RNAL OF THE
`ll ENGINEERING SOCIETY
`
`SPEECH PROCESSING
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`REPRODUCING ELEMENTS
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`RECURDING TECHNIQUES
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`JOURNAL OF THE
`
`7S
`I
`
`EY
`
`VOLUME 15 NUMBER 4
`
`OCTOBER 1967
`
`Tone Generation with Multiple Synchronous and Non-Synchronous RC
`
`Oscillators—Robert E. Owen
`
`+
`
`*#
`
`©
`
` & «
`
`&
`
`«2
`
`«
`
`SOG
`
`ARTICLES
`
`Acoustical Measurements by Time Delay Spectrometry—
`Richard C. Hevyser
`.
`‘
`‘
`‘
`a
`x
`z
`i
`.
`i
`
`i
`
`»
`
`F
`
`8 BY
`
`. 2... 383
`An Audio Noise Reduction System—Ray M. Dolby
`Factors Affecting the Needle/Groove Relationship in Phonograph
`7?
`ope
`;
`Playback Systems—C.
`RK. Bastiaans
`.
`‘
`7
`;
`.
`389
`Survey of Methods for Measuring Speech Quality—Michael H. L. Hecker
`and Newman Guttman
`,
`i
`‘i
`A Comparison of Two Types of Digitized Autocorrelation Vocoders—
`404
`Calvin F. Howard, HaroldJ. Manley and James C. Stoddard.
`.
`Information Content of a Sound Spectrogram—Tiong Suy Yu... 407
`soled
`A Limited-Vocabulary Adaptive Speech-Recognition System—
`Paul W. Rosy
`i
`‘ a ee
`‘
`414
`Directional Microphones—Harry F, Olson ee ee 7)
`A New Concert Violin—Carleen Maley Hutchiny and John C. Schelleng
`432
`_
`:
`Miniature Audio Amplifiers—William H. Greenhaum . 2... 438
`sh
`aN
`.
`:
`onograph
`Turntables to
`Normal
`Loads—T. 8.
`Cole,
`Sr.
`.
`¢
`Sensitivity of Pho
`h Turntabl
`| Loads—T.
`8. Cole,
`446
`
`400
`
`N
`
`S.
`
`DEPARTMENTS
`~
`:
`Letters to the Editor
`452
`Membership Information .
`}
`. 1. 464
`Obituaries
`.
`2.
`Sound Track
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`m
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`Convention Exhibits Preview .
`466
`Shopping the Audio Market
`|
`ial News of the Sections
`i
`«
`472
`Editorial
`:
`‘
`3
`‘
`a Available Literature...)
`477
`Index to Volume 15
`-
`
`.
`.
`
`,
`.
` .
`
`480
`483
`485
`488
`489
`
`EDITORIAL BOARD
`
`Donald M. Black
`Frank A. Comerci
`John D. Colvin
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`David L. Klepper
`Donald S. McCoy
`John G, MeKnight
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`Robert E. Owen
`Editor: Harry F. Olson
`‘
`‘
`‘
`j
`_ Managing Editor: Jacqueline Harvey
`.
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`;
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`imciretace
`Journal of the Audio Engineering Society, Volume 15, No. 4, October, 1967. Published quarterly by
`the Audio Engineering Society and supplied to all members in good standing. Publication office,
`104 Liberty Street, Utica, N. ¥. 13502. Executive office, Room 428, Lincoln Building, 60 East 42nd
`Pp
`i
`Street, New York, N. ¥. 10017, Entered as second class mail at the
`post office at Utica, N.Y. Sub-
`scription to nonmembers, $11 per year. Copyright 1967 by the Audio Engineering Society. The
`Journal is indexed in the Applied Science & Technology
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`in part, any
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`author(s) and this journal, An author, or his research affiliate may reproduce his paper in full cred-
`iting this journal. This permission is not assignable.
`“
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`The “Journal of the Audio Engineering Society" and its cover design has been registered as a
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`2
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`ec
`ICERS 1966-67
`;
`Preside
`.
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`p.R. eaeyeaa
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`Teo L. Beranck
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`

`

`Directional Microphones
`
`HARRY F. OLSON
`
`=
`
`RCA Laboratories, Princeton, New Jersey
`
`A comparison of gradient, end-fired line, and cross-fired surface wave microphones
`has been carried out. The subjects considered include the directivity as a function of
`the dimensions and of frequency, the problem of obtaining a uniform directional pattern
`with respect to frequency, and the ambient noise response and relative pickup distances
`of directional microphones.
`
`an
`A directional microphone is
`INTRODUCTION
`transducer
`for
`converting acoustic
`acousto-electronic
`vibrations into the corresponding electrical undulations
`which exhibits a variation in response to sounds arriving
`from different directions with respect
`to some reference
`axis of
`the system. The main reason for
`the use of
`directional microphones is to pick up desired sounds and
`discriminate against unwanted sounds such as reverbera-
`tion and noise. Directional microphones may be divided
`into two main categories, namely the gradient types which
`depend for directivity upon the difference in pressure,
`or powers of
`the difference in pressure, between two
`points in space, and wave types which depend for direc-
`livity upon some form of constructive and destructive
`wave interaction. The purpose of this paper is to describe
`the construction, operation, and performance of gradient
`and wave type directional microphones.
`
`GRADIENT MICROPHONES
`
`a microphone in
`A pressure gradient microphone is
`which the electrical output corresponds to a component
`of the gradient or space derivature of the sound pressure.
`A first-order pressure gradient microphone is a micro-
`phone in which the response corresponds to the difference
`in pressure between two points in space. The first-order
`pressure gradient response resembles the particle velocity
`in a sound wave and as a consequence this
`type of
`microphone is termed a velocity microphone. A first-
`order pressure gradient microphone may be depicted as
`consisting of
`two pressure-sensitive elements separated
`by a distance that is small compared to the wavelength,
`connected in phase opposition as shown in Fig. 1. The
`directional characteristic of a first-order pressure gradient
`microphone is of the cosine type, given by the equation
`e€; = e, cos
`(1)
`
`where ¢, = output of the microphone for the angle 4,
`= angle between the direction of the incident sound
`and the line joining the two elements, and e, = output
`of the microphone for #@ = 0. The directional charac-
`teristic of the first-order pressure gradient microphone
`is also shown in Fig, 1,
`420
`
`a microphone that
`A unidirectional microphone is
`responds predominantly to sound incident from a single
`solid angle of a hemisphere or less. The most common
`
`PRESSURE
`
`ELEMENTS OUTPUT
`
`1eoe
`
`first-order bidirectional gradient
`a
`1, Elements of
`Fig.
`microphone and corresponding directional characteristic.
`
`type
`the one of gradient
`unidirectional microphone is
`in which the directional characteristic is a cardioid, A
`unidirectional microphone may be depicted as two pres-
`sure-sensitive elements separated by a distance that
`is
`small compared to the wavelength, connected in phase
`opposition through a delay network. The directional
`characteristic of
`the first-order gradient unidirectional
`microphone is given by the equation
`
`é, = e,(Dot+ D,cosA)
`
`(2)
`
`Where ¢, = output of the microphone for the angle #.
`6 = angle between the direction of the incident sound
`and the line joining the two elements, e, = output o!
`the microphone for #= 0, D, = distance between the
`elements, and D, = path length of the delay. For D, =
`D,
`the directional characteristic is a cardioid, as show!
`in Fig. 2. The directional characteristic for 2D, = Di
`and D, = 2D, are also shownin Fig. 2.
`
`JOURNAL OF THE AUDIO ENGINEERING SOCIETY
`
`
`
`3
`
`

`

`
`
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`RECORDING INSTRUMENTS COMPANY
`A Division of DICTAPHONE CORPORATION
`421
`OCTOBER 1967, VOLUME 15, NUMBER 4
`
`
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`
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`
`(203) 335-5146
`Makers of the renowned Scully lathe, since 1919
`Symbol of Precision in the Recording Industry.
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`

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`
`JOURNAL OF THE AUDIO ENGINEERING SOCIETY
`
`A second-order pressure gradient unidirectional micro-
`phone is depicted in Fig. 3.
`In this form,
`the second-
`order gradient unidirectional microphone consists of two
`gradient microphones of the first order connected in
`phase opposition combined with a delay line. The direc-
`tivity pattern of the second-order gradient unidirectional
`microphone is given by
`
`@s = @y( Dot D,cosé) cost
`
`(3)
`
`where e¢. = output of the microphone for the angle #.
`@ = angle between the direction of the incident sound
`and the line joining the two elements, e, = output of the
`microphone for # = 0, D, = distance between the two
`first order gradient elements, and BD, = path length of
`the delay.
`The directional characteristics for D; = Ds and 2D,
`= D, are shown in Fig. 3, A consideration of the direc-
`tional characteristics of Fig. 3 shows that these are much
`sharper than one lobe of one of the cosines of Fig. 1.
`
`WAVE MICROPHONES
`Line Microphones
`A line microphone is a wave-type directional micro-
`phone consisting of a single straight-line element or of
`an array of continuous or spaced electroacoustic trans-
`ducing elements disposed on a straight line.
`In the end-
`fired line microphone the maximum response occurs for
`sound arriving along the axis of the microphone, Typical
`
`PRESSURE
`ELEMENTS}
`
`
`
`|
`y
`
`
`
`
`A
`
`TRANS,
`|
`TRANS.
`ELEMENT| ELEMENT
`Ly
`B
`
`HARRY F. OLSON
`
` PRESSURE
`
`ELEMENTS
`
`QUT PUT
`
`Fig. 2. Elements of a unidirectional gradient microphone
`
`and directional characteristics for various ratios of D, and D.,
`
`In
`end-fired line microphones are depicted in Fig. 4.
`Fig. 4a the line microphone consists of a number of
`small pipes with the open end as pickup points, equally
`spaced on a line, and with the other end connected to
`a transducing element.
`In Fig. 4b the line microphone
`consists of a tapered tube connected to the transducing
`element.
`In Fig. 4c the holes of Fig. 4b are replaced
`4
`rn
`
`
`
`
`
`iC
`
`TAPERED
`/ PIPE
`
`SLOT
`
`TRANS,
`ELEMENT
`a
`
`
`
`|| TAPPERED
`Ur PIPE
`¥e
`
`HOLE
`
`ts
`
`,6
`
`Fig. 4. Different types of end-fired line microphones, The
`pickup systems are: a. A bundle of different
`lengths of pipe
`with the open ends as pickup points; b. A tapered pipe wilh
`holes as pickup points; c. A tapered pipe with a slot
`os
`pickup point,
`
`OUTPUT
`
`Fig. 3. Elements of a unidirectional second-order gradient microphone and directional characteristics for two different Tr
`of D, and Dz.
`422
`
`OS
`
`5
`
`

`

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`OCTOBER 1967, VOLUME 15, NUMBER 4
`
`3COMPANY
`
`423
`
`6
`
`

`

`
`
`7
`
`

`

`PP eS
`a
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`Ecae=CJICICI
`PetrHHOo: 2
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`In use at Pampa Recording, Detroit, Michigan. from the smallest component
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`ANOTIHER
`SVSTEMS INNOVATION
`by
`
`A
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`
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`8
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`

`

`= Ath Y 2 OOM
`
`by a continuous slot as the pickup line. Construction of
`the lines and transducers is beyond the scope of this paper
`and does not contribute to essential subject matter.
`The directional characteristic of an end-fired line mi-
`crophone is given by
`2
`=
`
`sin[(r/ 2) (L—Leosf) |
`(w/A)(L—Leos#)
`
`(4)
`
`output of the microphone for the angle 9,
`
`é) = output of the microphone for @ = 0, @ = angle
`between the direction of the incident sound and theline,
`L = length of the line, and ) = wavelength.
`The directivity characteristics of an end-fired line mi-
`crophone are shown in Fig. 5. The directivity is a func-
`tion of the length of the line and the frequency.
`
`Surface Microphones
`
`A surface microphone is a wave-type directional micro-
`
`where ¢, 180°
`
`LENGTH= 180%
`
`d@
`
`
`
`o MICROPHONES
`aFoeootBPsoonco@ooo6eooobséo#09 Bgi
`
`
`Fig. 5.
`length a.
`
`Directional characteristics of an end-fired line microphone for various values of the ratio of line length to wave-
`
`phone consisting of a surface element or a number of
`transducing elements disposed on a surface.
`In the cross-
`fired surface microphone the maximum sensitivily occurs
`on a line perpendicular
`to the surface. A cross-fired
`surface microphone consisting of elements approximately
`equally disposed on a circular surface is depicted in Fig.
`6a. The cross-fired circular surface may be built as a
`large electroacoustic microphone, as shown in Fig. 6b.
`The directional characteristic of this microphone for
`the front hemisphere may be expressed as
`2 [(2D/ i.) sind]
`(4D/%)siné
`
`ey
`
`ey
`
`“i
`
`a
`
`o
`a°
`o
`
`a
`
`SECTION X—x’
`A
`
`SECTION X—X'
`8
`
`®%
`Fig. 6. Two types of cross-fired surface microphones,
`A group of microphones located on a surface. b. A condenset
`microphone with the diaphragm as the pickup surface.
`
`where ¢, = output of the microphone for the angle 4,
`
`LA
`DIAMETER = >
`
`A
`DIAMETER = >
`
`DIAMETER =A
`
`DIAMETER = 2A
`
`Fig. 7. Directional characteristics of a eross-fired surface mic rophone for various values of the ratio of surface diameter !€
`wavelength i.
`426
`
`JOURNAL OF THE AUDIO ENGINEERING societ’
`
`
`
`9
`
`

`

`Might |
`other
`
`
`
`
`
`You're looking at a revolutionary
`concept in cardioid microphone design
`— actually two microphones in one.
`It is a microphone system with two
`independent capsules. Like a high-
`quality two-way speaker system,
`one capsule respondsto low and the
`other to high frequencies with a
`built-in crossover network at 500 cycles,
`
`Go ahead ... pick up the new
`AKG D-200E two-way microphone and
`try it! Then ask your most severe
`critic to listen.
`
`Look for this symbol! It signifies
`this exclusive concept — a product of
`AKG research, ¢
`
`MICROPHONES * HEADPHONES
`CISTRBUTED BY
`NORTH AMERICAN PHILIPS COMPANY, INC.
`100 RAST 43nd STREET, NEW YORK, NEW YORK 10017
`
`OCTOBER 1967, VOLUME 15, NUMBER 4
`
`427
`
`
`
`10
`
`10
`
`

`

`HARRY F. OLSON
`
`é) = output of the microphone for @ = 0, D = diameter
`of the circular surface, @ = angle between the direction
`of the incident sound and a center line normal
`to the
`
`surface, /, = Bessel function of the first order, and 4 =
`wavelength,
`The exact expression for the directional characteristic
`of the cross-fired circular surface microphone including
`the rear hemisphere is complex and is not presented here.
`The directional characteristics of a cross-fired surface
`
`microphone are shown in Fig. 7. The directivity is a
`function of the diameter of the surface and of frequency.
`
`COMBINATION LINE AND SURFACE
`MICROPHONE
`
`A combination line and surface microphone is a micro-
`phone in which the terminations of a large number of
`line elements are arranged on a circular surface as de-
`picted in Fig. 8. The microphone shownin Fig. 8 is one
`form of this combination system.
`The directional characteristic of such a microphone is
`the product of an end-fired line and a cross-fired surface.
`The directional characteristics for
`a combination end-
`fired line and cross-fired surface in which length of line is
`three times diameter of the surface are shown in Fig. 9.
`
`COMBINATION LINE AND CARDIOID
`MICROPHONE
`
`A combination line and cardioid microphone is a
`microphone in which a line system is combined with a
`gradient system, One simple form of a combination line
`and cardioid microphone is shown in Fig. 10.
`The directional characteristics of
`the microphone of
`Fig. 10 are shown in Fig. 11,
`In the low-frequency range
`the directional characteristic is a cardioid because the
`directivity pattern of a short
`line is practically omni-
`directional.
`In the high-frequency range the directivity
`pattern is that of a line microphone.
`
`@®&
`
`e° &
`
`e8
`FRONT VIEW
`
`LINE
`
`LINE
`
`TRANSDUCER
`ELEMENTS
`
`SIDE VIEW
`
`Fig. 8. Front and side views of a combination line and
`surface microphone.
`428
`
`11
`
`
`
`LENGTH © 3X
`
`LENGTH* $A
`plameTeR =
`
`LENGTHe 3A
`oiameer = %
`oF
`
`if
`
`Fig. 9. Directional characteristics of a combination line
`and surface microphone for various ratios of line length and
`surface diameter to wavelength \.
`In all cases the line length
`is three times the surface diameter.
`
`NOISE DISCRIMINATION AND RELATIVE
`PICKUP DISTANCE FOR MICROPHONES
`WITH DIFFERENT DIRECTIVITY PATTERNS
`
`The discrimination of a microphone against noise,
`reverberation, and unwanted sounds increases as its di-
`rectivity increases. The pickup distance for
`the same
`
`LINES
`
` OUTPUT
`
`
`
`TRANSDUCER
`ELEMENTS
`
`10. A combination line and cardioid microphone.
`Fig.
`For a cardioid directivity pattern for the gradient element.
`1=D,.
`
`reception of noise, reverberation, other unwanted sounds
`increases as directivity of microphone increases.
`The noise-to-signal
`ratios for various directivity pat
`terns are shown in Fig. 12, The ratio of noise-to-signal
`pickup is assumed to be unity for the omnidirectional
`microphone.
`Figure 12 also shows the pickup distances for the same
`reproduced noise andreverberation for various directivity
`patterns. The pickup distanc2 for
`the omnidirectional
`microphone is assumed to be unity.
`
`CONCLUSIONS
`The directivity characteristics of gradient-type micT
`phonesare essentially invariant with respect to frequency:
`The directivity characteristics of microphones consist
`ing of lines, surfaces, combinations of lines and surface
`JOURNAL OF THE AUDIO ENGINEERING society
`
`11
`
`

`

`Syll
`
`WhenStanton engineers get together, they drawthe line.
`ponse curve
`of the newStanton 681
`is virtually a straight
`line from
`
`The frequency
`Calibration Standard
`10-20,000 Hz.
`That’s a guarantee.
`In addition, channel separation must be 35 dB or
`sater at 1,000 Hz. Output must be 0.8 mv/cm/sec
`mini-
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`If a 681 doesn’t match these specifications whenfirst
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`Each 681 includes hand-entered s
`verily that your 681 matches the or
`
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`what has been cut into the grooves, Nc
`But you don’t have to be a profe
`“rence a Stanton 681 Calibration Standard will make,
`ally with the “Longhair” brush which provides the
`clean grooves so essential for clear reproduction. The im-
`provement
`in performance is immedi
`audible, even
`to the unpracticed ear.
`The 681 is
`completely new, fromits slim-line config-
`
`tem. The 681A with coni
`EE with elliptical
`
`_4>
`i call Wt9
`
`
`
`
`
`o engineers who use Stanton cartridges as their ref-
`
`netics, Inc., Plainview, L. I., N. Y.
`
`_ 3
`
`¢
`
`3
`
`
`
`
`
`CoatLOLOL aLCLair)
`
`12
`
`

`

`
`
`HARRY F. OLSON
`
`UP TO 350Hz
`oe
`
`
`550Hz
`
`NOOHZ
`o°
`
`2200Hz
`
`4400Hz
`0°
`
`eo*
`
`‘ao?
`
`sao?
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`to?
`
`10"
`
`Fig, 11. Directional characteristics of a combination line and cardioid microphone for various frequencies, The length
`of the line is 12 in,, and D,=Dy,,
`
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`
`
`
`
`
`
`ot MORE|...
`
`
`
`
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`
`
`~ Ser10°
`
`DIRECTIVITY
`INDEX
`DISTANCE
`RATIO
`NOISE
`RESPONSE
`
`‘
`
`MF
`Hs
`3
`
`.
`
`=
`x
`
`and combinations of lines and gradients vary with respect
`to frequency. The net result is frequency discrimination
`for sound sources located off the axis and in the reverber-
`ant sounds and noise,
`
`The length of a line must be relatively great in order
`to obtain some measure of directivity in the low-frequency
`range. For example,
`the length of an end-fired line
`microphone at 100 Hz must be 22 ft to equal the direc-
`tivity pattern of the second-order gradient microphone.
`The obvious conclusion is that a line microphone must
`be very long indeed in order to obtain any semblance of
`directivity in the low-frequency range.
`The diameter of an end-fired surface microphone must
`also be quite large in order to obtain directivity. For
`
`xample, at 160 Hz, the diameter of the end-fired surface
`microphone must be 8 ft in order to equal the directivity
`of the second-order gradient microphone, A disk of this
`diameter is very cumbersome.
`For the combination end-fired and cross-fired surface
`microphone the length must be 16.5 ft and the diameter
`5.5 ft at 100 Hz to equal the directivity pattern of the
`second-order gradient microphone. Here again the struc-
`ture is large and cumbersome.
`In the case of the combination line and cardioid micro-
`phone the directivity in the low-frequency range is that
`of a cardioid,
`In general,
`the range in which directivity
`is most important
`is the low-frequency range where the
`levels of both ambient noise and reflected sounds are high.
`
`90° igoe
`
`
` ‘A
`
`A
`
`12
`
`35
`
`\
`2
`
`25
`
`6
`
`a
`25
`
`
`
`Fig. 12. Relation of directivity patterns to the directivity index, distance pickup ratio,
`Parameters are asstimed to be unity for nondirectional or omnidirectional microphone.
`
`and noise
`
`response. All
`
`three
`
`
`
`THE AUTHOR
`
`Harry F, Olson received the B.S., M.S., Ph.D., and
`F.E. degrees
`from the University of
`Iowa, and an
`Honorary D.Sc. degree from Iowa Wesleyan College.
`He has been affiliated with the research department of
`Radio Corporation of America, the engineering depart-
`ment of RCA Photophone,
`the research division of
`RCA Manufacturing Company, and RCA Laboratories.
`Dr. Olson is Staff Vice President of the Acoustical and
`Electromechanical Research Laboratory of the RCA
`Laboratories.
`Dr. Olson, past president of both the Audio Engi-
`neering Society and the Acoustical Society of America,
`past chairman of the Administrative Committee IRE
`Professional Group on Audio, and currently Editor of
`the AES Journal, has received the Modern Pioneer
`Award of the National Association of Manufacturers,
`John H. Potts Medal of the Audio Engineering Society,
`
`Samuel L. Warner Medal of the Society of Motion Pic-
`ture and Television Engineers, John Scott Medal of the
`City of Philadelphia, Achievement Award of the Profes-
`sional Group on Audio of the Institute of Radio Engi-
`neers, John Ericsson Medal of the American Society of
`Swedish Engineers, Audio Engineering Society Award,
`The Emile Berliner Award and Mervin J, Kelly Award.
`He holds more than 100 U, §, Patents, has written
`numerous papers, and books including Elements of
`Acoustical Engineering, Acoustical Engineering, Dy-
`namic Analogies, and Musical Engineering.
`A member of Tau Beta Pi, Sigma Xi, and the Na-
`tional Academy of Sciences, Dr, Olson is also a Fellow
`of the Society of Motion Picture and Television Engi-
`neers,
`the American Physical Society,
`the Institute of
`Electrical and Electronics Engineers,
`the Acoustical
`Society of America and an Honorary Member of AES.
`
`
`
`430
`
`JOURNAL OF THE AUDIO ENGINEERING SOCIETY
`
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`13
`
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`13
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`