Neumann.Berlin Pressure Microphone With Sperical Acoustic Surface User's Manual
Contents
1. 12230 80601 Pressure Microphone with Spherical Acoustic Surface Dipl Ing Stephan Peus Paper read at the 16th Tonmeistertagung Karlsruhe 1990 In studio practice there are practically only two forms of the modern single diaphragm pressure microphone the cylindrical rod type and the flat boundary layer or pressure zone type Generally speaking studio microphones are faced with two diametrically opposed re quirements On the one hand the diaphragm surface should be as small as possible for the best transmission characteristics over the entire audible range independently of frequency and direction On the other hand the diaphragm has to deliver an adequate output level for a high s n ratio which for its part stipulates a certain minimum diaphragm area Rod microphones generally compromise with a diameter of about 20 mm Since a microphone is used in free space in the studio and is required to pick up sound from all directions it is not only the diaphragm diameter which has to be taken into con sideration but also the dimensions of the microphone body as a whole which by its very presence has an influence on the sound field With a microphone diameter of 20 mm for instance the result in the upper frequency range is both a pressure build GEORG NEUMANN GMBH CHARLOTTENSTR 3 D 1000 BERLIN 61 TEL 030 25993 0 TLX 184595 NEUAK D TELEFAX 030 25993 108 Printed in Germany GN 11 up characteristi2. shows the development of the sound pressure at the marked locations on the surfaces of various bodies in free sound field Here the overall dimensions of the bodies are coordinate wise equal thus permitting a qualitative comparison of the frequency responses It is intended to demonstrate that at the end face of a cylinder for instance fundamentally other pressure build up conditions are manifested than on bodies with other shapes In addition it is apparent that the sound pressure level in front of a rod type microphone rises by up to 10 dB and it is not only a doubling of the pressure which occurs The most even development is achieved on a spherical surface It is also interesting to note how the sound pressure changes on the front side of the sphere when there is no rear side The same applies for bodies of other shapes Thus it is not the same thing whether a microphone diaphragm forms the end face of a long cylinder or con stitutes a disc in free sound field example dual diaphragm capsule Fig 2 depicts various sound impingement angles for two of the bodies illustrated in the foregoing cylinder and sphere It was apparent in Fig 1 that the pressure build up in front of a sphere is very even and extends only as far as a doubling 6 dB rise but it is seen in Fig 2 that this smooth development also applies to other sound impinge ment angles Furthermore it is obvious that level variations among various impingement angl
3. 0 series The microphone is the TLM 50 Literature 1 B M ller A new Type of Boundary Layer Microphone Paper read at the 88th AES Convention Montreux 1990 2 H Grosskopf Neue Kondensatormikrophone f r Rundfunk Studios FTZ 1951 No 9 Fig 1 and 2 are from the book Acoustical Engineering Chapter I by Harry F Olson D van Nostrand Comp INC Princeton New Jersey SPHERE HEMISPHERE RESPONSE IN DB RESPONSE IN Hi 600 FREQUENCY CYLINDER RESPONSE IN DB RESPONSE IN DB 200 300 600 1000 2000 4000 1000 FREQUENCY FREQUENCY DOUBLE CONE CONE a a 2 z N o z z c c 5 600 1000 200 300 600 1000 FREQUENCY FREQUENCY DOUBLE PYRAMIO PYRAMID RESPONSE IN DB RESPONSE IN DB 200 300 600 1000 2000 4000 5 600 1000 FREQUENCY FREQUENCY Schalldruckpegelverl ufe an unterschiedlich geformten K rpern Response frequency characteristics at objects of different shapes GEORG NEUMANN GMBH 8 E AVANT TW am SS 120 DIRECTION OF PLANE WAVE NORMAL A RE VN Winkelabhangigkeit des Schalldruckpegelverlaufs an Zylinderstirnflache und Kugel f The diffraction of a sound wave by a cylinder and sphere GEORG NEUMANN GMBH T m gt O Z Z lt 2 Ka ad O LL O euoudouoru eqni 09 IN 34 jo suods i Aduenbe4 0 0S IN suoud
4. asured impulse widths had values between 7 9 5 11 6 us whereby an impulse width of 10 us corresponds to a limit frequency of 50 kHz No doubt it would be possible to deliver a lecture just on the differences in sound produced by these five microphones but on the other hand the lecturer would have to define what an audible difference actually is As the above mentioned measured results indicate the audible differences were naturally very small and therefore the selection of the reference capsule was almost a matter of personal preference For each one of the five microphones we received descriptions of the sound which had reference to distinctive characteristics in various frequency ranges Naturally such statements did not result in any clear order of merit but nevertheless pointed the way to a decision in favour of the 2 25 um diaphragm which delivered the shortest measured impulse width The Housing Already prior to these detailed listening tests for the selection of the diaphragm material it had been established in several series of measurements and again in subjective tests that it is not necessary to mount the capsule arrangement in such a sizeable housing as was axiomatic in the age of the vacuum tube Accordingly the smaller and handier housing and grill form shown in Fig 6 was evolved The Circuitry The microphone capsule and the first amplifier stage are built into the plexiglass sphere Therefore the signal path in the
5. c for frontal sound impingement and the occurrence of direction de pendent shadowing and diffraction effects The microphone therefore does not reproduce just the original sound field but the original plus alterations superimposed on it by the fact of its own presence As for the pressure build up caused by the movement of air particles in front of the al most sound reflecting diaphragm it is endeavoured to shift this into the region of the highest possible frequencies by making the end face of the cylindrical microphone very small or by dint of incorporating the diaphragm in a large surface to shift the pressure build up into such a low frequency region that the microphone is subjected to it over its entire range of response It is then acting as a boundary layer microphone and is thus again able to function independently of frequency At the same time a microphone of this type by incorporation in a limiting surface which is present in any case is withdrawn out of the sound field and is no longer able to cause any interference 1 The Capsule Body Now it is certain that the dream of both microphone manufacturers and users will con tinue to be pursued of producing an almost non present and a noise and distortion free transducer Alternatively one can regard a microphone body as a necessary evil and consider whether other practical designs might be evolved which would not inter fere with the sound field as much as a cylinder Fig 1
6. es are greater with a cylindrical design that with a spherical body The Archtype At the beginning of the Fifties a microphone was produced by NEUMANN in which a 12 mm diameter condenser capsule was located in a spherical body 40 mm in diameter This configuration somewhat removed from the tube amplifier was free mounted behind a wire gauze grille the diaphragm being parallel to the microphone housing Fig 3 2 What we have here then is a combination of the bodies discussed in the foregoing cylinder end face microphone capsule and spherical body the characteristics of which corresponding to the ratio of wavelength to body dimensions transposed are rediscovered for example in the 0 frequency response of this microphone Fig 4 the flat rise from 1 kHz upwards with a maximum lift of 6 dB sphere characteristics the fall off above 10 kHz cylinder surface and capsule characteristic and a dip around 4 kHz caused by diffraction and reflection at the amplifier housing After 20 years of manufacture this microphone was and still is extremely popular partly because of some of the characteristic properties described above For this reason we have decided to take up this concept once again Another special feature of this microphone not previously mentioned is the capsule By the use of an 8 um thin metallic diaphragm and a diaphragm spacing of only 10 um with condenser pressure microphones this is twice as much a
7. grill down to the other circuitry is low impedance This means that the wire gauze grill could be made particularly open in the acoustical sense without any danger of hum or RF interference In the lower part of the microphone is a switchable high pass filter stage and the dc converter which generates the voltage necessary for the amplifier and capsule from the 48 V phantom powering circuit The output level of the microphone can be reduced by 10 dB by means of a second Switch Summary A condenser pressure microphone has been developed which by the nature of its cap sule design has a particularly short transient response and exploits the acoustical pres sure build up and diffraction properties of a spherical surface Fig 7 the smooth rise in the pressure build up range for frequencies above 1000 Hz amaximum pressure rise of 6 dB in the 8 kHz region the early and even increase in the directivity index with rising frequency whereby likewise due to the spherical surface the frequency responses for side and rear sound incidence are almost identical In the middle and upper frequency range the microphone exhibits directional proper ties which are almost comparable with those of a pressure gradient microphone while at the same time featuring as a pressure transducer a sensitivity which is linear all the way down to the lowest frequencies The amplifier circuitry is transformerless and corresponds to that of our fet 10
8. hragm mass The thinner or the lowest in mass the diaphragm the shorter the attack time i e the width of the first impulse It should be borne in mind here though that the diaphragm cannot act on its own both the cushion of air in front ofthe diaphragm and the enclosed volume behind it make a considerable contribution to the mass amounting in fact to some 30 to 75 depending on the capsule design However since the volume be hind the diaphragm like the diaphragm spacing of this particular capsule is very small the average width of the reproduced impulse is extremely short no more than some 8 5 us Following these objective measurable values which do not by themselves provide any evidence of quality we proceeded to the much more complicated question of subjec tive assessment Since hearing and the interpretation of things heard is not an absolute process as everyone knows we decided to make use of a comparison capsule For this purpose it seemed only logical to use an original capsule from past production We therefore had five microphones of identical construction the capsules of which were covered with five different diaphragms compared by sound engineers Diaphragms of aluminium nickel and titanium were used with thicknesses between 2 25 um and 10 um Differences output level were smaller than 0 5 dB below 5 kHz and somewhat larger above 5 kHz depending on the diaphragm used but in no case greater than 1 dB The me
9. onuue1uoH sop 608020611964 3 0 RA 00001 0005 0002 0001 006 00 001 ORTE Rus 23 83 1 14 10 23 10 77 STE Hur 2 25 pm Nickelmembrane d 44 444 ag a 10 p Nickelmembrane eppure Glaserklirren Clinking of glasses 50 ps Div 2 20 kHz Impulsantworten des TLM 50 mit zwei verschiedenen Membranen Transient response of the TLM 50 with two different membranes NEUMANN 1 619 93039 AMI EHHH o PO ART 010100110000000001 LOL Wd MN o c o a o x B o G o 75 x lt 2 a o 75 Ba En N o 2 c o Aen L Response frequency characteristics of the TLM 50 pressure condenser microphone GEORG NEUMANN GMBH
10. s a rule a high sen sitivity and low equivalent noise level have been attained In the case of the sphere zero degrees is defined in practice only by the circumstance that a microphone diaphragm can regrettably not be constructed as a breathing sphere but is a flat sur face section of the sphere with a surface normal Selection of Diaphragm Material Unfortunately there are no metal foils on the world s markets which meet our require ments regarding thickness constancy freedom from porosity and surface quality For this reason we decided to make the diaphragms for this pressure transducer by our own electroplating methods This afforded us the freedom to determine the thickness ofthe diaphragm ourselves i e to examine with the aid of acoustic measurements and listening tests whether there were still any audible differences below 10 um diaphragm thickness Metrologically the tests were carried out with the aid of the transient response of the various capsules Parameters were the thickness of the diaphragm its mechanical ten sion and the diaphragm material Fig 5 shows the transient response diagrams of two extremes as far as the diaphragm thickness is concerned 2 25 um top and 10 um centre At the bottom for purposes of comparison and as a supplementary criterion is the oscillogram of a breaking glass with a maximum frequency of 20 kHz As might be expected the transient response reflects directly the diap
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