the bidirectional microphone

PAPERS

0 INTRODUCTION

In the beginning of electrical recording there was theomnidirectional microphone, and it was good, for it pickedup sound equally well from all directions, was relativelyeasy and inexpensive to build, and, perhaps most impor-tant, it freed the performer from having to stare down themouth of the large black horn that made a direct acousticalconnection to the cutting head on the recording lathe.

Next came the bidirectional microphone, and it too wasgood, for it offered the user directionality, that is, morecontrol over what it heard. By picking up sound equallyfrom the front and the back it enabled the performers towork on either side, facing each other. At the same time itrejected unwanted sound coming from the sides, such asother performers and room noises. It also was relativelyeasy and inexpensive to build.

Through the generations these two begot all the rest.Their progeny has not always been as good, however, andit certainly is neither easy nor inexpensive to build them.Progress moves in mysterious ways.

The omnidirectional and bidirectional microphonestogether can be considered as the matriarch and the patri-arch of all other polar patterns. Why, then, is the bidirec-tional microphone the forgotten patriarch in the micro-phone locker?

Every sound engineer knows intuitively that a cardioidmicrophone picks up what it is aimed at, and this is goodenough for most sound-reinforcement and recording appli-cations. The omnidirectional microphone, which gatherssound from all directions equally, is most often used forrecording, but rarely is it used for sound reinforcementbecause of its relative inability to reject feedback whenused at a distance from the sound subject. It is these twomicrophone types, and their common variations, thataccount for nearly 90% of all microphones sold. Amongthe remainder are the shotgun and, yes, the bidirectional.

Although the bidirectional microphone has been withus almost since the earliest days of electrical recording, itremains the least appreciated and used of the polar pat-terns available in the modern microphone locker becauseit is not well enough understood how to take advantage ofits unique polar pattern. How do you handle the rear-lobepickup? What can you do with the null plane? This is amicrophone that makes you think about how to use it!

1 MICROPHONE POLAR RESPONSE––A BIT OFHISTORY AND THE BASICS

1.1 Omnidirectional MicrophoneAll microphones respond to the motion of air particles

from which they generate analogous electric signals. Thusthey are transducers, converting one form of energy (airmotion) into another (electric). This may seem simpleenough, but how they go about this task is anything butsimple.

J. Audio Eng. Soc., Vol. 51, No. 3, 2003 March 211

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 remainssomewhat of a mystery to many recording and sound-reinforcement engineers. The bidirec-tional microphone is examined from historical, technical, and operational perspectives. It isreviewed how it was developed and exists as a fundamental element of almost all other single-order microphone patterns. In the course of describing how this unique pattern responds tosound waves arriving from different angles of incidence, it is shown that very often it can beemployed successfully where other more commonly used microphones cannot.

* Presented at the 113th Convention of the Audio EngineeringSociety, Los Angeles, CA, 2002 October 5–8. Specific facts forthis paper were derived from Harry F. Olson in [2, pp. 219–228].

STREICHER AND DOOLEY PAPERS

Like a barometer, the first practical microphonesresponded to the changes in air pressure caused by thecompressions and rarefactions of a sound wave as it radi-ated outward from the source and impinged on the micro-phone diaphragm (Fig. 1). (Compressions exist where theair particle density is greater than the average pressure;rarefactions are where the density is less than the averagepressure.) These were called, naturally enough, pressuremicrophones.

A microphone diaphragm moves only when there is adifference in the air particle density between its front andback. As a sound wave reaches the microphone, it causesthe diaphragm to move in direct response to thesechanges in air pressure. With a pressure microphone, thediaphragm covers a sealed chamber. The air within thischamber has a fixed air particle density. Thus no matterfrom what direction it approaches the microphone, thesound wave will cause the diaphragm to move inward ifthe pressure is greater, or outward if it is less than the

density inside the chamber. Because they respond equallyto sound coming from all directions, pressure micro-phones became known as omnidirectional. The polarequation for the omnidirectional pattern is ρ � 1. This isa scalar function, because it indicates magnitude, irre-spective of direction.

One of the earliest commercial microphones, theWestern Electric model 618A (developed in the mid-1920s), was a moving-coil-type transducer. Fig. 2 shows asimplified functional schematic. A lightweight coil of wirewas glued to the back of a very thin diaphragm and sur-rounded by a magnet. As the sound wave caused thediaphragm to move, the coil was moved similarly withinthe magnetic field. This is the essence of a small motorgenerator which, in turn, creates a very small electric cur-rent corresponding to the original sound wave.

Also developed in the mid-1920s, the Western Electricmodel 394 and the RCA model 11A were the first capacitor-type microphones. These also were pressure transducers.

212 J. Audio Eng. Soc., Vol. 51, No. 4, 2003 April

Fig. 2. Simplified drawing of a typical moving-coil-type pressure sound wave omnidirectional microphone. The output is directly pro-portional to the motion of the diaphragm caused by the variations in air pressure as the sound wave passes by.

Diaphragm

Voicecoil

Magnet Structure

Motion of Diaphragmwhen excited by thechanges in air pressurecaused by Soundwaves

Microphone Housing

2 1

3

Soundwaves can approachthe diaphragm from anydirection.

Fig. 1. As sound waves radiate outward from the source, they produce alternating compression (positive pressure) and rarefaction (neg-ative pressure) in the air.

-+

- RAREFACTION(negative pressure)

+ COMPRESSION(positive pressure)

- - - - - -+ + +

++

+

PAPERS BIDIRECTIONAL MICROPHONE

1.2 Bidirectional MicrophoneIn the early 1930s a fundamentally different type of

microphone was developed, the pressure-gradient micro-phone. Like the omnidirectional microphone, this alsomoved in response to the difference in pressure betweenthe front and the back of the diaphragm as the sound wavepassed by. However, in this microphone the diaphragm(which in these early versions was a very thin aluminumribbon) was exposed on both sides. Thus as the soundwave moved past it, it created a very slight but nonethelessdistinct difference in the air pressure on either side of the

ribbon. This ribbon was suspended in a magnetic field andthus generated a small electric current in direct response toits movement. The ribbon microphone, like the moving-coil type, proved to be environmentally stable, easy tomaintain, and more reliable than capacitor microphones ofthe period (Fig. 3).

Inherent to this design is the fact that sound waves com-ing toward the microphone directly from either the frontor the back will be picked up with equal sensitivity. Theonly difference will be the absolute polarity of the electricoutput: sounds arriving from the back will produce polar-ity opposite to those arriving from the front (Fig. 4). This

J. Audio Eng. Soc., Vol. 51, No. 4, 2003 April 213

Fig. 4. (a) As the sound wave approaches from the front of the diaphragm, positive pressure produces a positive voltage at the outputof the microphone. (b) As the sound wave approaches from the back of the diaphragm, positive pressure produces a negative voltageat the output of the microphone.

(a) (b)

PositiveAir

Pressure

Positive VoltageOutput

2 1

3 PositiveAir

Pressure

Negative VoltageOutput

2 1

3

Fig. 3. (a) Simplified drawing of a typical ribbon-type pressure-gradient bidirectional microphone. (b) The output of the pressure-gradient ribbon microphone is directly proportional to the differences in pressure induced on the front and back of the ribbon as thesound wave (compressions and rarefactions) passes by.

(b)

As the Soundwave passesthe Ribbon Diaphragm theCompressions andRarefactions result in adifference in pressure onthe front and back of theribbon.

2 1

3

(a)

Simplified Side View(shown without side

Pole Pieces for clarity)

Simplified Front View

N S

Ribbon Diaphragm

Magnet Structure

Pole Pieces

Ribbon clampsalso serve ascontact terminals


two-sided response led these to be termed bidirectionalmicrophones. (They also are commonly called figure-of-eight microphones because of the obvious shape of theirpolar response.) The polar equation for the bidirectionalpattern is ρ � cos θ, where θ signifies the angle of inci-dence of the sound as it approaches the microphone.Because it indicates both magnitude and direction, this isa vector function. The term velocity microphone also isoften applied to ribbon microphones because the currentin the ribbon is directly proportional to the velocity of itsmotion in the magnetic field.

The significant operational difference between the bidi-rectional microphone and the omnidirectional one is thatwhile the omni responds to sounds arriving from any andall directions with equal sensitivity, with a properlydesigned single-diaphragm bidirectional microphone aresponse null of almost �90 dB will occur at precisely 90degrees from the principal pickup axis. Fig. 5 shows thatthis null exists both vertically and horizontally because asound wave approaching the microphone along the planeof the diaphragm will produce equal pressure on bothsides of the diaphragm. If there is no difference in pressureon the front and the back of the diaphragm, there will beno output signal. Because this null plane affects both sidesof the diaphragm equally, the figure-of-eight polarresponse will be uniform with respect to frequency.

1.3 Deriving Other Polar PatternsIt is not within the scope or intent of this paper to dis-

cuss in detail the wide variety of other microphone polarpatterns. Suffice it to say here that all first-order micro-phone patterns can be represented mathematically as somecombination of omnidirectional (pressure) and bidirec-tional (pressure-gradient) components. In fact, the firstpractical cardioid microphone was developed by HarryOlson in 1931 and released in 1933 as the RCA 77A uni-directional ribbon. As shown in Fig. 6(a), it utilized a long

ribbon clamped in the middle. The lower half was exposedon both sides, functioning as a conventional pressure-gradient pickup, and the upper half was coupled at the rearto an acoustically damped chamber so that it operated likea pressure-response pickup.1 Thus the two halves of theribbon responded to both the pressure and the particlevelocity components of the sound wave, and because bothhalves worked within a common magnetic field, theircombined output resulted in a cardioid pickup pattern.

The RCA model 77D shown in Fig. 6(b) was developedin the early 1940s and employed a rotating shutterbetween the ribbon and a damped chamber. This “polydi-rectional” microphone offered selectable patterns byadjusting the amount of damping on the ribbon to achievean omnidirectional, a unidirectional, or a bidirectionalpolar pattern. The final version, the RCA 77DX, remainedin production until the mid-1970s.

At about the same time a very different approach wasemployed by Western Electric to create a cardioid micro-


1 The diagrams of Fig. 6 were taken from Harry Olson [1],who was head of the RCA acoustical research division from1934 to 1967. This paper, first published in 1976 and included inthe Audio Engineering Society’s anthology [2] provides detaileddescriptions of many of the evolutionary developments in micro-phone technology.

Fig. 5. As the sound wave approaches directly along the plane ofthe ribbon (that is �90 degrees from the front), it produces equalpressure on both sides of the diaphragm. Because this results inno pressure gradient, there will be no output.

Fig. 6. (a) RCA model 77A unidirectional ribbon microphone.(b) RCA model 77D multipattern ribbon microphone.

(b)

(a)

Equal Pressureon both sidesof the ribbon

No Output

OR

2 1

3


phone pattern. They used separate omnidirectional moving-coil and bidirectional ribbon transducers summed electri-cally and enclosed within a common housing. This was theclassic model 639A cardioid microphone. As shown inFig. 7, later versions offered the user the ability to selectfrom among multiple patterns––omnidirectional, bidirec-tional, and three variations of cardioid––by adjusting therelative amounts of the pressure and velocity componentsin the combined output. Released in 1939, the Altec(Western Electric) model 639B was the first commercialswitch-selectable, multipattern microphone.

1.4 Polar Equations for Microphone PolarPatterns

As noted earlier, the polar equation for an omnidirec-tional pattern is ρ � 1, and the polar equation for a bidi-rectional microphone is ρ � cos θ. These are scalar andvector functions, respectively, and as such they describethe essential components of any sound wave measured at

a point in space. Fig. 8 shows that by combining these twopatterns equally, the result is a cardioid pattern. The polarequation for the cardioid can be expressed as ρ � 1⁄2 (1 �cos θ).

In general terms, the polar equation for any first-ordermicrophone polar pattern can be represented by the equa-tion ρ � a � b cos θ, where a � b � 1 and the values ofa and b represent the relative amplitudes of the omnidi-rectional and bidirectional components, respectively. Fig.9 illustrates some of the most commonly used microphonepatterns. Note that the pickup characteristics termed ran-dom energy response, distance factor, and directivityindex describe how the various polar patterns relate totheir sonic environment.

The random energy response figures describe how eachpattern compares to the omnidirectional pattern in thepickup of the entire sonic environment. For example, ifexposed to the same total acoustic power coming from alldirections, the output of a cardioid will be about one-third


<<< Photo and Line Drawing ofAltec/Western Electric 639>>>

Fig. 7. (a) (Altec Western Electric) model 639B, the first commerical multipattern microphone, which derived its polar pattern by com-bining a moving-coil pressure transducer with a ribbon pressure-gradient transducer together in a common housing. (b) Simplifiedschematic diagram of the 639B. (From [3, pp. 177–178, fig. 4-66].)

(b)

(a)

Moving-coilPressurePickup

RibbonPressure-gradientPickup

+ -

+

-

2 1

3


that of an omnidirectional one. This is particularly usefulwhen determining the ratio of direct to reverberant soundin a microphone pickup.

The directivity index is a measure of the increased sen-sitivity on axis versus off axis for the various polar pat-terns, again stated relative to the omnidirectional micro-phone as the reference.

The distance factor is simply another way of express-ing the directivity index. Here it is stated as a measure of

the relative distance between the microphone and thesound source. For example, to achieve the same direct-to-ambient ratio, a cardioid can be used at 1.7 times the dis-tance as an omnidirectional microphone.

1.5 Not All Bidirectional Microphones AreCreated Equal

The ribbon microphone referred to is a single-diaphragmtransducer, and so are some capacitor microphones. While


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.)

Fig. 8. Cardioid pattern, the result of combining an omnidirectional and a bidirectional pickup equally.

ρ = 1 ρ = cos θ

ρ = a + b cos θ


there are both ribbon and capacitor microphones that offervariable patterns utilizing purely acoustical means, mostmodern studio capacitor microphones that provide multi-ple switch-selectable patterns accomplish this by combin-ing the electric outputs of two cardioid diaphragms mountedback to back on a common back plate. The German engi-neers Braunmühl and Weber obtained a patent for thistechnology in 1935 (Fig. 10). While single-pattern dual-

diaphragm capacitor microphones were manufactured byNeumann in the early 1930s, the first commercial switch-selectable multipattern capacitor microphone utilizing thisdesign was the Neumann model U47, issued in 1949.

Combining the signals from the back-to-back cardioidsof a Braunmühl–Weber capsule produces the basic pat-terns shown in Fig. 11. In addition to the three primarypatterns shown, by adjusting the relative amplitudes of


Fig. 11. By adding the front and back diaphragm signals from a dual-diaphragm microphone capsule a multipattern microphone canbe created. This is the principle behind the original multipattern capacitor microphone developed by Braunmühl and Weber. There arethree basic polar patterns. (a) Adding no signal from the back diaphragm leaves just the front cardioid pattern. (b) Adding the backdiaphragm signal in phase produces an omnidirectional pattern. (c) Adding the back diaphragm signal in reverse phase produces a bidi-rectional pattern.

DIAPHRAGMDIAPHRAGM

BACKPLATE(FIXED ELECTRODE)

INSULATOR

ELECTRODE LEADELECTRODE LEAD

ACOUSTICAL PORTSTHROUGH BACKPLATE

BACKPLATE LEAD

Fig. 10. Braunmühl–Weber multipattern capacitor microphone capsule, first described in 1935. It has two diaphragms on either sideof a common back plate. Each is essentially a cardioid pattern, and when their signals are combined electrically, all first-order polarpatterns may be created.

(a)

(b)

(c)


these two cardioid signals, a multitude of intermediatepatterns can also be generated. It is important to under-stand that it is not actually cardioid patterns per se that arebeing combined but their respective omnidirectional andbidirectional components. Also, because they are facing inopposite directions, these components are in antipolaritywith respect to each other. Although these microphonesare combining cardioid signals electronically, it is actuallythe omnidirectional and bidirectional components of thesesignals that are being added and subtracted to achieve allof the polar patterns produced by the dual-diaphragmmultipattern microphone.

When done with precision, the polar patterns thus cre-ated can be nearly as uniform as their single-pattern coun-terparts. When the response becomes less than ideal, it ismost often as the sound wave approaches from close to theplane of the capsule diaphragms, that is, near 90 degreesoff axis. Here, because of the physical construction of themicrophone housing and the spacing between the twodiaphragms, minute differences exist between the respec-tive electric signals of the diaphragms, so that when theyare combined, some interference cancellations occur. As aresult, the off-axis response of these microphones athigher frequencies may be compromised. The omnidirec-tional pattern tends to become constricted at the sides, thecardioid becomes irregular, and the bidirectional patternsimilarly resembles a less than perfect figure-of-eight(Fig. 12).

2 USING THE BIDIRECTIONAL MICROPHONE

2.1 Taking Advantage of the NullsMost people, when using a directional microphone, just

aim it at the subject, giving little thought to the overall polarresponse pattern. While this point-and-shoot approachmight work in a simple recording or public-address (PA)situation, there is much more to consider when the goinggets rough. Careful aiming of the nulls of a microphonepattern often can be more significant to the quality of thesound pickup than where the principal axis is pointing.Offending intrusive sounds such as PA, monitor, or rein-forcement loudspeakers, other nearby instruments, noisyair-conditioning equipment, or other environmental noisescan often be minimized by proper aiming of the nulls ofthe microphone. By reducing these unwanted sounds, theclarity of the pickup will increase dramatically.

2.2 Minimizing FeedbackAs shown in Fig. 9, the bidirectional microphone has the

deepest null of all patterns, nearly �90 dB in the plane ofthe diaphragm with a well-designed model. It is importantto realize that this null plane extends both laterally and ver-tically 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 situ-ations, where feedback is always threatening.


Fig. 12. Polar response diagrams for the Neumann U87 multipattern capacitor microphone. (From [5, pp. 633–634].)


Fig. 13 shows a typical concert setup, where a per-former is downstage front and a central loudspeaker clus-ter is directly overhead. In this situation the loudspeakercluster will be 90 degrees off axis (vertically) to the micro-phone. Because a cardioid microphone is only �6 dBdown at 90 degrees, the potential for feedback will behigh. By using a bidirectional microphone, however, withthe deep null plane aimed directly at the cluster, the poten-tial for feedback can be almost completely eliminated.With the performer directly on axis, the rear lobe will beaimed out into the audience, which is relatively much far-ther away so that the inverse square law will prevail toreduce their pickup by the microphone.

Similarly, if side-fill stage monitors are used becausethese also are at 90 degrees to the performer, a bidirec-tional microphone again will provide optimum rejectionof these for the prevention of feedback.

2.3 Reducing Environmental NoiseOut of doors or in large interior spaces such as sound

stages, factories, or warehouses, environmental or generalbackground noise tends to approach a microphone alongthe plane of the horizon if its source is either reasonablydistant or random in nature. Because this sound wave will,in effect, produce equal pressure on both sides of thediaphragm of a vertically oriented bidirectional micro-phone (that is, the diaphragm is horizontal), this noise willcancel and sound sources that are closer and more directlyon axis will prevail. Jim Tannenbaum, a very active film

dialog mixer in Hollywood, explained at an AES workshophow he uses this to good effect in recording actors in anoisy environment. By placing a bidirectional microphonejust below the camera shot and orienting its pattern verti-cally, the actor’s voice is picked up by the front lobe whilethe rear lobe is aimed at his feet, which presumably are notmaking any noise at all. The result is that the environmen-tal noise pickup is significantly less than the direct soundof 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 contemporarystudio recording is minimizing leakage from nearby instru-ments into the various microphones. While gobos can bevery effective in isolating performers from each other, theycan introduce their own set of problems. To be effective,gobos usually are very bulky and occupy valuable floorspace. They also inhibit the ability of the musicians to heareach other easily, thus requiring complex and often cum-bersome headphone monitor mixes for the musicians.

One solution to this problem is to use bidirectionalmicrophones and arrange the musicians so that they are atright angles to each other, thus placing nearby musiciansin the null of their neighbor’s microphone, and vice versa.Although this cannot eliminate the need for gobosentirely, it will reduce their number significantly. As aresult, the studio can be less crowded, and because themusicians now will be better able to hear each other


Fig. 13. By placing a bidirectional microphone so that the overhead central loudspeaker cluster is 90 degrees off axis, it will be in thenull plane of the pickup. Similarly, side-fill monitors also will be at �90 degrees to the microphone. In this arrangement, maximumgain before feedback can be achieved.

OVERHEAD CENTRALLOUDSPEAKER CLUSTER

SIDE-FILLMONITOR

SIDE-FILLMONITOR

90° off-axisto microphone




directly, the need for numerous monitor headphones canalso be reduced.

Another common problem in both recording and sound-reinforcement situations occurs when a singer also is play-ing an instrument, such as guitar or piano. The need toprovide good isolation between the singer’s voice and theinstrument usually leads to the use of separate micro-phones for each, but this can lead to problems of balanceand phase interference between the microphones. In bothof these situations the use of a single bidirectional micro-phone can provide the solution.

Placing a vertically oriented bidirectional microphonebetween the performer’s mouth and the guitar and adjust-ing its position to achieve a proper balance between thetwo can provide an excellent pickup of both and, at thesame time, rejection of other nearby instruments. Ofcourse, if there is a monitor loudspeaker directly at theperformer’s feet, this technique will not work and separatemicrophones will be required.

When the performer is seated at a piano, a bidirectionalmicrophone can be placed above and in front of his or herhead, aimed such that his or her voice will be directly onaxis to the front and the null plane aimed into the piano.This will provide a clean vocal pickup with maximumrejection of the piano which, then, can be miked sepa-rately. The rear lobe of the vocal microphone will beaimed upward toward the ceiling, so you need to be surethere are no hard reflections (or loudspeakers) to be heardfrom this area.

In concert recording, when there is a chorus placedbehind the orchestra, it often is difficult to keep theinstruments at the back of the orchestra, usually brass orpercussion, from leaking into the choral pickup. The useof bidirectional microphones, placed high above andaimed downward toward the chorus and with their nullplanes aimed directly at the back of the orchestra, oftenwill solve this problem. The front lobe of the microphonespicks up the chorus and the rear captures the immediate

reflection from the canopy over the stage, adding an extradegree of fullness to their sound.

The exception always proves the rule. On two occasionsthe author has had the opportunity of recording theSymphony No. 8 by Gustav Mahler. By coincidence bothtimes a concert tuba had been seated directly in front ofthe boys’ choir. Even a bidirectional microphone placeddirectly in front of the choir, with the null plane aimedstraight down the bell of the tuba, was not sufficient tokeep this very powerful low-frequency instrument fromintruding on the pickup of the choir.

3 WORKING BOTH SIDES OF THE MICROPHONE

Ever since the golden days of radio in the 1930s and1940s actors have appreciated working with bidirectionalmicrophones such as the RCA models 44 and 77. Not onlydo these have an unsurpassed quality with the humanvoice, the two-sided pickup helped to create the art of radioacting because it allowed the actors to work on either sideof the microphone so that they were able to face and act toand with each other. Coming into a scene meant doing lit-tle more than starting with one’s head turned slightly awayfrom the microphone and then turning back toward themicrophone as dialog began. If a more distant approachwas required, beginning the scene just a step or two backand then moving toward the microphone would producethis effect. Coming in from an even greater distance couldbe accomplished by starting the dialog from the side of themicrophone and then moving around to be on axis.Throughout all of this, the script could be held directly tothe side of the microphone, allowing the actors to read, yetminimizing the sound of the pages rustling as they werechanged or, as was common practice, let fall to the floor.

Vocal ensembles, such as duets, trios, and quartets, alsoused these microphones to good advantage by groupingaround the microphone and balancing their voices toachieve a proper natural blend. No need to rely on a mix-


Fig. 14. The sound wave from a distant or random noise source approaches the microphone and actor as a horizontal plane wave. If themicrophone is a vertically oriented bidirectional, the noise will be reduced significantly relative to the closer, on-axis actor’s voice.

Vertically-orientedBidirectional Microphone

Distant or RandomNoise Source

Actor


ing engineer to make them sound right––the musiciansdid it themselves!

For working in stereo, two bidirectional microphones,oriented at 90 degrees with respect to each other, createthe classic crossed bidirectional pair (also known as aBlumlein pair, in recognition of Alan Blumlein who firstproposed this technique in his seminal patent of 1934).This technique provides what many engineers have said isthe most natural sounding stereophonic image of anymicrophone configuration because it provides an extre-mely even spread with precise and accurate localizationwithin the stereo stage.

As with the single bidirectional microphone, a Blum-lein pair can be worked from opposite sides with equaleffect. This allows multiple actors or musicians to groupin the front and back quadrants of the microphone pair fora full stereophonic performance. Notice, as shown in Fig.15, that the stereo channels in the back quadrant arereversed with respect to the front, and this must be kept inmind when arranging the stereo stage perspective. It alsois important to realize that the two side quadrants are outof phase with each other, so any direct sound should beavoided here, lest it become vague and difficult to localizeor cancel entirely when summed to mono.

4 PROXIMITY EFFECT

The proximity effect, or “bass tip-up” as the British callit, is a characteristic of all directional microphones, but

none exhibits more than the single-diaphragm velocitymicrophone. In fact, it is the bidirectional component inall directional microphones that renders them susceptibleto the proximity effect. Pressure microphones, on theother hand, are not subject to it.

This rising low-frequency response at closer workingdistances can be used to good effect, in particular withmale voices to give them an almost superhuman richnessand depth. Like most things in audio, however, the poten-tial tradeoff is reduced articulation or clarity, which canresult from excessive bass response. The proximity effectshould be treated like another form of equalization and, assuch, used with care and moderation.

5 BIDIRECTIONAL MICROPHONES FORSTEREO AND SURROUND SOUND

We already have introduced the crossed bidirectionalmicrophone pair shown in Fig. 15, but there is anotherimportant stereophonic microphone configuration thatBlumlein defined in his 1934 patent, the mid/side tech-nique, and this too has the bidirectional microphone at itscore. In fact, it is the bidirectional component that pro-vides all of the directional information in this stereophonicpickup technique.

The mid/side system employs two vertically coincidentmicrophones: a forward-facing (mid) microphone and alaterally oriented bidirectional (side) microphone. By com-bining their signals via a sum-and-difference matrix, the


Fig. 15. The Blumlein microphone configuration is comprised of two coincident crossed bidirectional microphones, where the princi-pal axis of each is coaligned with the null axis of the other.

Left +

Left -

Right +

Right -

0°

45° 45°

90° 90°

180°


left channel traditionally is the mid � side signal and theright is the mid � side. Although a cardioid is shown asthe mid microphone in Fig. 16, any polar pattern can beused. (In fact, in his original research Blumlein used anomnidirectional as the mid component.) Further, the ratioof mid to side also can be varied in the matrix to adjustthe width of the resulting stereophonic image. Varyingboth the polar pattern of the mid microphone and themid-to-side ratio can produce a rich variety of stereophonicperspectives.

By using the mid/side technique, an extremely naturaland versatile stereophonic image can be produced. Notonly can this rival or surpass any other conventional stereopickup, it also is the only one that is capable of providingan almost infinite variety of stereo perspectives whileremaining fully mono compatible. Carrying this principleeven further, by employing velocity patterns oriented alongthe three cardinal axes––lateral, fore/aft, and vertical––and then matrixing these with the pressure component, thecomplete spherical sound field, as captured at a point inspace, can be described. This is the essence of the Sound-Field microphone system, developed by Michael Gerzonin the 1970s. Originally developed as a remote-controlledmicrophone for stereophonic and ambsionic recordings,this unique system is capable of providing a fully discreteand completely adjustable multichannel surround soundpickup. (For an in-depth discussion of the Blumlein,mid/side, and SoundField microphone techniques refer to[6]. The complete Blumlein patent of 1934 is reproducedin its Appendix.)

6 PRACTICAL MATTERS––DO’S AND DON’T’SOF RIBBON MICROPHONES

As observed earlier, although both ribbon and capaci-tor transducers can be true velocity pickups, it is the

ribbon microphone that is the more common. Most bidi-rectional capacitor microphones are dual-diaphragmdesigns. Therefore a few comments on the proper han-dling and treatment of ribbon microphones seem to be inorder.

The first, and perhaps most important, rule with ribbonmicrophones is, don’t connect them to a powered input.Either phantom or T power can convert a ribbon micro-phone instantly into a blown fuse. With T power (a remotepowering system where a 12-V dc differential existsbetween pins 2 and 3 of the conventional XLR input con-nector) this damage will be guaranteed. With phantompower systems (where there is supposed to be no voltagepotential between pins 2 and 3), if everything is in perfectorder, there will be no problem. However, all it takes is apoor cable, a loose connector, or an intermittent short cir-cuit to create a slight differential voltage, just enough todamage a ribbon microphone. Therefore it is strongly rec-ommended that any powering on a microphone preampli-fier input be turned off for about 5 minutes before a ribbonmicrophone is connected. This will allow sufficient timefor the preamplifier’s internal blocking capacitors to dis-charge fully.

A second and equally important rule is never to blowdirectly into a ribbon microphone to test it. “Poof, poof, isthis microphone working?” Not as well as it was a minuteago. Strong air turbulence can stretch the ribbondiaphragm, and while it may not break, it will nonethelesschange the tension of the ribbon and degrade the micro-phone performance significantly. Here the rule at hand isin fact to use the back of your hand. If you can feel the airmotion on the back of your hand, do not put the micro-phone there unless you first provide some form of windprotection, such as a Popper Stopper.2 Obviously outdooruse requires special care so that the ribbon is not damagedby wind. Indoors, however, it is also important to avoid airturbulence. Open windows, air-conditioning systems, oreven rapid movement of the microphone, such as whencarried about or panned on a studio boom, all can be suf-ficient to stretch the ribbon.

While on the subject, it should be emphasized that it isnever wise to blow into a microphone, no matter what typeit is. Not only does this force dirt and moisture inside, ifthe microphone is connected to a live sound system, thisstrong blast of acoustical energy, when amplified, mightbe sufficient to launch the loudspeaker cones right out oftheir boxes.

Normal high sound-pressure-level (SPL) sound sourcesdo not usually pose a problem because most ribbon micro-phones can handle 130-dB SPL or more without difficulty.It is only those explosive sources that produce a stronggust of air, such as an electric bass amplifier, a guitarbeing plugged (or unplugged) while the amplifier levelcontrol is turned up fully, a kick drum, or even a very closetalking or singing voice with a lot of plosive sounds, thatrequire special protection. Again, just apply the back-of-the-hand test. If the microphone is stored in a cabinet orbox, do not slam the door. This strong acoustic pressure


2 Popper Stopper is a registered trademark of Shure, Inc.

Fig. 16. Basic mid/side to left/right conversion. Mid � side pro-duces the left channel and mid � side produces the right. Notethat the mid microphone may be of any polar pattern.


impulse could be sufficient to stretch the ribbon.Remember also that most ribbon microphones contain a

magnet that produces a fairly strong magnetic field. Thisfield can attract any ferric objects toward the microphone,and, if they are small enough, they can penetrate the outerscreening and work their way inside the microphone.Minute iron particles, sometimes known as “tramp iron,”exist everywhere within our environment. When in closeproximity of a ribbon microphone, these can be drawninside and over time can build up sufficiently in the mag-netic gap to rub against the ribbon, causing distortion. Thebest prevention is just to keep the microphone coveredwith a plastic bag when it is not actually in use. This sim-ple procedure also protects the microphone from the airturbulence problems discussed.

When storing the microphone, common sense is allthat is needed to protect it from excessive mechanicalshock and air turbulence. If it will be left in storage forextended periods of time, it is a good idea to keep themicrophone upright so that the ribbon is vertical. Thiswill minimize the tendency of the ribbon to sag due to thepull of gravity. Again, it is best to keep it covered until itis being used.

7 A FEW OF THE INDUSTRY’S LEADING MIXINGENGINEERS TALK ABOUT HOW THEY USE THEBIDIRECTIONAL PATTERN

When I first started recording I was lucky enough towork at a studio that had multipattern RCA 77s, and figure-of-eight only 44, 74, and B&O ribbons. I used them onhorns, electric guitars, lead vocals, strings, woodwinds,just about everything. More recently the Royer 121 joinedmy microphone cabinet as my electric guitar mike ofchoice.

The figure-of-eight pattern works well in live trackingsituations to isolate neighboring players, for instance,keeping an adjacent percussionist out of an acoustic guitarplayer’s microphone. I recently had a great experiencewith the AEA R44 on jazz guitarist Peter White’s nylonstring guitar.

The figure-of-eight pattern also works well with back-ground singers who are good at balancing themselves on asingle mike, such as a Neumann U87 or an AKG 414. I’vehad wonderful experiences using this technique whileworking with great singing groups like Poc, the WilsonSisters, and the Nitty Gritty Dirt Band performers whounderstand the art of blending harmonies. The figure ofeight also works superbly with solo instrumentalists. Theback side does a good job of capturing the room tone of asolo sax. I’ve had good experiences with people likeGrover Washington Jr., Dave Koz, Wayne Shorter, GatoBarbieri on a single mike. I’ve also used the figure-of-eight pattern as part of an AKG C-24 MS miking setup forrecording strings. I’ve used this technique on sessions forElton 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-eightpattern is recording saxes and/or woodwinds in a Big

Band setting. It helps the separation between instrumentsbecause the side-to-side rejection is important. The factthat the back is open is not really an issue because any-thing leaking into the back is too far away to be con-cerned about. In this particular case I’m using NeumannU67s or 87s.

Leslie Ann Jones, Skywalker Ranch,Marin, California, 2002 June

The first time I used a figure-of-eight was on a back-ground vocal track using U67s or 87s. The singers likedthe eye contact and did their own balancing on the phonemonitors. The sound was cleaner, as you might expectwith fewer electronics in the signal path.

Next I experimented with Blumlein stereo. Using aU67, 87, or C414 pair head to head as a crossed-eightarray sounded good on classical piano, lead vocals, elec-tric guitar, and drum overheads.

Figure-of-eight ribbon microphones are often my firstchoice now. Most of my electric guitar, bass guitar, anddrum recordings are done with AEA and RCA 44s andColes 4038s. I use ribbons almost exclusively on brass andwoodwinds. A recent Brian Setzer Orchestra session inStudio A at Capitol had five saxes at the conductor’s left,four trombones in the middle, and four trumpets at hisright. The musicians formed a shallow arc, and each faceda figure-of-eight ribbon set 2 to 3 feet in front. Thesedelivered great sound and exceptional isolation.

Figure-of-eight ribbons also excel as room mikes ondrums. The side nulls are aimed toward the drums and themikes are placed 10 to 12 feet apart and 6 to 8 feet out.This delivers the room sound with little direct sound. It’ssimilar to the sound effects trick for recording a gunshot.The weapon is fired in a 44’s side null, so the fat roomsound is dominant.

Jeff Peters, Los Angeles, California, 2002 June

Using a pair of figure-of-eight microphones in a crossed90 degree Blumlein pair is one of my favorite microphonetechniques, especially when recording a really good choir.I’m amazed how few music-recording people are familiarwith this excellent technique.

I’m often asked: what is the effect you used on the choiron Michael Jackson’s “Man in the Mirror” on his BADalbum? My answer is: there is NO effect on the choir!Then I explain that the recording was done with only asimple Blumlein pair. I have to admit that the rest of thiswinning combination was Andre Crouch’s gospel choir,one of the best in the world, and Westlake Audio’s gor-geous Studio D in Hollywood. This wonderful piece ofmusic has a graceful, natural sounding, dynamic curve toit. From the transparent, burnished brass synthesized bellsin the intro, to the Andre Crouch choir that comes in at themodulation, to the climax with the huge ending, I feel that“Man in the Mirror” is the musical centerpiece of thealbum.

My favorite pair of Neumann M-49s, vertically coinci-dent and angled 45 degrees to either side of the centerline,captured this great choir recording. It’s why Blumlein isone of my favorite stereophonic microphone techniques



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 exten-sively by all Capitol mixers. I could go on and on aboutusing them. The 44 was our standard kick drum microphonefor pop orchestras. The bass drum would be used with thefront skin removed and a sandbag inside. The 44 was laidhorizontally atop the sandbag, and this combination deliv-ered a very tight thud sound. When doing pop-type jazzorchestras we used the 44s for sax and woodwinds, placedso that the dead sides did not pick up too much brass.

In symphony work I always used a 44BX for the doublebass. For Angel Classical sessions, depending on hallacoustics, I often set the forward-facing capsule on myNeumann SM69 or AKG C24 stereo microphones to figure-of-eight and then used a mid/side decoder to provide vari-able angle control for the resultant virtual Blumlein pair. Iwas the only mixer who frequently used the figure-of-eightpattern on stereo mikes. The full story is quite long, butroom acoustics and reverb time strongly influenced whetherI used the stereo mikes in XY or whether I used them withdifferential circuits (M/S) to further control the balance. Inthe studio at Capitol I used M/S with differential balancecontrols a great deal.

Carson Taylor, Danville, California, 2002 April

8 REFERENCES AND RECOMMENDEDREADINGS

[1] H. F. Olson, “A History of High-Quality StudioMicrophones,” presented at the 55th Convention of theAudio Engineering Society, J. Audio Eng. Soc. (Abstracts),vol. 24, p. 862 (1976 Dec.), preprint 1150, also publishedin Microphones [2, pp. 219–228].

[2] Microphones, An Anthology of TechnicalPapers (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-ComputerInterface Limited, UK, 2001).

[6] R. Streicher and F. A. Everest, The New StereoSoundbook, 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).


Ron Streicher received a B.A. degree in music from theUniversity of California and an M.A. degree in communi-cations arts from Loyola University, both in Los Angeles.Pursuing a lifelong involvement in music, his interest inaudio began in 1963 while serving as a volunteer for themusic department of a public radio station in Los Angeles;that avocation subsequently evolved into his career. Hismany broadcast projects include sound design and pro-duction of radio plays, national syndication of the LosAngeles Philharmonic Orchestra concerts, and chambermusic concerts from throughout California. His work hasbeen heard over National Public Radio and the PublicBroadcasting System networks.

Continuing to be involved with live music performanceand production, Mr. Streicher joined the engineering staffand faculty of the Audio Recording Institute at the AspenMusic Festival and School in 1988; since 1997 he has

served as its Audio Production Manager. For eleven sum-mers prior to Aspen, he designed and supervised concertsound reinforcement for the Philadelphia Orchestra, theMetropolitan Opera, and the New York City Opera pro-ductions at the Mann Music Center in Philadelphia. Hisrecording projects have taken him as far afield as Karachi,Shanghai, throughout Europe, and twice to Moscow torecord the Bolshoi Theatre Orchestra. He has engineeredrecordings for Angel, Brio, CMS Desto, CRI, Discovery,Koch International, Omega Record Classics, RCA, Pilz,Protone, and SAZ Records. He also produced two projectsfor the AES: the CD “Graham Blyth in Concert” and thevideo “An Afternoon with Jack Mullin.”

A fellow of the Audio Engineering Society, Mr.Streicher just completed eleven years as the secretaryof the AES and is currently president-elect. He isactively involved with the Society’s educational activi-

R. Streicher W. Dooley

THE AUTHORS



ties and has given numerous presentations at local andinternational meetings. In recognition of his long-termservice to the Society, he was awarded the AES BronzeMedal in 1995.

●

Wes Dooley’s speciality is on-location recording, andhis experiences in the United States, Europe, Africa, andNew Zealand led him to develop portable recording toolssuch as multichannel microphone arrays, mid/side stereoprocessors, stereo phase displays, and very tall micro-phone stands. His company is dedicated to creating prod-ucts that further the art and science of recording.

Ribbon microphones are what Mr. Dooley has becomebest known for. He has represented and serviced Coles’4038 ribbon microphones in the United States for the pasttwo decades. During the 1990s he observed that RCA 44“collectors” were taking these microphones out of circula-tion, making it difficult for recording studios to own or usea 44. Its rebirth became Mr. Dooley’s crusade and resultedin the AEA R44, a faithful recreation of this classic micro-phone. 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, wasintroduced at the Fall 2002 AES convention in Los Angeles.

Mr. Dooley and Mr. Streicher previously have coau-thored two papers about stereo microphone techniquespublished in the AES Journal and the StereophonicTechniques Anthology. Mr. Dooley is a fellow of the AES.He has chaired workshops on microphone techniques andmixing strategies for compatible multiple releases for cin-ema, broadcast, and videocassette, has presented sectionmeetings on stereo techniques and forensic audio, and hasparticipated on panels at many meetings of the AES andother technical organizations. A former governor andvice-president (Western Region) of the Society, heremains involved with AES standards work and currentlyserves on the SC-03-12 Working Group on ForensicAudio, where he heads a writing group on Forensic AudioStandards. He is also a member of the SC-04-04 WorkingGroup on Microphone Measurement and Characteriza-tion. Also an amateur audio historian, Mr. Dooleycochaired the Audio History Room at the Fall 2002 AESconvention in Los Angeles.

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