A MICROPHONE performs two essential functions; it first transforms the sound energy it receives into vibrations, and then transforms the vibrations into a voltage whose pattern matches the original sound. A microphone is, in fact, a generator of electricity, but is driven by sound energy instead of mechanically. In the days when the author first began to study microphones. he conceived an idea that he thought would help reduce the cost of electricity. He proposed the installation of giant exponential horns on noisy railway stations which would "capture" the wasted sound energy and turn it into electricity through the medium of huge microphones loaded to the throats of the horns! His tutor was pleased that the basic idea of microphones had been grasped, even though he didn't seem very impressed with the scheme! Excluding the "lo-fi" carbon microphone, there are four basic types which are employed in hi-fi work: (1) crystal, (2) moving-coil, (3) ribbon, (4) condenser. With all types there is a diaphragm, or a moving member, which is caused to vibrate by the sound energy and which actuates the generating system.
There are two methods by which the sound energy can evoke sympathetic vibrations of the diaphragm. Except in the case of the ribbon microphone, which has a thin ribbon instead of a diaphragm, the sound is applied to one side of the diaphragm only and the pressure component of the sound radiations is utilized. The rear of the diaphragm is cut off from the pressure wave by the microphone housing and is thus at normal atmospheric pressure, which results in the diaphragm being deflected inwards and outwards by the pressure variations in accordance with the pattern of the sound radiations. Micro phones adopting this principle are said to be pressure operated.
In addition to the pressure component of a sound wave, there is another component called the particle velocity (see Section 1). Since it is not possible to produce a ribbon of sufficiently slender dimensions to couple with high efficiency to the velocity component of a sound wave, there can be no such thing as a purely velocity-operated microphone, though this term is sometimes used in regard to ribbon microphones. Ribbon microphones represent an approximation to velocity operation, since both sides of the ribbon are exposed to the sound field, and movement of the ribbon is caused by the sound pressure difference between the two sides. Such microphones are known as pressure gradient types, and since the pressure gradient of a sound wave is proportional to the particle velocity, there is some excuse for the term "velocity operated".
CRYSTAL MICROPHONES
The generating system of a crystal microphone, as of the crystal pick-up, is Rochelle salt crystal which produces an electrical potential difference when subjected to changes of pressure. The crystal is cut from a big crystal at a critical angle and placed between metallic plates arranged so that the pressure on the crystal is varied in sympathy with the vibrations of the diaphragm.
The idea is shown in Fig. 8.1. The voltage thus generated varies in accordance with the sound vibrations.
There is another type of crystal microphone, generally referred to as the sound-cell microphone, in which the sound pressure operates the crystal directly without a conventional diaphragm. This type is considerably less sensitive than the diaphragm-actuated type, but its frequency response is much better since there is no coloration from the diaphragm.
Crystal microphones are high-impedance devices and can, therefore, be connected direct to the grid circuit of a valve. They are not affected by magnetic hum fields, are fairly light in weight and, in the case of the diaphragm type, provide a fairly high output voltage, thereby avoiding the necessity of high-gain microphone amplifiers. They are used extensively by home tape-recordists, and are useful for a number of applications which do not need long connecting cables--these, because of the necessary screening, would be likely to attenuate the higher frequencies.
MOVING-COIL MICROPHONES
This microphone is basically the same as the moving coil loudspeaker (see Fig. 8.2). The diaphragm is mechanically coupled to the coil which operates in the air gap formed by the poles of a permanent magnet. The winding thus cuts) ...
FIG. 8.1. Crystal microphone and its connection to a high-impedance
circuit.
FIG. 8.2. The moving-coil (dynamic) microphone. Its low impedance requires
the use of a matching transformer.
... the lines of force of the magnet when it is actuated by the diaphragm. A voltage is, therefore, produced whose magnitude is proportional to the rate of cutting of the lines of force, or the velocity of the coil. Moving-coil units are of low impedance (around 30 ohms), and usually give a smaller output than the crystal unit. However, they lend themselves to operation at greater distances from the amplifier than crystal types, since a low-impedance line is less subject to losses of all kinds. A matching transformer is required, and this is usually mounted in or near the amplifier.
RIBBON MICROPHONES
Fig. 8.3 shows the basic construction of the ribbon microphone. It operates in the same way as the moving-coil unit, in that the ribbon represents the moving conductor. Both the output voltage and the impedance are very low, and to bring the impedance up to a reasonable figure a small transformer is often incorporated in the stem of the microphone. The Reslo unit embodies this feature.
(Left) FIG. 8.3. The ribbon microphone. Its very low output impedance
usually re quires the use of a microphone transformer in the housing
to produce a reasonable line impedance. (Right) FIG. 8.4. The condenser
microphone requires a polarizing voltage, which is obtained from the
h.t. line through a resistor.
CONDENSER MICROPHONES
As with the electrostatic loudspeaker, the condenser microphone (the term electrostatic microphone is rarely used) is essentially a condenser formed of two plates separated by the air as a dielectric. One plate is fixed, while the other serves as the diaphragm and is caused to vibrate by the incident sound pressure. The capacitance across the two terminals thus varies in accordance with the sound pattern. A polarizing voltage is required, and is connected in series with the microphone by way of a high-value resistor which, irrespective of diaphragm movement, holds the charge on the microphone at a fairly constant value. Thus, as the capacitance alters in value due to sound pressu1 e, the potential across the capacitor varies accordingly, since this is equal to the charge divided by the capacitance. This varying potential is applied to the grid circuit of the microphone amplifier valve, as shown in Fig. 8.4.
This type of microphone is used with certain Continental tape recorders, and also for laboratory tests and studio applications. Since the output impedance is extremely high, and the output voltage is affected by cable capacitance, a small pre-amplifier is sometimes built into the microphone housing so that the line impedance can be reduced to a workable value.
FIG. 8.5. The omnidirectional response of pressure microphones tends
to be come unidirectional when the wave length of the sound is comparable
with the microphone dimensions.
FIG. 8.6. The figure-of-eight response of pressure-gradient microphones.
POLAR RESPONSES
Within the limitations of frequency, the pressure-operated microphone is essentially omnidirectional, i.e., it is responsive to sounds arriving from any direction within its range. Its polar response thus has a spherical distribution. However, at frequencies where the wavelength of the sound becomes comparable with the size of the housing, it tends to become unidirectional, and will have greater sensitivity to sound arriving at the front. This is illustrated in Fig. 8.5.
The pressure-gradient microphone, on the other hand, has a figure-of eight polar response, as shown in Fig. 8.6. This kind of microphone does not respond at all to sounds arriving at the sides but has a usable response over about 100 deg. both at the front and rear.
The polar response can, however, be modified to suit the prevailing acoustical conditions by partially closing the rear of the microphone by means of small acoustic filters (pads). A cardioid (heart-shaped) polar response can be obtained from a microphone which combines the output from a pressure-operated unit with the output from a pressure-gradient unit. Combined microphones of this kind, known as cardioid microphones, are used extensively for broadcasting work. The cardioid response diagram is given in Fig. 8.7.
Because they combine a high-quality pressure-operated unit with a pressure-gradient unit, whilst maintaining a sensitivity and acoustical balance over the greater part of the sound spectrum, true cardioid microphones are rather costly instruments, and are usually too expensive for the average enthusiast. However, they are sometimes employed by the "serious" amateur tape and disk recordist, and by organizations operating sound-reinforcement services.
It is interesting to note that a semi-cardioid response can be obtained from a pressure-gradient microphone by closing the back half of the ribbon with an acoustical filter. Two responses are thus obtained, circular and figure-of eight, which combine to give the cardioid response. A cardioid response can also be obtained from specially constructed condenser microphones, in which two diaphragms are used, separated by a perforated electrode.
MICROPHONE SENSITIVITY
The sensitivity of a microphone is usually expressed in decibels relative to a fixed reference level. The reference level chosen is invariably 1 volt (equals 0 db) with a sound pressure of 1 dyne per square centimeter (1 dyne/ cm^2). Thus, a microphone quoted as having an output level 60 db below 1 volt/dyne/cm2 would generate about 1 millivolt when subjected to a sound pressure of 1 dyne/cm^2. A sound pressure or sound intensity of twice the value would increase the output voltage by a factor of 2, while a sound pressure of half the value would decrease the output voltage by a factor of 0.5.
FIG. 8.7. A cardioid or heart-shaped response is obtained by combining
the principles of the pressure and pressure-gradient units.
The overall sensitivity is somewhat governed by the output impedance.
For instance, the Lustraphone Full-Vision microphone, is quoted as having an output of - 88 db at 25 ohms, and an output of - 54 db at 50,000 ohms.
In the latter case an impedance step-up transformer is used, which also increases the output voltage.
CHOICE OF MICROPHONE
No hard-and-fast rules can be given in this connection, since the final choice depends not only upon the particular application, but also upon economic factors. Nevertheless, no one microphone does everything equally well; the diversity of situations for which microphones are required calls for different types if results of the highest order are desired. In this case, consider able knowledge of microphone techniques is essential; the amateur may well obtain better results from the use of one versatile ribbon unit than by the unskilled arrangement of an array of more specialized instruments.
Indeed, the ribbon microphone of modem design can be used for almost all applications if reasonable thought is given to its positioning; the response pattern can easily be varied to suit special conditions by the inclusion of small acoustical filters, as already described. The modern unit has a good sensitivity, and can thus be connected direct to most amplifiers and tape recorders without the need for pre-amplification. The majority of commercial units employ inbuilt transformers providing an output impedance sufficiently low for connection to long lines, whilst also providing a good match to the input impedance of most amplifiers. The Resto Type RB miniature ribbon microphone can be obtained with output impedances ranging from 30 ohms to several thousands of ohms (i.e., high impedance). The Lustraphone range of ribbon microphones are also available with various impedance values.
Ribbon microphones have excellent frequency-response characteristics, often maintained substantially level up to 14-15 k hz. This type of microphone is therefore usually ideal for the recording or reproduction of music in all its aspects. It is not always suitable for outdoor work, where the delicate ribbon may be affected by wind pressure, though it is possible to employ so-called windshields to minimize this disturbance which manifests itself in the form of a roar from the loudspeaker.
Owing to the bi-directional characteristic, the orientation of the ribbon microphone can be adjusted so as to discriminate against unwanted pick-up off the main axis. This feature can be used to advantage to provide a fair degree of balance when a single unit is employed for the reinforcement or recording of an orchestra.
With its general freedom from response peaks, which are inherent in less exacting instruments, the ribbon microphone can also be used for sound reinforcement applications in rooms which are liable to produce acoustic feedback (the howl effect when the amplifier gain-control is advanced). In spite of the inherently lower sensitivity as compared with, say, moving-coil microphones, improved acoustical efficiency is often possible by the use of a ribbon unit.
The recording operator or sound-reinforcement engineer should always make a special point of instructing the artist or speaker in the use of the microphone. A few minutes spent in the serious consideration of this point is well worth while. Incidentally, considerable accentuation of the lower frequencies, resulting in a "boominess" of reproduction, occurs if a ribbon microphone is used too close to the sound source. If this type of microphone isused closer than about 3 ft. a suitable degree of bass cut should be applied at the amplifier. Perhaps this is the reason why crooners favor the ribbon microphone! The moving-coil or dynamic microphone is more sensitive than the ribbon unit; it is also more robust, less expensive, and suitable for outdoor as well as indoor functions. It is a popular unit with tape recordists generally and with public-address operators (it should be observed that the term "sound-reinforcement" has been taken throughout this guide to mean "hi-fi public address"). The average frequency response of this type of microphone usually falls short of that of the ribbon unit and ranges about 8 k hz. Its omnidirectional characteristic makes it difficult to avoid acoustic feedback effects in some applications.
The crystal microphone is also used by tape recordists, though it is losing favor with public-address and sound-reinforcement operators because of its high output impedance. It is usually less expensive than the other types considered. The output voltage is a little higher than for moving-coil units, and both its frequency response and response characteristics are rather like those associated with moving-coil units, though they vary widely in different designs. This microphone is also employed in office dictating machines.
The condenser microphone is rarely seen in amateur circles, but, as already mentioned, is sometimes employed with Continental (i.e. Grundig) tape recorders. It has an excellent frequency response, and certain specialized types have been produced which respond to frequencies up to 100 k hz! There is a great diversity of designs of the three basic units. There are microphone heads of various types for screwing to a floor or table stand, microphones complete with table stand, hand microphones, so-called full vision microphones designed to avoid hiding the artist (these are often seen on television), lapel microphones, noise-cancelling microphones and others.
It is outside the scope of this guide to describe the merits and demerits of all these types, but in all cases the functional units are similar to those described.
MICROPHONES AND MIXERS MICROPHONE MIXERS
There always comes a time when it is necessary to use more than one microphone. Microphones can be connected in parallel and then to a common microphone input socket on the amplifier, but this practice is not to be recommended. It is far better to use a microphone mixer so as to maintain optimum matching of the microphones to the input impedance, whilst at the same time having full control over the gain setting of each microphone channel.
A circuit of a microphone mixer (Pamphonic Sound Equipment) is given in Fig. 8.8. It will be seen that each microphone is fed into its own pre amplifier valve, and the outputs are combined, at a level determined by the setting of the appropriate "gain" or volume controls, and then fed to a common voltage amplifier, and thence to the common output transformer.
The five input transformers and the output transformer ensure that the correct load is presented to the microphones and the microphone input channel of the main amplifier or pre-amplifier; which in turn results in the maximum transfer of signal with the minimum generation of noise. whilst exploiting the frequency-response characteristics of the equipment to the full.
The 330k resistors connected to the sliders of the volume controls avoid heavy loading on the grid circuit of the output triode when only one channel is in operation; i.e., when four of the controls are backed right off. A degree of frequency correction is also applied to this stage through frequency selective feedback being given by the 680k resistor and the 0ยท1 mF capacitor connected between grid and anode. Further correction is applied across the primary of the output transformer T6.
The mixer has its own power supply, which uses a Mullard EB91 (usually employed as a signal detector) as the h.t. rectifier. In order to keep the valve within its limits of operation, the circuit is arranged in the form of a voltage-doubler, and the potential between the heater and cathode of the valve is reduced by the heater being connected to a point of positive potential.
The author has had frequent occasion to use this instrument, and it has always proved reliable and has given virtually no trouble at all.
Another neat little four-channel mixer is the Grundig Type GMU3.
This is designed essentially for use with Grundig tape recorders, and two of the channels cater for the Grundig condenser microphone by having the necessary 100-volt polarizing voltage available to these circuits. Of the other two channels, one is suitable for a low-impedance microphone, such as the Grundig ribbon unit, and the other is intended to accept a fairly high-level (approximately 300-mV) signal, such as that given by a radio receiver, amplifier control unit or another tape recorder. A magic-eye signal-level indicator is also included on the front panel.
Fig. 8.8. Circuit diagram of the Pumphonic- mixer unit, Model S W/600.
TRANSISTOR UNITS
Although power transistors are now used in the output stages of public address amplifiers of the semi-portable variety, they have not yet found their way into the comparable stages of hi-fi equipment. Whilst transistors are capable of delivering some 10-20 watts or more of audio, the distortion con tent is above hi-fi acceptance (it is difficult to keep harmonic distortion below about 5 percent). However, in low-level audio stages the transistor is now beginning to be exploited. One application is in microphone amplifiers and mixers. Recent introductions in this field are the transistorized pre-amplifier units by Lowther and the transistorized mixer unit by Lustraphone.
The mixer unit has four channels, two of high impedance and two of low impedance. The output circuit is suitable for direct connection to the high-impedance socket of almost all hi-fi amplifiers or control units. The power is provided by a single miniature mercury cell, which has an estimated life of some 1,000 hours. The frequency response is substantially flat from 50 hz to 14,000 hz.
Since transistors are inherently free from hum and microphony trouble that are always present to some degree with high-gain valve amplifiers, they are ideally suited to high-gain "front-ends". Small transistor amplifiers can easily be built into the housing of low-level microphones, including the battery power supply. Since a signal of high level can thus be distributed from the microphone circuit, the need for high-gain settings on the main amplifier is precluded, and the danger of hum and noise pick-up on the micro phone cable is considerably alleviated.
Another advantage of the transistor is that its input impedance can be arranged to match the low impedances of high-quality electromagnetic pick-ups and microphones, without the need for a matching transformer.
The transistor thus serves admirably as an impedance-matching device.
A circuit of a pre-amplifier suitable for electromagnetic pick-ups or microphones of from 100 to 1,000 ohms impedance is given in Fig. 8.9.
A transistor can be looked upon as two crystal diodes formed between the emitter and base and the collector and base. In the circuit the letters B, C and E around the transistor symbol represent base, collector and emitter respectively. These three points are often likened to the electrodes of a triode valve as follows: collector = anode, base = grid, and emitter = cathode.
The transistor is biased in the forward direction in the emitter/base circuit and in the reverse direction in the collector/base circuit. With the emitter/base bias disconnected there is theoretically no current (a very small amount in practice) in the collector/base circuit. When bias is applied to the base/emitter circuit, however, current flows in the base/collector circuit, and when the current in the base/emitter circuit increases, the current in the base/collector circuit also increases, but to a greater extent. This action is promoted by the emission of so-called positive holes from the emitter to the collector circuit with a consequent lowering of the resistance of the base/ collector circuit.
The signal is applied to the transistor so as to cause variation of the negative current in the base/emitter circuit, which, depending upon how the circuit is arranged, results in an equal or greater variation of current in the base/collector circuit. Generally speaking, there occurs a power gain because the applied signal promotes a current change in a low-resistance circuit (base/ emitter) while reflecting a similar or greater current change in a high resistance circuit, represented by the base/collector junction.
There are a number of methods by which the transistor can be connected into the circuit, as with the triode valve. The arrangement in Fig. 8.9 is usually referred to as a "common base" circuit (the SmF capacitor connected to the base makes the base common to the input and output circuits, or earthed base circuit, and it corresponds roughly to an earthed-grid valve circuit. The input signal is fed into the emitter by way of the 25mF coupling capacitor (such a large value being necessary to maintain a low-frequency response in a low-impedance circuit), and the output signal is taken by way of the 0.1mF capacitor from between the collector and positive line (chassis). The circuit is thus given a low input and a high output impedance, which is ideal for feeding a signal from a low-impedance microphone or pick-up to a high-impedance input circuit of an amplifier. There is no reversal of phase between the input and output signal voltages.
The circuit is capable of delivering approximately 1 volt r.m.s. of signal for an input of 16mV r.m.s., and thus has a voltage gain of some 62 times.
Power is derived from a 6-volt battery, and due to the very low current drain (400 microamps quiescent) a very small battery is all that is required. The response is about 3 db down at 100 hz and 20,000 hz relative to 1,000 hz.
The 6.8k resistor connected to the collector can be considered as the output load, while the resistor connected to the emitter and the two resistors whose junction is connected to the base serve to stabilize the circuit from the d.c. aspect.
FIG. 8.9. Circuit diagram of a transistorized microphone pre-amplifier.
One or two points regarding the servicing of transistorized equipment will be useful.
Transistors are very sensitive to heat, and if overheated when operating are liable to be destroyed in a very short time. They should therefore be kept clear of soldering irons and heat producing devices such as valves and resistors of the main amplifier.
Soldering in and out of the circuit should be performed as rapidly as possible. A miniature low-power soldering iron is desirable, but even then a heat "sink" should be produced by holding the transistor wires with long nose pliers while soldering is being done. The transistor wires should never be bent close to the seal on the transistor.
Reversing the polarity of the supply voltage will almost certainly result in immediate failure of the transistor. This must be borne in mind when performing in situ voltage, current and resistance checks. It is worth remembering that the negative terminal of most multi-range meters is usually in connection with the internal battery positive connector when used as ohmmeters.
So as to avoid disturbing the balance of voltage in a transistor circuit, voltage measurements are best made on a high-resistance instrument of at least 1,000 ohms-per-volt. It is not a good thing to make or break a connection in a transistor circuit while the supply voltage is connected. If it is required to perform a current test, the supply voltage should be disconnected and then the milliammeter inserted where required. The power can then be re-connected and the measurement taken. The same applies on removing the milliammeter.
SERVICING MICROPHONES
Generally speaking, it pays to let the maker have the microphone back if the need for servicing arises. Replacement diaphragms and ribbon elements can, however, be obtained for most quality microphones, as can crystal inserts for sound cell units. It is only a little more expensive to let the maker replace the faulty parts, whilst at the same time ensuring that the performance of the equipment will be up to the normal standard. In a number of cases, the microphone fixing screws are sealed so as to avoid unnecessary tampering, and if these seals are broken the manufacturer may charge for the correction of a fault, even during the guarantee period.
With moving-coil and ribbon units, continuity should be registered across the terminals, and a low-value resistance reading is obtained if the test is directed across the moving coil or ribbon element. A higher resistance reading will, of course, be obtained if the test is made across the secondary of an internal transformer. A crackling is usually heard from the microphone when this test is made, due to the battery in the test-meter causing the micro phone to act as a loudspeaker. Crackling noises are also heard from crystal units when subjected to this test, in spite of the normal lack of continuity as indicated on the ohmmeter.
Microphone switches are a constant source of trouble on certain micro phones, but this is usually fairly easy to remedy. Broken conductors in micro phone cables also represent a frequent source of trouble, particularly if the microphone is in constant employment and moved around a lot. These faults are quickly located by means of simple continuity or resistance checks.
MICROPHONE BALANCE
As we have already seen, the choice of microphone is somewhat governed by the polar characteristic and the frequency response, with due regard also to such things as sensitivity, the type of material to be amplified or recorded and (most important) the depth of one's pocket. The beginner invariably commences operations with a relatively cheap crystal microphone-very often the one supplied with the tape recorder or amplifier, though some manufacturers are now wisely leaving the choice of microphone to the user.
With a little experience, the beginner soon realizes that something more elaborate in the way of a microphone is desirable, if only to cut out the squeak of the door, the tick of the clock or the crackle of the fire. It is amazing how such noises assume prominence on a tape recording. While the ear is able to discriminate against unwanted noises, since there are two of these organs (stereo helps in this respect), the microphone responds to every noise and brings both wanted and unwanted sounds into focus at the loud speaker. If one microphone is used at some distance from the sound source, then the ambient sounds are going to be recorded at almost equal intensity.
The novice gradually discovers such things for himself, and possibly experiments with various microphones, combinations and orientations.
This is a good thing, because it provides the necessary experience in micro phone technique for which words can never be used as a substitute.
When using more than one microphone, particularly if the microphones are connected in parallel across the amplifier's common microphone input socket, care must be taken to ensure that they are not placed equidistant from the sound source. This is because there is the possibility of the micro phones being out-of-phase (there is an analogy with out-of-phase loud speakers), in which case serious distortion would occur as the result of cancellation effects at certain frequencies. If this trouble is suspected, the connections on one of the microphones should be reversed, or one of the microphones should be turned through 180 deg. if it is of the ribbon type with a figure-of-eight response.
In general, however, even if the microphones are phased correctly, it is not good policy to use them close together, because interference effects of the nature described may result at certain frequencies. If the response characteristics of the microphones are known (they can usually be estimated fairly accurately), they should be orientated with regard to each other so that their polar responses do not overlap to any large degree.
When acoustic feedback is troublesome (with, for example, sound reinforcement work), the placing of the microphone is of great importance.
If it is found that insufficient audio power can be obtained before the feed back point on the volume control, and it is impossible to re-position the microphone, other microphones should be tried, such as the ribbon or cardioid. Just before reaching the setting of the volume control which evokes the characteristic howl, a slight ringing sound may be heard when the micro phone is being used. At this point distortion may also be at a high level, and for this reason the volume-control setting must be retarded.
Intelligent use of the treble and bass controls may allow a greater volume setting to be used, since the acoustic feedback is a function of the room acoustics. A "live" room, for example, will reflect the higher-frequency sounds and possibly promote feedback conditions, while a "dead" room will tend to absorb certain frequencies of the sound and thus prevent it bouncing back into the microphone. Line-source loudspeakers assist in this respect also, as we have already seen, by concentrating the sound over the required area of coverage and leaving little for spilling into the microphone.
The reverberation of a room has an appreciable effect on a recording. If the microphone is placed a reasonable distance away from the sound source, then it is going to pick up not only direct sound, but also quite a lot of reflected sound; the recording will be colored by the room acoustics. If the micro phone, on the other hand, is placed fairly close to the sound source, the room acoustics will have less influence since most of the sound will be picked up direct, and only a small proportion will be reflected sound. It is, in fact, possible to arrange the position of the microphones to secure almost any required degree of recorded reverberation effect.
Too close a position of the microphone in relation to the sound source should be avoided for most applications, however, since this tends to promote a "bass heavy" effect, but is possibly useful for the recording of dance bands and rhythm groups, where plenty of bass may be required. It should also be remembered that the sound radiation from musical instruments varies considerably with frequency. With a piano, for example, the maximum treble occurs to the right-hand side of the keyboard, and diminishes progressively towards maximum bass in an arc towards the rear of the instrument. With string instruments, the maximum treble is confined to a narrow angle from the major dimension of the instrument.
It is obviously impossible to explore microphone balance in relation to all musical instruments, and from the point of view of the home recordist and enthusiast it often comes down to a matter of trial and error, aiming for overall balance without introducing undue coloration.