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PRIVATE AND CONFIDENTIAL 




RESEARCH DEPARTMENT 



A.K.G. ELECTROSTATIC MICROPHONES TYPES C26 AND C28 



Report No. L-048 

( 196 1/12) 



THE BRITISH BROADCASTING CORPORATION 
ENGINEERING DIVISION 



RESEARCH DEPARTMENT 



A.K.G, ELECTROSTATIC MICROPHONES TYPES C26 AND C28 



Report No. L-048 

( 196 1/12) 



H.D. Harwood, B.Sc, A.Inst.P,, A.M. I.E. E, / X^^ 4 ^' 

J.R. Chew, B.Sc. (Eng. ), Grad. I.E.E, *~t^ 

R.L. Deane 

R.J, Packer (T. Somerville) 



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Report No. L-048 

A.KoG„ ELECTROSTATIC MICROPHONES TYPES C26 AND C28 

Section Title Page 

SUMMARY ............................... 1 

1 INTRODUCTION 1 

2 DESCRIPTION 2 

<j a JL. a OcHSrSX • •eaaaoaoaooeaeseasesaaaasas <j 

2.3. Type CK26 Capsule ....................... 3 

2.4. Type CK28 Capsule ....................... 3 

2. 5. Amplifier and Cables ... ............. 4 

3 PERFORMANCE .............................. 5 

3.1. Method of Measurement ..................... 5 

3.2. Frequency Characteristics ................... 6 

3.3. Impedance ................ ...... 7 

3.4. Sensitivity .................. 9 

4 NOISE ........... ........ ........ ..... 9 

4.1. Internally Generated Noise .... ....... 9 

4. 2. Interference from Magnetic Fields ........ 10 

4.3. Interference from Radio-Frequency Signals 10 

4.4. Wind Noise ...... ......... . 10 

4. 5. Interpretation of Noise Measurements ..... 11 

4.6. Listening Tests ........ ... 11 

5 CONCLUSIONS . 11 

o REFERENCE ••••••••#••••••»•••••**•■»*•••* -L^ 



PRIVATE AND CONFIDENTIAL 



July 1961 



Report No. L-048 

( 1961/12) 



A.K.G. ELECTROSTATIC MICROPHONES TYPES C26 AND C28 



SUMMARY 

This report describes tests on two electrostatic microphones manufactured by 
the Akustische und Kino-Gerate Gesellschaft (A.K.G. ). The first, type C26, has 
omnidirectional and the second, type C28, cardioid characteristics. The microphones 
are of small dimensions and provision is made in each case for introducing an extension 
piece between the capsule and the head amplifier. Two types of windshield are 
provided; one can be fitted over the end of the microphone itself, whilst the other 
is designed to protect the microphone capsule when an extension piece is used. 

Measurements have been made of the microphone frequency characteristics with 
and without the various fittings, and the degree of interference from wind and from 
magnetic fields has also been determined. The quality of reproduction obtained, 
particularly from the type C28, is good and the degree of interference is low. The 
susceptibility to radio-frequency fields has also been measured and a method of 
reducing it has been indicated. 



1. INTRODUCTION 

The Akustische und Kino-Gerate Gesellschaft of Vienna has introduced two new 
electrostatic microphones, type C86 which has a nominally omnidirectional character- 
istic and type C28 with a cardioid characteristic. The same head amplifiers, mains 
units and interconnecting leads are used in both types, the only differences being in 
the internal construction of the capsules types CK26 and CK28, which have identical 
external dimensions. The capsule in each case is surrounded by a layer of fine wire 
gauze which forms part of the microphone case and affords an appreciable measure of 
protection against interference from wind. For more severe conditions a windshield,* 
type W28, is provided, which fits over the microphone head. To make the instrument 
less conspicuous, the portion of the case surrounding the capsule can be removed and 
slender extension pieces, types C29 or C30 of differing lengths, can be inserted 
between the capsule and head amplifier so that the latter may be concealed; when 
either of these extension pieces is used, a different windshield, type W17, is 
normally employed to protect the capsule. 

The type C28 has potential applications in places such as footlights where 
an inconspicuous microphone having a high degree of suppression of sound from the rear 



This windshield, which bears no type number, is described by the maker as type W26 or type 
W28 , according to the microphone with which it is supplied. Throughout this report, 
however, it will be referred to as the type V28 . 



is required. When the type C30 extension piece and the type W17 windshield are 
employed, the height is suitable and the size small enough to enable the microphone to 
be used on the stage by crooners. 

The type C26 microphone might be of interest as a replacement for the 
Philips type EL3921/00 microphone which is now obsolete. 

Two experimental models of the type C28 microphone were examined in June 
1957 and another pair, incorporating improvements, in March 1958. Some minor 
modifications were suggested and production units were received in May 1958. At the 
request of S.B.S.B. 's Department, two type CK26 capsules were tested in May 1959. 

The price to the B.B.C. is £90 for either the type C26 or the type C28. 
The approximate cost of the type G29 extension piece is £4, of the type C30 £4. 10s. Od. , 
and of the type W17 and type W28 windshields £4. 5s. Od. and £7. 13s. Od. respectively. 
A case is available to house all these items and costs an additional £8. 



2. DESCRIPTION 

2. 1. General 

Fig. 1 gives an external view and dimensions of the microphone, a capsule 
type CK28, the two extension pieces types C29 and C30, and the windshields types W17 
and W28; as previously mentioned, the type C26 instrument is externally identical. 
The windshields are designed to enclose the entire capsule; they are constructed of 
two layers of wire gauze sprayed with flock on the inside and spaced about 1/8 in. 
(3 mm) apart. This form of construction is quite effective but if the flock became 
wet it is doubtful whether the flow resistance would return, on drying, to its 
previous value. 






Fig. I-A.K.G. microphone type C28 with extension pieces types C29 and 
C30 and windshields types WI7 and W28. External view and dimensions 



2.2. Vfeight 

The weight of either microphone without cable is 4 lb (0*2 kg) and of the 
mains unit 9 lb (4 kg). 



2.3. Type CK26 Capsule 

The construction of the pressure type capsule CK26 is on conventional lines. 
The size is similar to that of the Philips microphone type EL3921/00 and the direc- 
tional properties are therefore similar. 



2.4. Type CK28 Capsule 

The design of the type CK28 capsule, which is the subject of a patent, is 
novel in that two apertures spaced along the capsule length are provided instead of 
the usual one; these communicate with different acoustic networks, each designed to 
operate over a different part of the frequency range. Fig. 2 shows the main con- 
structional details. Sound can reach the rear of the diaphragm either through 
aperture 1 and network la, lb, lc and Id, or through apertures 2 and network 2a, 2b, 
2c, 2d, 2e and 2f . It is claimed that each of the elements forming these networks 
may be adjusted separately to give the desired directional characteristics. This 



|C lb la APERTURES /GAUZE 

V 







Fig. 2 - A.K.G. microphone type C28. Constructional details of capsule type CK28 



arrangement has advantages over the usual construction since if only one rear sound 
aperture is employed it is difficult to design a microphone which maintains a constant 
cardioid characteristic over the vhole audio-frequency range. 

In the prototype CK28 capsules, holes were formed in the plate P and these, 
together with the enclosed volume of air, V, affected the phase of the sound pressure 
at apertures 1 and 2 and therefore the directional characteristics. As this fact had 
not been appreciated by the designer, and as the volume V in the different extension 
pieces had not been closely controlled in production, the directional characteristic 
was dependent on the particular fitting used. In the production models, however, 
this variation was prevented by eliminating the holes in the plate P. The type C28 
response curves which are given later in the report all refer to capsules modified in 
this way. 

2.5. Amplifier and Cables 

Fig. 3 shows the electrical circuit diagram of the head amplifier. The 
transformer may be connected to give a nominal output impedance of 50 or 200 ohms; 
the manufacturer suggests that the change from one output impedance to the other 
should only be undertaken in the factory. 

The microphone is connected by a plug and 65 ft (20 m) of cable to the mains 
unit type N28K, from which the audio-frequency output is taken through a 3-pin plug 
and socket. In an emergency a faulty mains unit may be replaced by one of the type 
N12K normally used for the A.K.G. type C12 microphone. 



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3. PERFORMANCE 

3. 1. Method of Measurement 

All the frequency characteristics of the type C26 microphone and those of 
the type C28 microphone above 200 c/s were measured by comparison with a pressure 
standard in a dead room. The characteristics of the type C28 microphone at frequen- 
cies below 200 c/s were measured in a travelling-wave duct by comparison with the same 
standard; the characteristics of the type C28 microphone with the type C29 extension 
piece were obtained in a similar manner. Generally the accuracy of comparison is 
+ i 63; errors of ± 1 dB are, however, possible in the measurements of the types C28 
and C29 instruments for sound incident at angles greater than 90 . 

All the characteristics given in this section were measured with the 
transformer connected to give a nominal output impedance of 200 ohms. 



































































































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200-ohm connection for sound incident at various angles 



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Fig. 5 - A.K.G. microphone type C26. Axial frequency characteristic 
measured at 200-ohm connection 



3.2. Frequency Characteristics 

Pig. 4 shows the open-circuit frequency characteristics of a type C26 
microphone for sound incident at various angles, and Fig. 5 the axial response when 
fitted with another type CK26 capsule. For comparison, Fig. 6, taken from Research 
Department Technical Memorandum M. 1008, shows the corresponding curves for a Philips 
microphone type EL3921/00. If the windshield type W28 is fitted to the type C26 
microphone the overall frequency response is not changed by more than ± 1 dB. When 
the microphone capsule is mounted on either the type C29 or the type C30 extension 
piece so that the smaller windshield type W17 has to be used, the change in response, 
shown in Fig. 7, is slightly greater. 



Fig. 8 shows the open— circuit frequency characteristics of a type C28 
microphone for sound incident at various angles, and Fig. 9 the difference in axial 
frequency response of three capsules. The change in response caused by the type W28 
windshield is remarkably small for a microphone with a cardioid polar characteristic 
and is not more than ± i dB for sound incident at 0° and 90 and 2 dB at 180 . 
Fig. 10 shows the response of the capsule referred to in Fig. 8 when mounted on the 
type C29 extension piece; when the longer extension piece type C30 is employed, the 
same frequency response is obtained. Fig. 11 shows the response of a microphone with 
and without the type W17 windshield; it will be noted that although the shield does 

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Fig. 7 - A.K.G. microphone type C26/C30. Change in response produced by 
windshield type WI7 for sound incident at various angles 



not greatly change the axial frequency response, it does appreciably degrade the 
directional characteristics at low frequencies. 

The above figures refer to early production models. However, measurements 
made on current microphones show a loss at low frequencies, varying from —5 dB to 
-10 dB at 50 c/s, much greater than that shown in Fig. 9. In addition there is some 
evidence that under service conditions the bass response decreases still further with 
time. This matter is being further investigated. 

3.3. Impedance 

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200 ohms transformer connection, lies between 160 ohms and 240 ohms and is thus 



































































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outside the maker's tolerance of + 10$; the value of this impedance is affected to 
some extent by feedback through the grid-anode capacitance of the valve, which forms a 
potential divider with the capacitance of the particular capsule and extension piece 
in use. The impedance is, however, substantially constant over the whole audio- 
frequency band so that the on-load frequency characteristics do not differ signifi- 
cantly from those shown. 

3.4. Sensitivity 

The high tension supply from the mains unit is unstabilised and as a result 
the capsule polarising potential, and therefore the microphone sensitivity, changes 
with the voltage of the mains supply. For nominal mains voltage the open— circuit 
sensitivity of the type C26 microphone in the mid-band region is -56 dB with reference 
to 1 volt/dyne/cm 2 when using capsule No. 310 and -57 dB for capsule No. 311; the 
maker's figure is stated to be "about 1-3 mV", i.e. -57«5 dB. 

Four type CK28 capsules were tested in the same head amplifier; the 
microphone open-circuit sensitivity lay between -54 dB and -55 dB with reference to 
1 volt/dyne/cm 2 , the maker's figure again specifying approximately -57*5 dB. When 
the type C29 and C30 extension pieces are used, the capacitance-to-earth of the 
additional length of lead involved shunts the signal from the capsule, and the 
sensitivity of the microphone is thereby reduced. The loss for the type C29 

extension piece is 1 dB and for the type C30 approximately 3 dB; the maker does not 
appear to have appreciated this effect as the same sensitivity is quoted for all 
three conditions. 



4. NOISI 

4.1. Internally Generated Noise 

The internally generated noise appearing at the output of the microphone is 
a combination of flicker effect in the valve and of thermal agitation in the resistive 
component of the grid circuit impedance. 

The open-circuit noise when weighted by an aural sensitivity network type 
ASN/3 is -105 dB with reference to 1 volt both for the type G86 and for the type C28. 
The mid-band sound pressures required to give the same output levels are +25 dB and 
+23 dB respectively with reference to 0*0002 dyne/cm 2 ; if the rise in axial response 
of the type C26 microphone at high frequencies were equalised, the noise level would 
be somewhat reduced. For comparison, the weighted noise figure for the Philips type 
EL3921/00 microphone is +27 dB. 

It appears that the noise figure for the types C26 and C28 microphones is 
not as good as it could be, because the head amplifier employs a triode operating 
under conditions of high gain producing a large effective input capacitance — about 
65 I4J&; as the type CK28 capsule has a capacitance of about 25 jj/JF, the signal at the 
valve grid is reduced to some 11 dB below the open-circuit output, resulting in an 
appreciable reduction in signal-to-noise ratio. The level delivered by the amplifier 
is too high to mix directly with that from most other types of microphone used in the 
Corporation; hence an attenuator of about 15 dB, designated AT. 2/6 and provided by 



10 



Equipment Department, is normally fitted in series with the output. It seems 
probable that if, instead of using external attenuation, the amplifier gain, and thus 
the input capacitance, were reduced, the same effect would be achieved but with a 
better signal— to-noise ratio. 

4.2. Interference from Magnetic Fields 

Measurements were made of the maximum open— circuit voltage induced in the 
microphone by a uniform magnetic field. The unweighted mid-band sound levels, with 
reference to 0»0002 dyne/ cm 2 , required to give an output equivalent to that produced 
by uniform fields of 1 milligauss at 50 c/s, 1 kc/s and 10 kc/s are +1 dB, +13 dB and 
+23 dB for the type C26 microphone using capsule No. 311, and -2 dB, +10 dB and +20 dB 
for the type C28 using capsule No. 2. These levels are regarded as extremely low and 
should cause no trouble under normal studio conditions. 

4.3. Interference from Radio-Frequency Signals 

As with many electrostatic microphones, radio— frequency signals may be 
picked up and demodulated in the head amplifier. Although the manufacturer has 
endeavoured to reduce this effect to a minimum, considerable interference has been 
experienced on some occasions. An investigation carried out by Research Department 1 
revealed the means by which the radio— frequency currents enter the amplifier screening 
and by paying careful attention to the method of earthing the microphone case the 
interference has been very considerably reduced. Details of the modification have 
already been issued as a memorandum by S.E.S.B. on 9th June 1960. It is carried out 
by connecting together the two tags shown in Fig. 12. However, when these micro- 
phones are to be used in places where appreciable radio— frequency fields are known to 
exist, tests should still be made in advance to ensure that interference is not likely 
to be troublesome. 




Fig. 12 - Wiring modification to 

reduce radio-frequency 

interference 



OLD CONNECTIONS 
■ NEW CONNECTIONS 



4.4. Wind Noise 

Measurements were made of the wind noise generated when the microphone was 
placed at various angles to a streamlined air flow of 10 m.p.h. both for the normal 
condition and when equalised so that the axial response was uniform from 1 kc/s down 
to 50 c/s. The open-circuit noise was weighted by the standard A.S.A.* network and 
measured by a V.U. meter; the results are given in the following table in terms of 
the level, with respect to 0*0002 dyne/cm 2 , of a 1 kc/s tone calculated to give an 
equal signal. 



American Standards issoeiation, Standard Z.24.3 
Noise and Other Soands". 



1944, "8onnd Level Meters for Measurement or 



11 



0° 


45° 


90° 


135° 


180° 


dB 


dB 


dB 


dB 


dB 


92 


85 


85 


96 


83 


102 


96 


97 


107 


96 


77 


61 


59 


79 


64 


84 


77 


75 


98 


81 


106 


100 


98 


96 


87 


117 


112 


111 


108 


101 


70 


69 


70 


71 


76 


85 


86 


88 


91 


90 



TABLE 1 
Microphone condition Angle 



Normal C28: Capsule No. 216. Amplifier No. 352 

IMequalised 
Equalised 

With W28 windshield, unequalised 
With W28 windshield, equalised 

Bare on G89 extension, unequalised 
Bare on C29 extension, equalised 

On C29 extension with W17 windshield, unequalised 
On C29 extension with W17 windshield, equalised 

Normal C26: Capsule No. 311. Amplifier No. 352 

Unequalised 
Equalised 

With W28 windshield, unequalised 
With W28 windshield, equalised 

Bare on C29 extension, unequalised 
Bare on C29 extension, equalised 

On C29 extension with W17 windshield, unequalised 
On C29 extension with W17 windshield, equalised 

The wind noise levels with the types W17 and W28 windshields measured without bass 
equalisation are low for both types of microphone. 

4.5. Interpretation of Noise Measurements 

In applying these results it should be remembered that the aural sensitivity 
weighting where used is intended to give an indication only of the loudness of the 
noise. The subjective assessment of the annoyance caused depends on such factors as 
the degree to which it may blend with the studio "atmosphere" and other background 
noises. 

4.6. Listening Tests 

Listening tests were carried out on speech from non— reverberant surroun- 
dings. The results obtained were in agreement with the objective tests. 



5. CONCLUSIONS 

The type C26 microphone appears to be a reasonable replacement for the 
Philips type EL3921/00. The electrical noise is a little lower and the microphone is 
slightly less directional at high frequencies; as with the Philips microphone, a 
certain amount of equalisation is desirable. 



50 


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45 


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51 


49 


43 


43 


46 


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38 


38 


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37 


37 


76 


63 


66 


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71 


58 


57 


60 


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42 


48 


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12 

The axial frequency response of the type C28 microphone is free from serious 
irregularities and is well maintained over the frequency band. The directional 
characteristic is also nearly independent of frequency. The electrical noise is low 
but could probably be made even lower by a modification of the head amplifier. 

The facility of mounting the type CK28 capsule on one of the extension 
pieces types C29 or C30 has proved useful on occasion. The windshields provided are 
effective with both the types CK26 and CK28 capsules and produce appreciably less 
degradation in the frequency characteristics than is usual with commercial shields. 



6. REFIRENCI 

1. B.B.C. Research Department Report No. L-047, "Radio-Frequency Interference 
in Electrostatic Microphones". 



MM 



Printed by B.B.C. Research Department, Kings wo od Warr en, Tadwortn, Surrey