I PRIVATE AND CONFIDENTIAL

RESEARCH DEPARTMENT

VISIT TO GERMANY - SEPTEMBER 1959

Report No. A~057

( 1960/2)

THE BRITISH BROADCASTING CORPORATION

ENGINEERING DIVISION

RESEARCH DEPARTMENT

VISIT TO GERMANY ~ SEPTEMBER 1959

Report No. A~057

( 1960/2)

T. Somerviile, B.Se., M,I.E.E., F.lnstoP. C.L.S. Gilford, M.Se., F.lnst.P., A.M.I.E.L O.E.L. Shorter, B •• Sc,{Eng.), AoM.I.E.E. (T. Somervil1e)

This Report is the property ot the British Broadcasting Corporstion and aay Dot be reproduced or dlsolosed to a third party in any tora without the written permission ot the Corporation.

Report No. A-057

VISIT TO GERMANY - SEPTEMBER 1959

Section Ti tle

1 INTRODUCTION... 0 • 0 • • • • • • • • • • • • • • • • • • • • • •

2 INTERNATIONAL CONGRESS ON ACOUSTICS

2.1. Architectural Acoustics 2.1.1. 2.1.2.

2.1.3.

201.4.

2.1.5.

Discussion on Concert Hall Design Some Notes on the Acoustical Design of Concert Halls - L.L. Beranek 0 ••••• 0 ••••• 0

The Audience and Room Acoustics - A.F.B. Nickson and R. W. Muncey •.•.•••••••••.••• The Building-Up Process of Sound Pulses in a Room and its Relation to Concert Hall Quality - V.L. Jordan Acoustics of Television Studios -T. Somerville ••• 0 • • • • • •

2.1.6. Sound Absorption Coefficients and Acoustical Design--

Page

1

1

2

2

3

3

3

4

T. D. Northwood and C.G. Balachandran • • • . • . 4 2.1.7. Der Einfluss der Kanten auf die Schall-absorption

poroser Materialien -We Kuhl • • • • • • • 4 2.1.8. Computations on the Absorption of Sound by

Absorbent Patches - R.K. Cook. • • . • . • 4 2.1.9, Precision of Reverberation Chamber Measurements of

Sound Absorption Coefficients -R.V. Waterhouse 5 2.1. 10. Untersuchungen zur Ermi ttlung der Diffusi tat bei

Messungen des Schallabsorptionsgrades im Hallraum -G. VenZke and P. Dammig • • • • • • • • • • • . • . 5

2.1.11. Die Messung der Schallabsorption von Materialien -C. W. Kosten .••••••••• 0 • • • • • • • 5

2.1.12. Welcher Aufwand an Information ist erforderlich, urn einen Raum akustisch zu charakterisieren? - L. Cremer 6

2.1.13. Das Problem der Schallstreuung in der Raumakustik -J. Hol tsmark . . • • . . 0 • • • • • • • • • • • • • 7

2.1.14. Direct Reading Reverberation Time Measuring Device -LeX. Nepomuceno 0 • • • • • • • • • • • • • 7

2.1.15. A New Automatic Method for Measuring the Reverberation Time and the State of Diffusion of a Room - M. R. Schroeder 0.......... 7

Section

3

2.1.17.

Title

Wide Band Absorbers with Impermeable Facings -C.L.S@ Gilford and N.C.H. Druce • " ..•. The Damping of Eigentones in Small Rooms -F.J. van Leeuwen •• " .•.

2.1.18. Luftschalld8mmung - K. GOsele

2.2. Electra-Acoustics

2.2.1.

2.2.2.

Kurzer Uberblick uber die Vorschlage stereophoner Rundfunkubertragungen - K. Wilhelm " •.. Die Praxis der Regel technik bei kompatiblen Stereoaufnahmen - W. Schlechtweg Grenzen der Trickstereophonie - F. Enkel ••... Eigenschaften des naturlichen Richtungshorens und ihre Anwendung auf die Stereophonie - N. V. Franssen Stereophonic Sound Reproduction - H. F. Olson .• Measurements of Elastic Parameters of Paper for Loudspeakers ~ C. Bordone--Sacerdote • • ec • '0

Synthesis of BassrReflex Loudspeaker Enclosures -E. de Boer. '" • 0 " • • • 0 " • • 0 • , • • •

Miniature Condenser Microphones - C. Wansdronk Improvement of Acoustic Feedback Stability in Public Address Systems ~M.R. Schroeder

2.2.10. Some Engineering Problems Arising in the Sound

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7 8

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8

8

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9

9

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Equipment of the Television Service - D.E. L. Shorter 10

2.2.11. Wiedergabe von Magnetton-Aufzeichnungen mit Hilfe des Halleffektes - F. Kuhrt " "" .".... 10

203. Physiological and Psychological Acoustics

2.3.1.

2.3.3.

2.3.6. 2.3.6.

Uber die nervosen Prozesse beim Horen und der !ibrationsempfindung - G. von Bekesy Uber die Grundlagen der Entstehung der Erregung im Nervensystem ~ A. von Muralt •.•••.• Regelungsvorgange beim Sprechen und Horen -K. Kupfmuller • 0 0 •

On the Acoustics of Speech - G. Fant The Auditory Input-Qutput F1IDction - S. S. Stevens The Form of the Loudness Function Near Threshold -B. Scharf and J.C. Stevens Die Tragheit der Lautstarkeempfindung und ihre Nachbildung bei objektiven Lautstarkemessern - H. Niese

204. Measurements

2.4.10 A Logarithmic Attenuator - S.B Pederson

COLOCNE (WESTDEUTSCHER RUNDFUNK) . • • • • , . . • , . " . • . . • . .

10

10

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12

Section Title

4 STUTTGART (SUDWESTFUNK)

4.1. Sound Studios 0 •

4.2. Television Studios

4.3. Microphone Testing

5 MUNICH (BAYERISCHE RUNDFUNK)

6 BADEN-BADEN (SUDWESTFUNK)

6.1. Concert Studio

6.2. Chamber Music Studio

6.3. Entertainment Music Studio

6.4. The Drama Complex

6.5. Speech Studios

6.6. Monitoring Loudspeakers

7 FRANKFURT (HESSISCHE RUNDFUNK)

7.1. Sound Studios . 0

7.2. Television Studios

7.3. Stereophony 0

8 CONCLUSICNS • c • • • • • • 0 0 • • • 0 0 • • • • • • • • • • • • • • •

9 REFERENCES

Page

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17

January 1960

1, INTRODUCTION

Report No. A-057

( 1960/2)

PRIVATE AND CONFIDENTIAL

VISIT TO GERMANY ~ SEPTEMBER 1959

The Third International Congress on Acoustics took place in Stuttgart from 1st to 8th September 1959. These congresses are held every three years and are sponsored by the International Union of Pure and Applied Physics through the Inter­national Commission on Acoustics. The first one was in Delft in 1953 and the second in Cambridge, Massachusetts, in 1956. Research Department was represented by Messrs. Gilford, Shorter and Somerville; Mr. Alexander Brown of Building Department was also at the Congress. The opportunity was taken on the way to Stuttgart to visit sound and television studios in Cologne. Studios in Stuttgart, Munich and Baden-Baden were visited during the Congress, while on the return journey a visit was paid to Frankfurt.

2. INTERNATIONAL CONGRESS ON ACOUSTICS

The lecture theatres in the Physics Institute in Stuttgart were used for the Congress, except for the opening session which took place in the Liederhalle.

Because of extensive war damage in Stuttgart new buildings are being con­structed for the Physics Institute and two of the completed new lecture theatres were used for the Congress. They are equipped with every modern refinement, including sound reinforcement and blackboards that rise out of the floor, but it would have been an advantage if the lights could have been dimmed and not extinguished, so that notes might be taken when slides were being projected. The Liederhalle consists of three concert halls: the Beethoven-Saal, the Mozart-Saal and another small hall. These halls are all of the irregular shape that is common in Germany at the present day and are constructed mainly of reinforced concrete. Because of the rather rough concrete exterior they are known locally as the "Liederbunker".

The morning sessions all consisted of invited papers on which there was no discussion, and the afternoons were devoted to contributed papers which had to be dealt with in eight concurrent sessions because of the large number, It was impossible therefore to cover everything, even though the B.B.C. had four representatives. The same criticism was also levelled at the Second Congress in Cambridge, Massachusetts, although the position was even worse in Stuttgart. Each paper was allowed fifteen minutes for delivery and five minutes for discussion. It would be a great improve­ment in the organisation if the papers were pre-printed because it is almost impossible to make a valuable contribution in fifteen minutes and to compress the discussion into

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five minutes. In all, there were about 360 papers covering the following sUbjects:

Architectural acoustics and vibrations Electro-acoustics Musical acoustics and room acoustics Ultrasonics Molecular acoustics Physiological acoustics Psychological acoustics Measurements

In addition to the technical sessions, a reception for the delegates was given by the City of Stuttgart, there was a concert by a string quartet in the Mozart­Saal of the Liederhalle, a technical excursion to Munich and a banquet on the last evening in the Beethoven-Saal of the Liederhalle.

In view of the number of papers, a brief survey only will be given.

2.1. Architectural Acoustics

Of the invited papers, those of most interest in architectural acoustics were by GOsele on sound insulation, the contribution by Cremer on diagnostic methods and the measurement of absorption coefficients by Kosten.

One important topic which ran through the papers on architectural acoustics was the measurement of sound absorption by the reverberation method. The absorption coefficient of a material is not an invariant property of its structure, depending as it does on the size and shape of the sample in relation to the wavelength of the sound and on the nature of the incident sound. In particular, it will depend upon the angle of incidence or the distribution of incidence angles, and, if the absorber is measured in a room, the coefficient will vary with the position in the room.

The result is that the measurements of a given material carried out in different laboratories often differ considerably from one another and many efforts have been made in the past to agree on a standardised procedure for making rever­beration room measurements.

Hitherto, the arguments at national and international standardising com­mittees appear to have been mainly a matter of trying to get one's own particular method adopted without very much reference to the reason and nature of the differences between different methods. Work has been started fairly recently in Research Depart­ment to attempt to overcome this difficulty and it was extremely interesting to find the same tendency in the papers presented at the Congress.

2.1.1. Discussion on Concert Hall Design

The most interesting and controversial aspects of International Congresses on Acoustics are the discussions which inevitably arise on concert hall design. Dr. Beranek convened a meeting one evening of all those present at the Congress who were

3

interested in this subject. There were representatives from Germany, Belgium, Denmark, U.S.A., Holland, Finland, Canada, Australia, Poland, Czechoslovakia and Israel. The United Kingdom was represented by Mr. H,J. Purkis from the Building Research Station, and Messrs.Gilford and Somerville from the B.B.C.

Dr. Beranek has recently been acting as consultant for a new concert hall in Tel Aviv which is modelled on the lines of the Festival Hall and unfortunately, although not surprisingly, has similar acoustic properties. All modern halls, particularly when shaped like a fan, have short reverberation times and are therefore difficult for the performers and do not produce the best results for the audience. The B.B.C. has for many years objected to halls of this type. It was therefore interesting to find support coming, not only from Dr. Beranek himself, but from Scandinavians and, in particular, a German engineer, Dr. Winckel of Berlin. Although many experts still consider reflectors to be necessary, the tendency now is to use them to direct sound down into the orchestra and not so much to project sound into the hall. There are, however, die-hards of the "fan shape school", typified by Kosten from Holland, who still believe that reflectors are required to increase the sound level in the hall. This is surprising in the case of Kosten because one of the best halls of the "Leipzig" type is the Concertgebouw in Amsterdam.

2.10 2. Some Notes on the Acoustical Design of Concert Halls - L. L. Beranek

This paper was based on a book which Beranek is preparing from data provided by all the well-known authorities on the subject. He states in this book that his interest was stimulated by the performance of St. Andrew's Hall, Glasgow, and this decided him to try to analyse all available data. His main conclusions are that the older concert halls in the rectangular form, with much ornamentation which gives good diffusion, are better than modern halls, the majority of which are fan shaped. Some of the defects he puts down to the fact that audiences apparently provide greater absorption than was previously allowed for, so that the new halls are lacking in reverberation. He appears also to have come round to the view that the direction of sound away from the orchestra with the intention of increasing the intensity at the rear is not a good thing. There will be many who will disagree with his conclusions but the accumulation of data is an excellent thing in itself and will be of great value to future designers.

2.1030 The Audience and Room Acoustics - A.F. B. Nickson and R. W. Muncey

The theme of this paper was that in spite of all the investigation that hit's gone into architectural acoustics, there are still considerable variations in the halls which are being built. The authors pointed out that even when a hall was considered by the experts to be unsatisfactory, the users always considered it to be good. In the circumstances it was suggested that further development in the field of architectural acoustics as far as concert halls was concerned was unnecessary and that, as long as echoes were eliminated and the reverberation was not too short or too long, nothing more need be done. This paper should have raised much controversy but there was not time for adequate discussion.

20104. The Building-Up Process of Sound Pulses in a Room and its Relation to Concert Hall Quality -V.L. Jordan

Jordan is the best known of the acoustic consultants on the Continent. He was responsible for the acoustic design of the large concert studio in Copenhagen, the

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studio Which demonstrates most of the bad features of a fan shaped plan. The members of the orchestra have always complained that they could not hear each other. In this paper Jordan described experiments with Plexiglas reflectors to reflect sound energy back to the orchestra, and he has received favourable comment as a result of his experiments. He is now carrying out an investigation of the build up of sound pulses in rooms in the hope that he may be able to relate his investigations to concert hall quali ty.

2.1.5. Acoustics of Television Studios - T. Somerville

As a result of the expansion of television throughout Britain the B.B.C. has now had considerable experience in the acoustic treatment of television studios. In this paper B.B.C. experience was described and curves of optimum reverberation time were given. Details of the acoustic requirements for different types of programme were discussed and the techniques in television studios were compared with the corres­ponding techniques in sound studios.,

2.1.6. Sound Absorption Coefficients and Acoustical Design - T.D. Northwood and C. G. Balachandran

This paper described the investigation of diffusion in reverberation rooms using a correlation method. In the course of this work it was shown that a much better prediction of the effect of an absorber placed in a room with other absorbers was obtained by the Eyring formula than the Sabine formula which is used by many organisations for reverberation room work. This agrees with B.B.C. experience.

2.1.7. Der Einfluss der Kanten auf die Schall-absorption por8ser Materialien - W. Kuhl

Dr. Kuhl gave results on experiments on the edge effects of absorbers. He found that the effect of sub-dividing an absorber was to introduce additional absorp­tion proportional to the length of edge added by the process of sub-division. His measurements were made by dividing a patch originally 108 ft2 (10 m2) in area by stages into 25 patches. He also stated that edge effects could be eliminated by surrounding the sample with a frame projecting outwards from the wall to twice the thickness of the sample.

2.1.8. Computations on the Absorption of Sound by Absorbent Patches - R. K. Cook

Cook alsO described investigations of the edge effect of absorbers. On the basis that the edge effect of a sample amounted to the addition of a quantity propor­tional to the periphery p, the total absorption could be written as A ao + 'YP where A is the area, ao the true absorption coefficient and 'Y an edge effect constant. This may be written a. = ao + 'Y pi A where a. is the effective coefficient of the sample and it can be seen that the second term in the bracket approaches zero as A becomes large.

Hence for a number of determinations of a plotted against piA the graph immediately shows the infinite area absorption coefficient and the edge effect constant.

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2e109o Precision of Reverberation Chamber Measurements of Sound Absorption Coefficients - R. V. Waterhouse

Waterhouse discussed the precision of reverberation room measurements with special reference to the use of rotating fans to improve the diffusion. He gave some evidence supporting his preference for this method, though it does not appear to be shared by many other organisations.

2.1.10. Untersuchungen zur Ermittlung der Diffusit~t bei Messungen des Schallabsorptionsgrades im Hallraum -- G. Venzke and P. D1im:mig

Venzke and Dammig described a method of measuring directional diffusion in reverberation rooms using a parallel battery of Labor MD82 directional microphones.

2.1.11. Die Messung der Schallabsorption von Materialien ~ C. W. Kosten

This was the most important paper on this subject. The first part was a description of the various forms of standing-wave tube required to cover the whole audio-frequency range and a review of the various kinds of absorption coefficient, such as normal, random or oblique incidence. The author touched on the effects of sample size, diffraction, distribution and diffusion, and referred to a paper by Meyer and Kuttruff which arrived at results closely similar to those given in a recent Research Department report. 1 The German work was carried out in models, whereas the Research Department work was in full-sized rooms. He then referred to the efforts which had been made by various continental organisations to compare the results of reverberation measurements in different laboratories. In 1955 a pilot experiment was made by three German laboratories only, using the same sample for all measurements. This laid the basis of a more recent experiment embracing 25 laboratories, in which the B.B.C. Research Department took part. In this experiment each organisation made measurements with its normal arrangement of the sample or samples and normal measuring technique. All details were noted on the questionnaire forms. Kosten arranged the measurements of the five groups as follows:

1. Sample in one area only with adequate arrangement for diffusion. This he described as the I.S.O. condition.

2. Rectangular room, one sample with poor diffusion.

3. Non-rectangular rooms with one sample.

4. Rectangular room with three or more samples.

5. Non-rectangular room with three or more samples.

On the average the I.S.O.-condition results lay more or less in the middle of the whole population. They were also the most self-consistent. Groups 4 and 5 appeared to be the next most consistent but were on the whole higher than the I.S.O. group. Groups 2 and 3 were generally lower and more variable than the rest.

Kosten showed that the influence of diffusion was mainly to raise the higher­frequency part of the absorption curve. It will be noted also that sub-division of the sample into three or four patches would tend to have this effect besides intro­ducing additional edge effects. On both counts, therefore, one would expect them to be the highest.

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To obtain a graph unifying all the results, he plotted 1/12;' 3 as ordinates against the mean absorption coefficient above 500 c/s as abscissae, 1 being the frequency and 1 the volume of the room. This introduced a correction for the size of the reverberation room and it was found that the I.S.O.~condition results fell fairly closely on a single downward sloping line through the centre of the diagram. To the left of this line fell conditions 2 and 3 while conditions 4 and 5 were all to the right, including the B.B.C. Research Department results.

One implication of this diagram was that the Research Department condition gives higher results than those from any of the other laboratories co-operating. Nevertheless, careful checks which are made from time to time on the accuracy of prediction of reverberation times of studios show that at frequencies above 500 c/s B.B.C. measurements lead to lower rather than higher predictions of total absorption than are realised in practice. For organisations concerned only with the manufacture of acoustic materials for application in large areas, the I.S.O. condition may well be that most likely to lead to dependable standards, but in a broadcasting organ~sation in which all treatments must be applied with a view to good diffusion it would lead to studios which are deader than predicted.

2.1.12. Welcher Aufwand an Information ist erforderlich, um einen Raum akustisch zu charakterisieren? - L. Cremer

The most important contribution on diagnostic methods in acoustics was this paper by L. Cremer. His main theme was that methods for analysing the acoustic characteristics of an enclosure should aim at providing as much information as the ear can assimilate and not more. He confined himself to transient processes, ignoring steady-state transmission characteristics since they are not directly audible.

According to Seraphim2 there are 50 discernible increments of reverberation time between 0'5 and 3 seconds requiring at least 5 bits for description, and if we take 20 critical frequency bands or third-octave bands as the minimum number, 100 bits are needed for a reverberation/frequency diagram. This figure must be multiplied again by the number of significantly different measurement positions in the hall.

He referred to attempts made by Thiele and by Niese to express echo-patterns as a single figure containing time and frequency but said that such single figures could not take into account all audible peculiarities.

Intermediate stages are possible. Junius 3 restricted the information in the echograms to integrals over 5, 10 and 20 ms, while Niese 4 integrated the displays over a period of 23 ms, representing the time during which the energy of successive impulses is summed by the ear.

He then dealt at length with the pulsed glide displays of Somerville and Gilford,5 paying particular attention to the brightness modulated display which was introduced experimentally as an alternative to the usual type but which has not been used as a routine test. Here, to reduce the redundancy of the information, Cremer divided the frequency scale into twenty bands, and he showed an example of such a display obtained in a room with a prominent flutter echo. A suitable test sound for use in conjunction with this test would be a Gaussian pulse, COmbining 5 bits for level steps with 20 frequencies and a suitable number of loudspeaker and microphone positions. A typical large hall would, by this method, require 500,000 bits for characterisation.

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Lastly he touched on the additional factor which can be observed - the direction - and showed again that all attempts, such as the "hedgehog" diagraIILS of Meyer and Thiele, to characterise the directional properties of halls gave a greater increase in the quantity of information than that provided by two ears in place of one.

2.1.13. Das Problem der Schallstreuung in der Raumakustik - J. Hol tsmark

This paper described the measurement of diffusion in rooms by examination of the fluctuations in decay curves. The author operated on the decay curve with an amplifier whose gain increased exponentially with time. The exponential time­constant was adjusted to convert the decay curve into a fluctuating horizontal trace which was then examined by autocorrelation with a continuously increasing correlation interval. However, it may be shown that this process is equivalent to a simple frequency analysis of the trace. It did not appear to give any useful results.

2.1.14. Direct Reading Reverberation Time Measuring Device - L.X. Nepomuceno

Nepomuceno described an electronic timing device measuring the time taken for a given fall in the sound level. It will also divide the decay into two halves which are separately timed, thus enabling the diffusion "S factor" introduced in a recent Research Department report 1 to be rapidly evaluated.

2.1.15. A New Automatic Method for Measuring the Reverberation Time and the State of Diffusion of a Room - M.R. Schroeder

This paper broke almost entirely new ground. In a previous papers in 1954 it had been shown that the number of phase reversals in a given frequency interval, using a loudspeaker and microphone separated by a known distance in a room, was proportional to the reverberation time of the room. Research Department tried this at the time as a simple steady-state method of measuring reverberation time but obtained results in error by a constant factor of 1'4. Schroeder now finds by a more correct analysis of the wave statistics that his original equation was wrong by a factor of approximately this amount (actually I2i @ This method enables a continuous, even automatic, measurement to be made without recourse to high-speed displays or timing devices.

2.1.16. Wide Band Absorbers with Impermeable Facings - C.L.S. Gilford and N. C.H. Druce

This paper described sound absorbers, recently developed by the B.B.C., in which a high coefficient of absorption is maintained up to high frequencies in spite of the fact that the front surfaces are impermeable membranes. The principle of operation is to use the front facing membrane in conjunction with a thin layer of porous material immediately behind it as a resonant system with a high natural frequency. Specialised forms of the absorbers were described.

2.1.17. The Damping of Eigentones in Small Rooras - F.J. van Leeuwen

This interesting paper gave detailed calculations for the properties of slit resonators required for the optimum damping of room resonances, It seems that as a result of this, and work recently published elsewhere by W. WOfile; further trials of resonators in studios may be justified.

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2.1.18. Luftschalldammung - K. GOsele

K. GOsele's invited paper on sound insulation was a masterly survey paper but did not contain very much new material. The main theme of his lecture was the necessity of avoiding, over limited frequency bands, excessive transmission due to resonances. In the case of direct transmission through walls, transmission could be caused by the use of inappropriate filling blocks, such as the pre-cast hollow concrete blocks so widely used in Germany, or by the use of edge constraints of the wrong compliance.

In the case of indirect transmission from one room to another by flanking paths, spurious transmission can result from false ceilings of identical construction in the two rooms or by a floating floor common to both. GDsele gave a figure of 10 dB as the possible reduction in sound insulation at 500 cls for the first case and up to 16 dB for the second. Dips of the same order had been found also where a layer of foamed resin heat insulation had been applied to a wall before plastering.

The author ended by applying these principles to the design of transmission­measuring rooms, showing that the experimental partition should be mounted in a comparatively weak wall of the source room, the receiving room being isolated from it by an air gap.

2.2. Electra-Acoustics

This branch of the subject did not have any invited papers. The great interest in stereophony at present was, however, reflected in a number of papers. These were mainly theoretical in character, ~ich is not surprising because it is as yet a bit early for wide practical experience to have been gained.

2.2.1. Kurzer Uberblick uoer die Vorschlage stereophoner Rundfunkubertragungen - K. Wilhelm

This paper gave a survey of the various proposed systems of stereophonic broadcast transmission.

2.2.2. Die Praxis der Regeltechnik bei kompatiblen Stereoaufnahmen­W. Schlechtweg

This paper described in principle a system of studio control equipment for producing, simultaneously, stereophonic and monophonic versions of the same programme. Mixing is carried out in accordance with the requirements for monophonic transmission, the stereophonic controls being limited to width regulation and left-right bias for each microphone.?' 8 It appeared to have been overlooked that any problems of compati­bili ty ~ich may exist cannot be removed by a mere re-arrangement of circuits.

2.2.3. Grenzen der Trickstereophonie - F. Enkel

The author discussed methods of reducing the directional information required for stereophony to the point where it could be conveyed by a pair of narrow­band pilot signals transmitted at low level, using frequencies just outside the upper limit of the programme spectrum. 9 The system is similar in principle to that of Percival. It is understood that experiments on these lines initiated in the West­deutscher Rundfunk have in fact been abandoned.

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202.4. Eigenschaften des natUrlichen Richtungshorens und ihre Anwendung auf die Stereophonie-N.V. Franssen

The paper suggested an electrical analogue, involving a pair of tapped delay lines, whereby the main characteristics of directional hearing - including the res­ponse to both time and amplitude differences at the two ears - could be simulated.

2.2.5. Stereophonic Sound Reproduction - H. F. Olson

This contribution was extremely disappointing; it consisted of little more than technical platitudes.

2.206. Measurements of Elastic Parameters of Paper for Loudspeakers-­C. Bordone-Sacerdote

This paper on loudspeakers is worth noting. It described measurements of the complex modulus of elasticity of diaphragm materials and the significance of the result in relation to the overall performance.

2.2. 7. Synthesis of Bass-Reflex LOUdspeaker Enclosures - E. de Boer

This was a mathematical theory of the behaviour of loudspeaker enclosures at low frequencies and showed that it may sometimes be advantageous to interpose an acoustic mass between the loudspeaker diaphragm and the enclosure volume.

2.2.8. Miniature Condenser Microphones - C. Wansdronk

The author described a newly developed miniature cardioid microphone employ­ing a radio-frequency polarising circuit. The electronic arrangements are a modi­fication of a system proposed in 1947 by van Zelst 10 but later abandoned because of the difficulty of keeping the circuit in adjustment. The improved stability of the new arrangement is obtained at the cost of increased noise, but the signal/noise ratio is nevertheless claimed to compare favourably with the figures commonly obtained with the conventional d.c. polarisation.

2.2.9. Improvement of Acoustic Feedback Stability in Public Address Systems - M. R. Schroeder

An ingenious artifice for increasing the gain of public address systems wi thout producing "singing" was des cri bed, and its effects demonstrated by magnetic recordings. Theoretical studies have shown that the overall loop attenuation, and hence the stability margin, could be increased if all signals picked up by the micro~ phone were slightly shifted in frequency before being applied to the loudspeaker. The optimum value of frequency shift is equal to the mean interval between the peaks and troughs in the acoustic frequency response of the room - as measured between loudspeaker and microphone - and in typical cases would be of the order of 5 c/s. An increase of stability margin of some 11 dB has been obtained by this method; in practice, however, the permissible gain increase is limited to about 6 dB because of some unpleasant subjective effects which result from the progressive changes in pitch of a signal repeatedly traversing the acoustic feedback loop. Even 6 dB, however, could represent a considerable improvement in marginal cases and the artifice could have many applications in systems where the quality of the reproduced sound is of little importance.

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2.2.10. Some Engineering Problems Arising in the Sound Equipment of the Television Service -D.E.L. Shorter

The application of special types of loudspeakers and microphones to the solution of engineering problems in the television service of the B.B.C. was dis­cussed. Wide range monitoring loudspeakers with special directional characteristi cs, designed for use in mobile control rooms, were described, together with other direc­tional loudspeakers, having a restricted frequency range, employed to allow artists in different studios to hear each other without danger of acoustic feedback. Reference was made to the use of high-directivity microphones for operation at long range and to the special measures necessary to avoid unwanted noise produced by movement of the boom. It was pointed out that long range working may sometimes be avoided by the use of microphones worn on the person; the fundamental limitations of this artifice were discussed.

202.11. Wiedergabe von Magnetton-Aufzeichnungen mit Hilfe des Halleffektes - F. Kuhrt

The author described the application of the Hall effect to magnetic record­ing, giving details of reproducing heads constructed with slices of the active material., of the order of 10,Uthick, sandwiched between the two pole pieces. The magnetic effect on the tape of the direct current passing through the Hall element was stated to be negligible.

2.3. Physiological and Psychological Acoustics

The first of the two opening lectures, given by Professor Feldkeller, was a broad review of the present relationship between acoustics and communication engineer­ing, covering the analysis and synthesis of speech sounds and of the corresponding movements of the vocal organs, the mechanism of hearing and the nature of the infor­mation to be received.

2.3.1. Uber die nervosen Prozesse beim Horen und der Vibrationsempfindung­G. von Bekesy

This was an invited paper and was mainly concerned with the analogy between the nervous system of the cochlea and that of the skin in general. Experiments in the sensations produced by the direct vibratory excitation of various points on the body yield information which can be applied to the explanation of hearing phenomena. Although much of the work had been previously published, the lecture, which was admirably presented, was a valuable contribution to the proceedings of the Congress.

2.3.2. Dber die Grundlagen der Entstehung der Erregung im Nervensystem - A. von Mural t

This also was an invited paper which was an excellent survey of the present theory of transmission of impulses along nerves.

2.3.3. Regelungsvorgange beim Sprechen und Horen - K. Kupfmiiller

This paper described the various servo mechanisms which regulate the pro-cesses of speech and hearing. Data were given showing the displacement/time function

11

for voluntary movements of the jaw and other parts of the body and used to deduce the values of time constant to be assigned to the physiological feedback loops involved in speech production. Corresponding time constants for the hearing mechanism were arrived at from data on the time taken to recognise various speech sounds. Finally, the simultaneous operation of the speech and hearing systems was considered with reference to such effects as the time lag in repeating a given sound and the reaction of a talker to a delayed reproduction of his own voice.

2.3.4. On the Acoustics of Speech -G. Fant

Fant described in considerable detail methods of speech synthesis employing acoustic and electrical models of the vocal tract. A method of operating on a given speech waveform to obtain information on the required circuit constants was given and recorded examples of synthetic speech were demonstrated.

2.3050 The Auditory Input-Output Function - S. S. Stevens

Stevens gave a general paper comparing loudness with other sensations and showed that the sone scale is the most valid method of measurement. At levels appreciably above the threshold the loudness is proportional to stimulus to the power 0'3.

2.3.60 The Form of the Loudness Function Near Threshold -B. Scharf and J.C. Stevens

This continued the work of the above paper. It was shown that the curve could be restored to the "0"3 power" along its whole length if the threshold pressure is subtracted from the actual pressure before calculating the phon values.

203070 Die Tragheit der Lautstarkeempfindung und ihre Nachbildung bei obj ektiven Lautstarkemessern - H. Niese

H. Niese presented new data on the loudness of impulsive noises, a subject on which he has published a good deal lately. He concluded that the assumption of his previously given figure 4 of 23 ms for the "integration time" of the ear may be used as the basis of an objective noise meter which puts impUlsive noises in their correct relationship to steady noise, Research Department has recently been explor­ing the same possib ili ty in connection with the measurement of traffic and footsteps noise in studios.

2.4. Measurements

2.4.10 A Logarithmic Attenuator - S.B. Pedersen

This paper described logarithmic amplifiers using semiconductor diodes. The equipment did not, however, appear nearly as advanced as our own, having a very much smaller range of logarithmic law, (40 dB as compared with the 60 dB of our standard instruments). Conversation with Mr. Pedersen elicited the fact that he had confi~d his investigations to one type of diode only, whereas we select the best individual diodes from the most suitable types, thereby avoiding the rather complicated correct­ing circuits which he found necessary even for his limited range.

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3. COLOGNE (WESTDEUTSCHER RUNDFUNK)

A general description of the sound studios was given in a previous report. i1

Since that date there have been only small changes, and the majority of the available time was therefore spent in the television block.

This is a temporary building only a few hundred yards from the Funkhaus, which is situated right in the heart of the city. It will be replaced within a few years by a large television building to be constructed on a bombed site immediately adjacent to the Funkhaus. As the building is temporary, the studio had little of great interest in its acoustic treatment; materials such as Heraklith board and rockwool were mainly used, with perforated brick Helmholtz resonators for low-frequency absorption. The studio sounded rather more reverberant than desirable. Ferns eh television equipment and Mole-Richardson lighting were in use, and an Ampex video recording channel was in process of being commissioned.

4. STUTTGART (SUDWESTFUNK)

4.1. Sound Studios

The main studio centre in Stuttgart is situated in the grounds of an old palace called the Villa Berg. There are one or two studios in the old building but recently a new block of sound studios has been completed in the grounds. The largest studio is one of about 100,000 ft 3 (2,Soo m3 ) for symphony orchestra or other large com­binations. The studio is "dead" for its size, having large areas of porous absorbers covered with perforated metal, giving 3O~ open area. There is a curved wall behind the studio and an end wall, part convex and part splayed. Diffusion has been care­fully studied; below the architectural ceiling, which carries patches of absorbing material, there is a suspended ceiling with numerous steel and perspex plates. The small audience is confined to three separate irregular-shaped areas, which are them­selves intended to promote diffusion. The reverberation time is O"S sec.

An unusual feature is a storage space beneath the stage, to which it is connected by large openings through which rostra and musical instruments may be passed. The storage space is damped with slit resonators.

The drama complex is of the usual continental type and consists of four studios served by a common control cubicle. All the studios have tracks, stairs and other effects. There is a "dead" room with a deep rockwool treatment, about 20 cm in thickness; a small reverberation room with a stone floor; an ordinary room having the treatment covered with slotted metal; and a large studio with an unusual type of perforated finish. This consists of aluminium sheet with square perforations to which, prior to perforation, had been stuck a wallpaper with a wood-grain pattern. The grain pattern helps to prevent troublesome optical effects. Even although the open area is 44~, the reverberation time of this studio is O·S sec.

4.2. Television Studios

At Villa Berg there are, so far, no television studios, although a new block is being planned. The only television studio is situated at Killesberg Hohepark.

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Here the north-light roof carries wooden frameworks parallel to the roof sections, faced with fabric on which is laid 10 cm thick rockwool. The guide was unable to quote the reverberation time, but was of the opinion that as short a time as possible had been aimed at, mainly to reduce the intensity of unavoidable studio noises. There was no concern with achieving any particular optimum time, since artificial rever­beration was extensively used to achieve the acoustic results. A unique feature of this studio was an audience gallery running round three sides and separated from the studio by glass windows.

4.3. Microphone Testing

A method of comparing the frequency responses of two microphones in a reverberant room was demonstrated at Villa Berg. The system is unsuitable for application to two microphones differing in directional properties and as a routine test appears to offer little advantage over the method, adopted elsewhere in Germany, 12

of comparing the mean sensitivity to various bands of random noise. The instrumental arrangements were, however, of some interest as they involve the use of two logarithmic rectifiers covering a wide amplitude range with high accuracy of law. For this purpose, an early type 13 of electro-mechanical servo system has been adopted which employs a fluid potentiometer having the "slider" carried on the pointer of a moving­coil milliameter. In the original version the required law was obtained by graduating the cross-section of the trough containing the fluid; in the new arrangement, a series of moveable electrodes dipping into the fluid form a ladder attenuator, the characteristics of which can readily be adjusted. A range of 80 dB with an accuracy of 0·5 dB is claimed.

5. MUNIClI (BAYERISClIE RUNDFUNK)

The two television studios out at Freimann and the drama complex at Rundfunkplatz in the centre of Munich are unchanged since the visit in 1955. 11 It was hoped that it would be possible to make a more thorough examination and assessment of the lightweight sound-insulation doors between the television studios, described in the 1955 report, but unfortunately the studios were in use.

Most of the time was therefore spent at the sound centre. The old drama studios are of no interest, but in the new studios curved sheets of perspex were fitted in front of the cubicle windows to prevent sound reflections. They did not, however, overcome the difficulty of dazzle from reflected studio lights. Sound diffusers were fitted to the walls but were rather too small to be effective. Ven­tilation silencing, using multiple splitters, and nylon screens were of interest.

On the same site now being constructed. 400,000 ft3 (11,300 m3).

a large new building, devoted mainly to sound studios, is This will include a large orchestral studio of about

At present the Herkulessaal or the hall of the Deutsches Museum is used for orchestral concerts.

In the construction of the new building every precaution has been taken to isolate the studios from each other.

At a later date a television block is to be provided.

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6. BADEN-BADEN (SUDWESTFUNK)

This centre is devoted entirely to sound broadcasting and includes a concert studio, two smaller music studios and a drama complex, in addition to talks studios. There are many points of interest in their design and they will therefore be described in some detaiL

6~1. Concert Studio

This and the chamber music studio described below are in a separate block which was erected shortly after the war. The volume is similar to that of Maida Vale Studio 1 but the ceiling is higher. It was built in a very short time, only five months elapsing between the laying of the foundation stone and the first concert. The reverberation time is 1"6 sec, with a slight rise in the bass. This is somewhat lower than the B.B.C. optimum and it is said to be excellent for chamber music.

The ceiling is entirely covered with polycylindrical panels, though,' as they all run transversely, diffusion may not be particularly good.

There is a strong tendency among the programme staffs in the Sudwestfunk towards multi-microphone methods and synthesis of recordings from as many as eight separately recorded signals" This is naturally most marked with light music (where the final result is quite frankly regarded as a synthetic product), but it is also extending to symphonic music broadcast from the concert studio. assessed as being only fair.

6.2.., Chamber Music Studio

The studio was

This studio is somewhat live (O'S sec) for its size of about 6,000 ft S

(170 mS), having a plywood ceiling, cork floor and absorbing treatment only round the walls. It is of no special interest.

6.3. Entertainment Music Studio

This is the largest in the main block. Its volume is 90,000 ft S (2,500 mS) and reverberation time 1'5 sec at 1 kcls falling at both lower and higher frequenciess The auditorium ceiling is covered with rows of hemispherical Helmholtz resonator absorbers which also occupy the sides of the stage. The remainder of the low~ frequency absorption is provided by the loaded building~board absorbers which have been described in a Research Department report. 14 It appears that the conclusion, expressed in that report, that the masses carried by these absorbers are of no value as tuning elements, has since been confirmed by Dro Kuhl of the Rundfunktechnisches Institut in Hamburg.

The ceiling over the stage itself is traversed by lighting rolls similar to those in the Liverpool Philharmonic Hall 15 and the back is covered with heavy curtains. This is a result of discussions during a recent visit to Research Department of Dr. H.J. von BraunmUhl, the chief engineer.

A dip at 270 cls in the reverberation characteristic before the installation of the Helmholtz resonators was found to be due to the use of a porous concrete as the inside surface of the walls. The dip was first thought to be due to coincidence effect transmission but was finally attributed to porous absorption by the concrete.

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There are seats for a small audience.

A separate studio of 9,000 ft S (250 mS ) with a reverberation time of 0·8 sec is used by soloists, often in conjunction with an orchestra in the main studio. This practice is, of course, fairly common in the film industry.

6.4. The Drama Complex

This comprises a lar~e main studio with a reverberation time of 1 sec, a lar~e dead room (0-2 sec), a normally treated speech room (0-4 sec) and a common control cubicle with the surprisin~ly lon~ reverberation time of 0'8 sec.

Normal porous treatment is in ~eneral use, covered in many places with wooden materials not seen in this country. One, called wicker panel, consists of long threads or cylinders of wood held together by wefts of a textile material about 1 in. (2°5 cm) apart, and the other consists of thin strips of wood fabricated into a continuous sheet with the same appearance as expanded metal.

The dead room, which is 15,000 ft s (400 mS ) in volume, is treated all over the walls and ceiling with slit resonators. On the walls, shelves are built in front of this, 1 ft (30 cm) deep and stacked with pieces of rockwool of different width laid flat. A similar depth below the ceilin~, resonators are hung with rockwool in folds. '!he front covering is the wooden "expanded metal".

6.5. Speech Studios

In another buildin~ are two continuity suites of five rooms each, and two more general purpose studios. The general construction of these studios followed the methods of the rest; there were no principles new to us, but it was clear that the Germans here and elsewhere are prepared to spend several times as much on acoustic treatment as is the case in the BoB.C.

606. Monitoring Loudspeakers

The design of loudspeakers was discussed with Dr. von BraunmUhl and it was learned that as a result of dissatisfaction with the "omnidirectional" design of the (then) N.W.DoR., experiments are being carried out with radiating systems havin~ semi-directional characteristics. Demonstrations of the omnidirectional loudspeaker compared with an experimental semi-directional system ;;ere given and these confirmed the view that complete absence of directional effects is not, as has sometimes been assumed, the ideal for a monitoring loudspeaker.

7. FRANKFURT (HESSISClIE RUNDFUNIO

The sound and television studios are on the same site about three miles north of the city centre. The sound studios are as described in the previous report 11

but additional studios, including television studios, have been built in the central circular building. The site was first developed after the war as the temporary home of the new parliament, and this building was the assembly hall. It has a shallow dome of which more is said below. Herren Scheuermann and Heckmann ~ave us every opportunity to examine the studios in detail, and were generally very helpful and obliging.

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7.1. Sound Studios

The large concert studio was visited first. Variation in the acoustics of this studio may be effected by reversing large hinged panels, one side faced with plasterboard and the other with 4 in. (10 cm) of felt covered with a thin polythene

film.

After several years' experience in the use of the variable acoustics, it has been found that it is worth while adjusting the absorption to compensate for an audience but not to introduce deliberate changes in the acoustics between programmes. Most orchestral recordings are made with the studio empty, and the studio is also used for television shows with audiences.

The roof construction gives a mean insulation of 76 dB from external sounds. This figure was measured using a helicopter hovering 5 m (16 ft) above the roof as the source of sound. Details of the roof construction were noted.

The drama complex was unchanged from the previous visit. There is, how-ever, more use of recording techniques, and there is a Kuhl reverberation plate which is used for drama but is not favoured for music of any kind, as it is considered to be more objectionably coloured than an ordinary echo room.

7020 Television Studios

There are two television studios, both in the domed building. One, about the size of Alexandra Palace Studio B, is used for all productions and the other, which is considerably smaller, is used only for rehearsals. There has been con­siderable difficulty in avoiding unwanted acoustic effects from the dome. At present it is lined with rockwool enclosed in paper, below which are sheets of building board fixed vertically, the purpose of which was not obvious,. It is not yet entirely satisfactory. The control room was found to be treated with Coronet building· board all over the ceiling and had parts of the walls covered with small rectangles of ''Variantex'' with their edges clipped into wooden frames so as to bow them outwards in the form of shallow part-cylindrical shells. "Variantex" is a thin wafer-like material made from glass fibre bonded together with a porous resin to give it a flow resistance of the order of 50-200 c.g.s. units. It is used as the front facing of the Coronet building board referred to above.

Applied here as a self-supporting wafer at a distance from the walls of the control room, it appeared to absorb considerably at low frequencies. The manufacturers claim an absorption coefficient of 0"7 down to 200 cls and also that it may be redecorated with a special lacquer without effect on its absorbing properties.

Frankfurt will eventually become the centre of the German television net­work, and, with this in view, construction has started on three new television blocks behind the main buildings. One block will contain offices and a second will provide three large television studios. The third will be occupied by presentation studio suites and by technical areas generally. This building is already partly constructed; it has been necessary to bond all the girders, concrete reinforcing metal, piping, conduit, etc., to avoid interference from the radar of a nearby United States base.

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7.30 Stereophony

Enquiries were made about the present state of stereophony in Germany and particularly in Frankfurt. It appears that experiments have been conducted since 1955 using two transmitters radiating from the same mast. Most experiments were with direct studio transmissions as opposed to recordings. The sole concern had been so far with studio systems, no attempt having been made to tackle the problems of trans­mission channel economy or compatibility.

8. CONCLUSIONS

The most useful feature of these triennial congresses on acoustics is the opportuni ty they give for meeting all the princip al workers in this fi eld. They are also valuable in that visits to establishments in the host country can be made for discussion on subjects of specific interest to the Corporation. They do, however, have an inherent weakness because there are so many contributors that too many con­current sessions are required and the discussion on individual papers is reduced practically to zero. There are signs, however, that the International Commission on Acoustics is aware of this defect and it is hoped that it may be remedied in the next Congress, which will be in Copenhagen.

The visits to broadcasting premises in Germany have sho'Wl1 that architectural acoustics, sound insulation, and ventilation of studios is considered to be of such importance that no expense appears to be spared. Furthermore, the German architect is obviously most co-operative and anxious to incorporate the necessary features into his design. Similarly, the provision of electro-acoustics equipment of the highest quality appears to be taken for granted.

9. REFERENCES

1. "An Investigation of Reverberation Time Variations and Diffusion of Sound in &nall Rooms", Research Department Report No.B-070, Serial No. 1959/16.

2. Seraphim, H. P., "Untersuchungen uber die Unterschiedschwelle exponentiellen Abklingens von Rauschb and impulsen If, Acust ic a, Vol. 8, Beiheft No. 1, p. 280, 1958.

3. Junius, W., "Raumakustische Untersuchungen mit neueren Messverfahren in der Liederhalle Stuttgart", Acusti ca, vol. 9, No.4, P.289, 1959.

4. Niese, H., "Die Prufung des raumakustischen 'Echograd-Kri teriums I mi t Hilf~ von Silbenverstandlichkei tsmessungen", Hochfrequenztechnik und Elektroakustik, Vol.p6, No.3, p.70, 1957.

5. Somerville, T. and Gilford, C.L.S., "Composite Cathode Ray Oscillograph Displ ays of Acoustic Phenomena and Their Interpretation", B. B. C. Quarterly, Vol. VII, No.1, p.1, Spring 1952.

6. Schroeder, M.R., "Eigenfrequenzstatistik und Anregungsstatistik in Raumen ", Acustica, Vol. 4, Beiheft No. 1, p.456, 1954.

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7. Wo"hle, W., "Schallabsorption von Helmholtzresonatoren bei allsei tigem Schalleinfall und bei verschiedenenraumlichen Anordnungen", Tagungsberichte der Internationalen Fachtagung "Bau und Raumalrustik", P. 71, Dresden 1957.

8. ''Visit to Holland-May 1959", Research Department Report A-052, Serial No. 1959/17.

9. Enkel, F., "Die Ubertragung raumlicher Schallfeldstrukturen uber einen Kanal mi t Hilfe unterschwelliger Pilotfrequenzen", Elektronische Rundschau, No. 10, p. 347, 1958.

10. van Zelst, J.J.Z., "Circuit for Condenser Microphones with Low Noise Level", Philips Technical Review, Vol. 9, No. 12, p. 3f37, 1947/48.

11. "Visit to Germany - April 1955", Research Department Report A-03S, Serial No. 1955/37.

12. "Vi si t to Vienna, Hamburg and Cologne - November 1955", Research Department Report A-041, Serial No. 1956/6.

13. Meyer, E. and Keidel, L., "Ro"hrenvol tmeter mi t logari thmischer Anzeige", E.N.T., Vol. 12, No. 2, 1935.

14. "Measurements on Loaded Building-Board Panel Sound Absorbers", Research Department Report B-064, Serial No. 1956/18.

15. Somerville, T., itA Comparison of the Acoustics of the Philharmonic Hall, Liverpool, and St. Andrew's Grand Hall, Glasgow", B.B.C. Quarterly, Vol. IV, No. 1, April 1949.

MV MM

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