polydirectional” microphone offered selectable patterns by
adjusting the amount of damping on the ribbon to achieve
an omnidirectional, a unidirectional, or a bidirectional
polar pattern. The final version, the RCA 77DX, remained
in production until the mid-1970s.
At about the same time a very different approach was
employed by Western Electric to create a cardioid micro1 The diagrams of Fig. 6 were taken from Harry Olson [1],
who was head of the RCA acoustical research division from
1934 to 1967. This paper, first published in 1976 and included in
the Audio Engineering Society’s anthology [2] provides detailed
descriptions of many of the evolutionary developments in microphone technology.
1.3 Deriving Other Polar Patterns
It is not within the scope or intent of this paper to discuss in detail the wide variety of other microphone polar
patterns. Suffice it to say here that all first-order microphone patterns can be represented mathematically as some
combination of omnidirectional (pressure) and bidirectional (pressure-gradient) components. In fact, the first
practical cardioid microphone was developed by Harry
Olson in 1931 and released in 1933 as the RCA 77A unidirectional ribbon. As shown in Fig. 6(a), it utilized a long
(a)
OR
Equal Pressure
on both sides
of the ribbon
2
1
3
No Output
Fig. 5. As the sound wave approaches directly along the plane of
the ribbon (that is 90 degrees from the front), it produces equal
pressure on both sides of the diaphragm. Because this results in
no pressure gradient, there will be no output.
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(b)
Fig. 6. (a) RCA model 77A unidirectional ribbon microphone.
(b) RCA model 77D multipattern ribbon microphone.
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phone pattern. They used separate omnidirectional movingcoil and bidirectional ribbon transducers summed electrically and enclosed within a common housing. This was the
classic model 639A cardioid microphone. As shown in
Fig. 7, later versions offered the user the ability to select
from among multiple patterns––omnidirectional, bidirectional, and three variations of cardioid––by adjusting the
relative amounts of the pressure and velocity components
in the combined output. Released in 1939, the Altec
(Western Electric) model 639B was the first commercial
switch-selectable, multipattern microphone.
a point in space. Fig. 8 shows that by combining these two
patterns equally, the result is a cardioid pattern. The polar
equation for the cardioid can be expressed as ρ 1⁄2 (1 cos θ).
In general terms, the polar equation for any first-order
microphone polar pattern can be represented by the equation ρ a b cos θ, where a b 1 and the values of
a and b represent the relative amplitudes of the omnidirectional and bidirectional components, respectively. Fig.
9 illustrates some of the most commonly used microphone
patterns. Note that the pickup characteristics termed random energy response, distance factor, and directivity
index describe how the various polar patterns relate to
their sonic environment.
The random energy response figures describe how each
pattern compares to the omnidirectional pattern in the
pickup of the entire sonic environment. For example, if
exposed to the same total acoustic power coming from all
directions, the output of a cardioid will be about one-third
1.4 Polar Equations for Microphone Polar
Patterns
As noted earlier, the polar equation for an omnidirectional pattern is ρ 1, and the polar equation for a bidirectional microphone is ρ cos θ. These are scalar and
vector functions, respectively, and as such they describe
the essential components of any sound wave measured at
<<< Photo and Line Drawing of
Altec/Western Electric 639>>>
(a)
Moving-coil
Pressure
Pickup
Ribbon
Pressuregradient
Pickup
+
+
-
2
1
3
(b)
Fig. 7. (a) (Altec Western Electric) model 639B, the first commerical multipattern microphone, which derived its polar pattern by combining a moving-coil pressure transducer with a ribbon pressure-gradient transducer together in a common housing. (b) Simplified
schematic diagram of the 639B. (From [3, pp. 177–178, fig. 4-66].)
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that of an omnidirectional one. This is particularly useful
when determining the ratio of direct to reverberant sound
in a microphone pickup.
The directivity index is a measure of the increased sensitivity on axis versus off axis for the various polar patterns, again stated relative to the omnidirectional microphone as the reference.
The distance factor is simply another way of expressing the directivity index. Here it is stated as a measure of
the relative distance between the microphone and the
sound source. For example, to achieve the same direct-toambient ratio, a cardioid can be used at 1.7 times the distance as an omnidirectional microphone.
1.5 Not All Bidirectional Microphones Are
Created Equal
The ribbon microphone referred to is a single-diaphragm
transducer, and so are some capacitor microphones. While
ρ=1
ρ = cos θ
ρ = a + b cos θ
Fig. 8. Cardioid pattern, the result of combining an omnidirectional and a bidirectional pickup equally.
Fig. 9. Chart of first-order microphone polar patterns, showing polar diagrams, equations, and various technical data. (From [4],
derived from data by Shure, Inc.)
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there are both ribbon and capacitor microphones that offer
variable patterns utilizing purely acoustical means, most
modern studio capacitor microphones that provide multiple switch-selectable patterns accomplish this by combining the electric outputs of two cardioid diaphragms mounted
back to back on a common back plate. The German engineers Braunmühl and Weber obtained a patent for this
technology in 1935 (Fig. 10). While single-pattern dual-
diaphragm capacitor microphones were manufactured by
Neumann in the early 1930s, the first commercial switchselectable multipattern capacitor microphone utilizing this
design was the Neumann model U47, issued in 1949.
Combining the signals from the back-to-back cardioids
of a Braunmühl–Weber capsule produces the basic patterns shown in Fig. 11. In addition to the three primary
patterns shown, by adjusting the relative amplitudes of
BACKPLATE LEAD
DIAPHRAGM
DIAPHRAGM
INSULATOR
BACKPLATE
(FIXED ELECTRODE)
ELECTRODE LEAD
ELECTRODE LEAD
ACOUSTICAL PORTS
THROUGH BACKPLATE
Fig. 10. Braunmühl– Weber multipattern capacitor microphone capsule, first described in 1935. It has two diaphragms on either side
of a common back plate. Each is essentially a cardioid pattern, and when their signals are combined electrically, all first-order polar
patterns may be created.
(a)
(b)
(c)
Fig. 11. By adding the front and back diaphragm signals from a dual-diaphragm microphone capsule a multipattern microphone can
be created. This is the principle behind the original multipattern capacitor microphone developed by Braunmühl and Weber. There are
three basic polar patterns. (a) Adding no signal from the back diaphragm leaves just the front cardioid pattern. (b) Adding the back
diaphragm signal in phase produces an omnidirectional pattern. (c) Adding the back diaphragm signal in reverse phase produces a bidirectional pattern.
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these two cardioid signals, a multitude of intermediate
patterns can also be generated. It is important to understand that it is not actually cardioid patterns per se that are
being combined but their respective omnidirectional and
bidirectional components. Also, because they are facing in
opposite directions, these components are in antipolarity
with respect to each other. Although these microphones
are combining cardioid signals electronically, it is actually
the omnidirectional and bidirectional components of these
signals that are being added and subtracted to achieve all
of the polar patterns produced by the dual-diaphragm
multipattern microphone.
When done with precision, the polar patterns thus created can be nearly as uniform as their single-pattern counterparts. When the response becomes less than ideal, it is
most often as the sound wave approaches from close to the
plane of the capsule diaphragms, that is, near 90 degrees
off axis. Here, because of the physical construction of the
microphone housing and the spacing between the two
diaphragms, minute differences exist between the respective electric signals of the diaphragms, so that when they
are combined, some interference cancellations occur. As a
result, the off-axis response of these microphones at
higher frequencies may be compromised. The omnidirectional pattern tends to become constricted at the sides, the
cardioid becomes irregular, and the bidirectional pattern
similarly resembles a less than perfect figure-of-eight
(Fig. 12).
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2 USING THE BIDIRECTIONAL MICROPHONE
2.1 Taking Advantage of the Nulls
Most people, when using a directional microphone, just
aim it at the subject, giving little thought to the overall polar
response pattern. While this point-and-shoot approach
might work in a simple recording or public-address (PA)
situation, there is much more to consider when the going
gets rough. Careful aiming of the nulls of a microphone
pattern often can be more significant to the quality of the
sound pickup than where the principal axis is pointing.
Offending intrusive sounds such as PA, monitor, or reinforcement loudspeakers, other nearby instruments, noisy
air-conditioning equipment, or other environmental noises
can often be minimized by proper aiming of the nulls of
the microphone. By reducing these unwanted sounds, the
clarity of the pickup will increase dramatically.
2.2 Minimizing Feedback
As shown in Fig. 9, the bidirectional microphone has the
deepest null of all patterns, nearly 90 dB in the plane of
the diaphragm with a well-designed model. It is important
to realize that this null plane extends both laterally and vertically with respect to the principal axis of the pickup.
Deep nulls mean good rejection of unwanted sounds,
which can be most beneficial in sound-reinforcement situations, where feedback is always threatening.
Fig. 12. Polar response diagrams for the Neumann U87 multipattern capacitor microphone. (From [5, pp. 633–634].)
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Fig. 13 shows a typical concert setup, where a performer is downstage front and a central loudspeaker cluster is directly overhead. In this situation the loudspeaker
cluster will be 90 degrees off axis (vertically) to the microphone. Because a cardioid microphone is only 6 dB
down at 90 degrees, the potential for feedback will be
high. By using a bidirectional microphone, however, with
the deep null plane aimed directly at the cluster, the potential for feedback can be almost completely eliminated.
With the performer directly on axis, the rear lobe will be
aimed out into the audience, which is relatively much farther away so that the inverse square law will prevail to
reduce their pickup by the microphone.
Similarly, if side-fill stage monitors are used because
these also are at 90 degrees to the performer, a bidirectional microphone again will provide optimum rejection
of these for the prevention of feedback.
2.3 Reducing Environmental Noise
Out of doors or in large interior spaces such as sound
stages, factories, or warehouses, environmental or general
background noise tends to approach a microphone along
the plane of the horizon if its source is either reasonably
distant or random in nature. Because this sound wave will,
in effect, produce equal pressure on both sides of the
diaphragm of a vertically oriented bidirectional microphone (that is, the diaphragm is horizontal), this noise will
cancel and sound sources that are closer and more directly
on axis will prevail. Jim Tannenbaum, a very active film
dialog mixer in Hollywood, explained at an AES workshop
how he uses this to good effect in recording actors in a
noisy environment. By placing a bidirectional microphone
just below the camera shot and orienting its pattern vertically, the actor’s voice is picked up by the front lobe while
the rear lobe is aimed at his feet, which presumably are not
making any noise at all. The result is that the environmental noise pickup is significantly less than the direct sound
of the talent, producing clean and usable dialog (Fig. 14).
2.4 Minimizing Pickup of Nearby Instruments
(Some Case Studies)
A significant and ever-present problem in contemporary
studio recording is minimizing leakage from nearby instruments into the various microphones. While gobos can be
very effective in isolating performers from each other, they
can introduce their own set of problems. To be effective,
gobos usually are very bulky and occupy valuable floor
space. They also inhibit the ability of the musicians to hear
each other easily, thus requiring complex and often cumbersome headphone monitor mixes for the musicians.
One solution to this problem is to use bidirectional
microphones and arrange the musicians so that they are at
right angles to each other, thus placing nearby musicians
in the null of their neighbor’s microphone, and vice versa.
Although this cannot eliminate the need for gobos
entirely, it will reduce their number significantly. As a
result, the studio can be less crowded, and because the
musicians now will be better able to hear each other
OVERHEAD CENTRAL
LOUDSPEAKER CLUSTER
90° off-axis
to microphone
SIDE-FILL
MONITOR
90° off-axis
to microphone
SIDE-FILL
MONITOR
90° off-axis
to microphone
Fig. 13. By placing a bidirectional microphone so that the overhead central loudspeaker cluster is 90 degrees off axis, it will be in the
null plane of the pickup. Similarly, side-fill monitors also will be at 90 degrees to the microphone. In this arrangement, maximum
gain before feedback can be achieved.
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directly, the need for numerous monitor headphones can
also be reduced.
Another common problem in both recording and soundreinforcement situations occurs when a singer also is playing an instrument, such as guitar or piano. The need to
provide good isolation between the singer’s voice and the
instrument usually leads to the use of separate microphones for each, but this can lead to problems of balance
and phase interference between the microphones. In both
of these situations the use of a single bidirectional microphone can provide the solution.
Placing a vertically oriented bidirectional microphone
between the performer’s mouth and the guitar and adjusting its position to achieve a proper balance between the
two can provide an excellent pickup of both and, at the
same time, rejection of other nearby instruments. Of
course, if there is a monitor loudspeaker directly at the
performer’s feet, this technique will not work and separate
microphones will be required.
When the performer is seated at a piano, a bidirectional
microphone can be placed above and in front of his or her
head, aimed such that his or her voice will be directly on
axis to the front and the null plane aimed into the piano.
This will provide a clean vocal pickup with maximum
rejection of the piano which, then, can be miked separately. The rear lobe of the vocal microphone will be
aimed upward toward the ceiling, so you need to be sure
there are no hard reflections (or loudspeakers) to be heard
from this area.
In concert recording, when there is a chorus placed
behind the orchestra, it often is difficult to keep the
instruments at the back of the orchestra, usually brass or
percussion, from leaking into the choral pickup. The use
of bidirectional microphones, placed high above and
aimed downward toward the chorus and with their null
planes aimed directly at the back of the orchestra, often
will solve this problem. The front lobe of the microphones
picks up the chorus and the rear captures the immediate
reflection from the canopy over the stage, adding an extra
degree of fullness to their sound.
The exception always proves the rule. On two occasions
the author has had the opportunity of recording the
Symphony No. 8 by Gustav Mahler. By coincidence both
times a concert tuba had been seated directly in front of
the boys’ choir. Even a bidirectional microphone placed
directly in front of the choir, with the null plane aimed
straight down the bell of the tuba, was not sufficient to
keep this very powerful low-frequency instrument from
intruding on the pickup of the choir.
3 WORKING BOTH SIDES OF THE MICROPHONE
Ever since the golden days of radio in the 1930s and
1940s actors have appreciated working with bidirectional
microphones such as the RCA models 44 and 77. Not only
do these have an unsurpassed quality with the human
voice, the two-sided pickup helped to create the art of radio
acting because it allowed the actors to work on either side
of the microphone so that they were able to face and act to
and with each other. Coming into a scene meant doing little more than starting with one’s head turned slightly away
from the microphone and then turning back toward the
microphone as dialog began. If a more distant approach
was required, beginning the scene just a step or two back
and then moving toward the microphone would produce
this effect. Coming in from an even greater distance could
be accomplished by starting the dialog from the side of the
microphone and then moving around to be on axis.
Throughout all of this, the script could be held directly to
the side of the microphone, allowing the actors to read, yet
minimizing the sound of the pages rustling as they were
changed or, as was common practice, let fall to the floor.
Vocal ensembles, such as duets, trios, and quartets, also
used these microphones to good advantage by grouping
around the microphone and balancing their voices to
achieve a proper natural blend. No need to rely on a mix-
Actor
Vertically-oriented
Bidirectional Microphone
Distant or Random
Noise Source
Fig. 14. The sound wave from a distant or random noise source approaches the microphone and actor as a horizontal plane wave. If the
microphone is a vertically oriented bidirectional, the noise will be reduced significantly relative to the closer, on-axis actor’s voice.
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ing engineer to make them sound right––the musicians
did it themselves!
For working in stereo, two bidirectional microphones,
oriented at 90 degrees with respect to each other, create
the classic crossed bidirectional pair (also known as a
Blumlein pair, in recognition of Alan Blumlein who first
proposed this technique in his seminal patent of 1934).
This technique provides what many engineers have said is
the most natural sounding stereophonic image of any
microphone configuration because it provides an extremely even spread with precise and accurate localization
within the stereo stage.
As with the single bidirectional microphone, a Blumlein pair can be worked from opposite sides with equal
effect. This allows multiple actors or musicians to group
in the front and back quadrants of the microphone pair for
a full stereophonic performance. Notice, as shown in Fig.
15, that the stereo channels in the back quadrant are
reversed with respect to the front, and this must be kept in
mind when arranging the stereo stage perspective. It also
is important to realize that the two side quadrants are out
of phase with each other, so any direct sound should be
avoided here, lest it become vague and difficult to localize
or cancel entirely when summed to mono.
none exhibits more than the single-diaphragm velocity
microphone. In fact, it is the bidirectional component in
all directional microphones that renders them susceptible
to the proximity effect. Pressure microphones, on the
other hand, are not subject to it.
This rising low-frequency response at closer working
distances can be used to good effect, in particular with
male voices to give them an almost superhuman richness
and depth. Like most things in audio, however, the potential tradeoff is reduced articulation or clarity, which can
result from excessive bass response. The proximity effect
should be treated like another form of equalization and, as
such, used with care and moderation.
5 BIDIRECTIONAL MICROPHONES FOR
STEREO AND SURROUND SOUND
We already have introduced the crossed bidirectional
microphone pair shown in Fig. 15, but there is another
important stereophonic microphone configuration that
Blumlein defined in his 1934 patent, the mid/side technique, and this too has the bidirectional microphone at its
core. In fact, it is the bidirectional component that provides all of the directional information in this stereophonic
pickup technique.
The mid/side system employs two vertically coincident
microphones: a forward-facing (mid) microphone and a
laterally oriented bidirectional (side) microphone. By combining their signals via a sum-and-difference matrix, the
4 PROXIMITY EFFECT
The proximity effect, or “bass tip-up” as the British call
it, is a characteristic of all directional microphones, but
0°
45°
45°
Left +
Right +
90°
90°
Left -
Right -
180°
Fig. 15. The Blumlein microphone configuration is comprised of two coincident crossed bidirectional microphones, where the principal axis of each is coaligned with the null axis of the other.
221
left channel traditionally is the mid side signal and the
right is the mid side. Although a cardioid is shown as
the mid microphone in Fig. 16, any polar pattern can be
used. (In fact, in his original research Blumlein used an
omnidirectional as the mid component.) Further, the ratio
of mid to side also can be varied in the matrix to adjust
the width of the resulting stereophonic image. Varying
both the polar pattern of the mid microphone and the
mid-to-side ratio can produce a rich variety of stereophonic
perspectives.
By using the mid/side technique, an extremely natural
and versatile stereophonic image can be produced. Not
only can this rival or surpass any other conventional stereo
pickup, it also is the only one that is capable of providing
an almost infinite variety of stereo perspectives while
remaining fully mono compatible. Carrying this principle
even further, by employing velocity patterns oriented along
the three cardinal axes––lateral, fore/aft, and vertical––
and then matrixing these with the pressure component, the
complete spherical sound field, as captured at a point in
space, can be described. This is the essence of the SoundField microphone system, developed by Michael Gerzon
in the 1970s. Originally developed as a remote-controlled
microphone for stereophonic and ambsionic recordings,
this unique system is capable of providing a fully discrete
and completely adjustable multichannel surround sound
pickup. (For an in-depth discussion of the Blumlein,
mid/side, and SoundField microphone techniques refer to
[6]. The complete Blumlein patent of 1934 is reproduced
in its Appendix.)
6 PRACTICAL MATTERS––DO’S AND DON’T’S
OF RIBBON MICROPHONES
As observed earlier, although both ribbon and capacitor transducers can be true velocity pickups, it is the
Fig. 16. Basic mid/side to left/right conversion. Mid side produces the left channel and mid side produces the right. Note
that the mid microphone may be of any polar pattern.
222
PAPERS
ribbon microphone that is the more common. Most bidirectional capacitor microphones are dual-diaphragm
designs. Therefore a few comments on the proper handling and treatment of ribbon microphones seem to be in
order.
The first, and perhaps most important, rule with ribbon
microphones is, don’t connect them to a powered input.
Either phantom or T power can convert a ribbon microphone instantly into a blown fuse. With T power (a remote
powering system where a 12-V dc differential exists
between pins 2 and 3 of the conventional XLR input connector) this damage will be guaranteed. With phantom
power systems (where there is supposed to be no voltage
potential between pins 2 and 3), if everything is in perfect
order, there will be no problem. However, all it takes is a
poor cable, a loose connector, or an intermittent short circuit to create a slight differential voltage, just enough to
damage a ribbon microphone. Therefore it is strongly recommended that any powering on a microphone preamplifier input be turned off for about 5 minutes before a ribbon
microphone is connected. This will allow sufficient time
for the preamplifier’s internal blocking capacitors to discharge fully.
A second and equally important rule is never to blow
directly into a ribbon microphone to test it. “Poof, poof, is
this microphone working?” Not as well as it was a minute
ago. Strong air turbulence can stretch the ribbon
diaphragm, and while it may not break, it will nonetheless
change the tension of the ribbon and degrade the microphone performance significantly. Here the rule at hand is
in fact to use the back of your hand. If you can feel the air
motion on the back of your hand, do not put the microphone there unless you first provide some form of wind
protection, such as a Popper Stopper.2 Obviously outdoor
use requires special care so that the ribbon is not damaged
by wind. Indoors, however, it is also important to avoid air
turbulence. Open windows, air-conditioning systems, or
even rapid movement of the microphone, such as when
carried about or panned on a studio boom, all can be sufficient to stretch the ribbon.
While on the subject, it should be emphasized that it is
never wise to blow into a microphone, no matter what type
it is. Not only does this force dirt and moisture inside, if
the microphone is connected to a live sound system, this
strong blast of acoustical energy, when amplified, might
be sufficient to launch the loudspeaker cones right out of
their boxes.
Normal high sound-pressure-level (SPL) sound sources
do not usually pose a problem because most ribbon microphones can handle 130-dB SPL or more without difficulty.
It is only those explosive sources that produce a strong
gust of air, such as an electric bass amplifier, a guitar
being plugged (or unplugged) while the amplifier level
control is turned up fully, a kick drum, or even a very close
talking or singing voice with a lot of plosive sounds, that
require special protection. Again, just apply the back-ofthe-hand test. If the microphone is stored in a cabinet or
box, do not slam the door. This strong acoustic pressure
2 Popper
Stopper is a registered trademark of Shure, Inc.
PAPERS
impulse could be sufficient to stretch the ribbon.
Remember also that most ribbon microphones contain a
magnet that produces a fairly strong magnetic field. This
field can attract any ferric objects toward the microphone,
and, if they are small enough, they can penetrate the outer
screening and work their way inside the microphone.
Minute iron particles, sometimes known as “tramp iron,”
exist everywhere within our environment. When in close
proximity of a ribbon microphone, these can be drawn
inside and over time can build up sufficiently in the magnetic gap to rub against the ribbon, causing distortion. The
best prevention is just to keep the microphone covered
with a plastic bag when it is not actually in use. This simple procedure also protects the microphone from the air
turbulence problems discussed.
When storing the microphone, common sense is all
that is needed to protect it from excessive mechanical
shock and air turbulence. If it will be left in storage for
extended periods of time, it is a good idea to keep the
microphone upright so that the ribbon is vertical. This
will minimize the tendency of the ribbon to sag due to the
pull of gravity. Again, it is best to keep it covered until it
is being used.
7 A FEW OF THE INDUSTRY’S LEADING MIXING
ENGINEERS TALK ABOUT HOW THEY USE THE
BIDIRECTIONAL PATTERN
When I first started recording I was lucky enough to
work at a studio that had multipattern RCA 77s, and figureof-eight only 44, 74, and B&O ribbons. I used them on
horns, electric guitars, lead vocals, strings, woodwinds,
just about everything. More recently the Royer 121 joined
my microphone cabinet as my electric guitar mike of
choice.
The figure-of-eight pattern works well in live tracking
situations to isolate neighboring players, for instance,
keeping an adjacent percussionist out of an acoustic guitar
player’s microphone. I recently had a great experience
with the AEA R44 on jazz guitarist Peter White’s nylon
string guitar.
The figure-of-eight pattern also works well with background singers who are good at balancing themselves on a
single mike, such as a Neumann U87 or an AKG 414. I’ve
had wonderful experiences using this technique while
working with great singing groups like Poc, the Wilson
Sisters, and the Nitty Gritty Dirt Band performers who
understand the art of blending harmonies. The figure of
eight also works superbly with solo instrumentalists. The
back side does a good job of capturing the room tone of a
solo sax. I’ve had good experiences with people like
Grover Washington Jr., Dave Koz, Wayne Shorter, Gato
Barbieri on a single mike. I’ve also used the figure-ofeight pattern as part of an AKG C-24 MS miking setup for
recording strings. I’ve used this technique on sessions for
Elton John, Bon Jovi, The Cult, and the Kronos Quartet.
Joe Chiccarelli, Los Angeles, California, 2002 June
I think my most consistent use of the figure-of-eight
pattern is recording saxes and/or woodwinds in a Big
Band setting. It helps the separation between instruments
because the side-to-side rejection is important. The fact
that the back is open is not really an issue because anything leaking into the back is too far away to be concerned about. In this particular case I’m using Neumann
U67s or 87s.
Leslie Ann Jones, Skywalker Ranch,
Marin, California, 2002 June
The first time I used a figure-of-eight was on a background vocal track using U67s or 87s. The singers liked
the eye contact and did their own balancing on the phone
monitors. The sound was cleaner, as you might expect
with fewer electronics in the signal path.
Next I experimented with Blumlein stereo. Using a
U67, 87, or C414 pair head to head as a crossed-eight
array sounded good on classical piano, lead vocals, electric guitar, and drum overheads.
Figure-of-eight ribbon microphones are often my first
choice now. Most of my electric guitar, bass guitar, and
drum recordings are done with AEA and RCA 44s and
Coles 4038s. I use ribbons almost exclusively on brass and
woodwinds. A recent Brian Setzer Orchestra session in
Studio A at Capitol had five saxes at the conductor’s left,
four trombones in the middle, and four trumpets at his
right. The musicians formed a shallow arc, and each faced
a figure-of-eight ribbon set 2 to 3 feet in front. These
delivered great sound and exceptional isolation.
Figure-of-eight ribbons also excel as room mikes on
drums. The side nulls are aimed toward the drums and the
mikes are placed 10 to 12 feet apart and 6 to 8 feet out.
This delivers the room sound with little direct sound. It’s
similar to the sound effects trick for recording a gunshot.
The weapon is fired in a 44’s side null, so the fat room
sound is dominant.
Jeff Peters, Los Angeles, California, 2002 June
Using a pair of figure-of-eight microphones in a crossed
90 degree Blumlein pair is one of my favorite microphone
techniques, especially when recording a really good choir.
I’m amazed how few music-recording people are familiar
with this excellent technique.
I’m often asked: what is the effect you used on the choir
on Michael Jackson’s “Man in the Mirror” on his BAD
album? My answer is: there is NO effect on the choir!
Then I explain that the recording was done with only a
simple Blumlein pair. I have to admit that the rest of this
winning combination was Andre Crouch’s gospel choir,
one of the best in the world, and Westlake Audio’s gorgeous Studio D in Hollywood. This wonderful piece of
music has a graceful, natural sounding, dynamic curve to
it. From the transparent, burnished brass synthesized bells
in the intro, to the Andre Crouch choir that comes in at the
modulation, to the climax with the huge ending, I feel that
“Man in the Mirror” is the musical centerpiece of the
album.
My favorite pair of Neumann M-49s, vertically coincident and angled 45 degrees to either side of the centerline,
captured this great choir recording. It’s why Blumlein is
one of my favorite stereophonic microphone techniques
223
PAPERS
and perhaps the best known single-point stereo technique.
For me, figure-of-eight is really great!
Bruce Swedien, Ocala, Florida, 2002 June
Figure-of-eight pattern microphones were used extensively by all Capitol mixers. I could go on and on about
using them. The 44 was our standard kick drum microphone
for pop orchestras. The bass drum would be used with the
front skin removed and a sandbag inside. The 44 was laid
horizontally atop the sandbag, and this combination delivered a very tight thud sound. When doing pop-type jazz
orchestras we used the 44s for sax and woodwinds, placed
so that the dead sides did not pick up too much brass.
In symphony work I always used a 44BX for the double
bass. For Angel Classical sessions, depending on hall
acoustics, I often set the forward-facing capsule on my
Neumann SM69 or AKG C24 stereo microphones to figureof-eight and then used a mid/side decoder to provide variable angle control for the resultant virtual Blumlein pair. I
was the only mixer who frequently used the figure-of-eight
pattern on stereo mikes. The full story is quite long, but
room acoustics and reverb time strongly influenced whether
I used the stereo mikes in XY or whether I used them with
differential circuits (M/S) to further control the balance. In
the studio at Capitol I used M/S with differential balance
controls a great deal.
Carson Taylor, Danville, California, 2002 April
8 REFERENCES AND RECOMMENDED
READINGS
[1] H. F. Olson, “A History of High-Quality Studio
Microphones,” presented at the 55th Convention of the
Audio Engineering Society, J. Audio Eng. Soc. (Abstracts),
vol. 24, p. 862 (1976 Dec.), preprint 1150, also published
in Microphones [2, pp. 219–228].
[2] Microphones, An Anthology of Technical
Papers (Audio Engineering Society, New York, NY,
1979).
[3] H. Tremaine, Audio Cyclopedia (Howard W. Sams,
Indianapolis, IN, 1969).
[4] J. Eargle, The Microphone Book (Focal Press,
Boston, MA, 2001).
[5] C. Woolf, Ed., Microphone Data (Human-Computer
Interface Limited, UK, 2001).
[6] R. Streicher and F. A. Everest, The New Stereo
Soundbook, 2nd ed. (Audio Engineering Asso., Pasadena,
CA, 1998).
9 ASSOCIATED READING
J. Borwick, Microphones––Technology and Technique
(Focal Press, London, UK, 1990).
M. Gayford, Ed., Microphone Engineering Handbook
(Focal Press, London, UK, 1994).
THE AUTHORS
224
R. Streicher
W. Dooley
Ron Streicher received a B.A. degree in music from the
University of California and an M.A. degree in communications arts from Loyola University, both in Los Angeles.
Pursuing a lifelong involvement in music, his interest in
audio began in 1963 while serving as a volunteer for the
music department of a public radio station in Los Angeles;
that avocation subsequently evolved into his career. His
many broadcast projects include sound design and production of radio plays, national syndication of the Los
Angeles Philharmonic Orchestra concerts, and chamber
music concerts from throughout California. His work has
been heard over National Public Radio and the Public
Broadcasting System networks.
Continuing to be involved with live music performance
and production, Mr. Streicher joined the engineering staff
and faculty of the Audio Recording Institute at the Aspen
Music Festival and School in 1988; since 1997 he has
served as its Audio Production Manager. For eleven summers prior to Aspen, he designed and supervised concert
sound reinforcement for the Philadelphia Orchestra, the
Metropolitan Opera, and the New York City Opera productions at the Mann Music Center in Philadelphia. His
recording projects have taken him as far afield as Karachi,
Shanghai, throughout Europe, and twice to Moscow to
record the Bolshoi Theatre Orchestra. He has engineered
recordings for Angel, Brio, CMS Desto, CRI, Discovery,
Koch International, Omega Record Classics, RCA, Pilz,
Protone, and SAZ Records. He also produced two projects
for the AES: the CD “Graham Blyth in Concert” and the
video “An Afternoon with Jack Mullin.”
A fellow of the Audio Engineering Society, Mr.
Streicher just completed eleven years as the secretary
of the AES and is currently president-elect. He is
actively involved with the Society’s educational activiJ. Audio Eng. Soc., Vol. 51, No. 4, 2003 April
PAPERS
ties and has given numerous presentations at local and
international meetings. In recognition of his long-term
service to the Society, he was awarded the AES Bronze
Medal in 1995.
●
Wes Dooley’s speciality is on-location recording, and
his experiences in the United States, Europe, Africa, and
New Zealand led him to develop portable recording tools
such as multichannel microphone arrays, mid/side stereo
processors, stereo phase displays, and very tall microphone stands. His company is dedicated to creating products that further the art and science of recording.
Ribbon microphones are what Mr. Dooley has become
best known for. He has represented and serviced Coles’
4038 ribbon microphones in the United States for the past
two decades. During the 1990s he observed that RCA 44
“collectors” were taking these microphones out of circulation, making it difficult for recording studios to own or use
a 44. Its rebirth became Mr. Dooley’s crusade and resulted
in the AEA R44, a faithful recreation of this classic microphone. Introducing a widening circle of modern recordists
to ribbon mikes has been a fulfilling task. His latest opus,
the AEA R84 large ribbon geometry microphone, was
introduced at the Fall 2002 AES convention in Los Angeles.
Mr. Dooley and Mr. Streicher previously have coauthored two papers about stereo microphone techniques
published in the AES Journal and the Stereophonic
Techniques Anthology. Mr. Dooley is a fellow of the AES.
He has chaired workshops on microphone techniques and
mixing strategies for compatible multiple releases for cinema, broadcast, and videocassette, has presented section
meetings on stereo techniques and forensic audio, and has
participated on panels at many meetings of the AES and
other technical organizations. A former governor and
vice-president (Western Region) of the Society, he
remains involved with AES standards work and currently
serves on the SC-03-12 Working Group on Forensic
Audio, where he heads a writing group on Forensic Audio
Standards. He is also a member of the SC-04-04 Working
Group on Microphone Measurement and Characterization. Also an amateur audio historian, Mr. Dooley
cochaired the Audio History Room at the Fall 2002 AES
convention in Los Angeles.
225

The Bidirectional Microphone: A Forgotten Patriarch *

Transcription

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