Chapter 3

Microphones

A microphone is a transducer that converts acoustical sound energy into electrical energy, based on the principle described in Fact File 3.1. It performs the opposite function to a loudspeaker, which converts electrical energy into acoustical energy. The three most common principles of operation are the moving coil or ‘dynamic’, the ribbon, and the capacitor or condenser. The principles of these are described in Fact Files 3.2–3.4.

The moving-coil or dynamic microphone

The moving-coil microphone is widely used in the sound reinforcement industry, its robustness making it particularly suitable for hand-held vocal use. Wire-mesh bulbous wind shields are usually fitted to such models, and contain foam material which attenuates wind noise and ‘p-blasting’ from the vocalist’s mouth. Built-in bass attenuation is also often provided to compensate for the effect known as bass tip-up, a phenomenon whereby sound sources at a distance of less than 50 cm or so are reproduced with accentuated bass if the microphone has a directional response (see Fact File 3.5). The frequency response of the moving-coil mic tends to show a resonant peak of several decibels in the upper-mid frequency or ‘presence’ range, at around 5 kHz or so, accompanied by a fairly rapid fall-off in response above 8 or 10 kHz. This is due to the fact that the moving mass of the coil–diaphragm structure is sufficient to impede the diaphragm’s rapid movement necessary at high frequencies. The shortcomings have actually made the moving coil a good choice for vocalists since the presence peak helps to lift the voice and improve intelligibility. Its robustness has also meant that it is almost exclusively used as a bass drum mic in the rock industry. Its sound quality is restricted by its slightly uneven and limited frequency response, but it is extremely useful in applications such as vocals, drums, and the micing-up of guitar amplifiers.

One or two high-quality moving-coil mics have appeared with an extended and somewhat smoother frequency response, and one way of achieving this has been to use what are effectively two mic capsules in one housing, one covering mid and high frequencies, one covering the bass.

The ribbon microphone

The ribbon microphone at its best is capable of very high-quality results. The comparatively ‘floppy’ suspension of the ribbon gives it a low-frequency resonance at around 40 Hz, below which its frequency response fairly quickly falls away. At the high-frequency end the frequency response remains smooth. However, the moving mass of ribbon itself means that it has difficulty in responding to very high frequencies, and there is generally a roll-off above 14 kHz or so. Reducing the size (therefore the mass) of ribbon reduces the area for the sound waves to work upon and its electrical output becomes unacceptably low. One manufacturer has adopted a ‘double-ribbon’ principle which goes some way towards removing this dilemma. Two ribbons, each half the length of a conventional ribbon are mounted one above the other and are connected in series. They are thus analogous to a conventional ribbon that has been ‘clamped’ in the centre. Each ribbon now has half the moving mass and thus a better top-end response. Both of them working together still maintain the necessary output.

The ribbon mic is rather more delicate than the moving coil, and it is better suited to applications where its smooth frequency response comes into its own, such as the micing of acoustic instruments and classical ensembles. There are, however, some robust models which look like moving-coil vocal mics and can be interchanged with them. Micing a rock bass drum with one is still probably not a good idea, due to the very high transient sound pressure levels involved.

The capacitor or condenser microphone

Basic capacitor microphone

The great advantage of the capacitor mic’s diaphragm over moving-coil and ribbon types is that it is not attached to a coil and former, and it does not need to be of a shape and size which makes it suitable for positioning along the length of a magnetic field. It therefore consists of an extremely light disc, typically 12–25 mm in diameter, frequently made from polyester coated with an extremely thin vapour-deposited metal layer so as to render it conductive. Sometimes the diaphragm itself is made of a metal such as titanium. The resonant frequency of the diaphragm is typically in the 12–20 kHz range, but the increased output here is rather less prominent than with moving coils due to the diaphragm’s very light weight.

Occasionally capacitor microphones are capable of being switched to give a line level output, this being simple to arrange since an amplifier is built into the mic anyway. The high-level output gives the signal rather more immunity to interference when very long cables are employed, and it also removes the need for microphone amplifiers at the mixer or tape recorder. Phantom power does, however, still need to be provided (see ‘Phantom power’, below).

Electret designs

A much later development was the so-called ‘electret’ or ‘electret condenser’ principle. The need to polarise the diaphragm with 48 volts is dispensed with by introducing a permanent electrostatic charge into it during manufacture. In order to achieve this the diaphragm has to be of a more substantial mass, and its audio performance is therefore closer to a moving-coil than to a true capacitor type. The power for the head amplifier is supplied either by a small dry-cell battery in the stem of the mic or by phantom power. The electret principle is particularly suited to applications where compact size and light weight are important, such as in small portable cassette machines (all built-in mics are now electrets) and tie-clip microphones which are ubiquitous in television work. They are also made in vast quantities very cheaply.

Later on, the so-called ‘back electret’ technique was developed. Here, the diaphragm is the same as that of a true capacitor type, the electrostatic charge being induced into the rigid back plate instead. Top-quality examples of back electrets are therefore just as good as conventional capacitor mics with their 48 volts of polarising voltage.

RF capacitor microphone

Still another variation on the theme is the RF (Radio Frequency) capacitor mic, in which the capacitor formed by the diaphragm and back plate forms part of a tuned circuit to generate a steady carrier frequency which is much higher than the highest audio frequency. The sound waves move the diaphragm as before, and this now causes modulation of the tuned frequency. This is then demodulated by a process similar to the process of FM radio reception, and the resulting output is the required audio signal. (It must be understood that the complete process is carried out within the housing of the microphone and it does not in itself have anything to do with radio microphone systems, as discussed in ‘Radio microphones’, below.)

Directional responses and polar diagrams

Microphones are designed to have a specific directional response pattern, described by a so-called ‘polar diagram’. The polar diagram is a form of two-dimensional contour map, showing the magnitude of the microphone’s output at different angles of incidence of a sound wave. The distance of the polar plot from the centre of the graph (considered as the position of the microphone diaphragm) is usually calibrated in decibels, with a nominal 0 dB being marked for the response at zero degrees at 1 kHz. The further the plot is from the centre, the greater the output of the microphone at that angle.

Omnidirectional pattern

Ideally, an omnidirectional or ‘omni’ microphone picks up sound equally from all directions. The omni polar response is shown in Figure 3.1, and is achieved by leaving the microphone diaphragm open at the front, but completely enclosing it at the rear, so that it becomes a simple pressure transducer, responding only to the change of air pressure caused by the sound waves. This works extremely well at low and mid frequencies, but at high frequencies the dimensions of the microphone capsule itself begin to be comparable with the wavelength of the sound waves, and a shadowing effect causes high frequencies to be picked up rather less well to the rear and sides of the mic. A pressure increase also results for high-frequency sounds from the front. Coupled with this is the possibility for cancellations to arise when a high-frequency wave, whose wavelength is comparable with the diaphragm diameter, is incident from the side of the diaphragm. In such a case positive and negative peaks of the wave may result in opposing forces on the diaphragm.

Figure 3.2 shows the polar response plot which can be expected from a real omnidirectional microphone with a capsule half an inch (13 mm) in diameter. It is perfectly omnidirectional up to around 2 kHz, but then it begins to lose sensitivity at the rear; at 3 kHz its sensitivity at 180° will typically be 6 dB down compared with lower frequencies. Above 8 kHz, the 180° response could be as much as 15 dB down, and the response at 90° and 270° could show perhaps a 10 dB loss. As a consequence, sounds which are being picked up significantly off axis from the microphone will be reproduced with considerable treble loss, and will sound dull. It is at its best on axis and up to 45° either side of the front of the microphone.

Figure 3.1   Idealised polar diagram of an omnidirectional microphone

Figure 3.2   Typical polar diagram of an omnidirectional microphone at a number of frequencies

High-quality omnidirectional microphones are characterised by their wide, smooth frequency response extending both to the lowest bass frequencies and the high treble with minimum resonances or coloration. This is due to the fact that they are basically very simple in design, being just a capsule which is open at the front and completely enclosed at the rear. (In fact a very small opening is provided to the rear of the diaphragm in order to compensate for overall changes in atmospheric pressure which would otherwise distort the diaphragm.) The small tie-clip microphones which one sees in television work are usually omnidirectional electret types which are capable of very good performance. The smaller the dimensions of the mic, the better the polar response at high frequencies, and mics such as these have quarter-inch diaphragms which maintain a very good omnidirectional response right up to 10 kHz.

Omni microphones are usually the most immune to handling and wind noise of all the polar patterns, since they are only sensitive to absolute sound pressure. Patterns such as figure-eight (especially ribbons) and cardioid, described below, are much more susceptible to handling and wind noise than omnis because they are sensitive to the large pressure difference created across the capsule by low-frequency movements such as those caused by wind or unwanted diaphragm motion. A pressure-gradient microphone’s mechanical impedance (the diaphragm’s resistance to motion) is always lower at LF than that of a pressure (omni) microphone, and thus it is more susceptible to unwanted LF disturbances.

Figure-eight or bidirectional pattern

The figure-eight or bidirectional polar response is shown in Figure 3.3. Such a microphone has an output proportional to the mathematical cosine of the angle of incidence. One can quickly draw a figure-eight plot on a piece of graph paper, using a protractor and a set of cosine tables or pocket calculator. Cos 0° = 1, showing a maximum response on the forward axis (this will be termed the 0 dB reference point). Cos 90° = 0, so at 90° off axis no sound is picked up. Cos 180° is −1, so the output produced by a sound which is picked up by the rear lobe of the microphone will be 180° out of phase compared with an identical sound picked up by the front lobe. The phase is indicated by the + and − signs on the polar diagram. At 45° off axis, the output of the microphone is 3 dB down (cos 45° represents 0.707 or 1/√2 times the maximum output) compared with the on-axis output.

Traditionally the ribbon microphone has sported a figure-eight polar response, and the ribbon has been left completely open both to the front and to the rear. Such a diaphragm operates on the pressure-gradient principle, responding to the difference in pressure between the front and the rear of the microphone. Consider a sound reaching the mic from a direction 90° off axis to it. The sound pressure will be of equal magnitude on both sides of the diaphragm and so no movement will take place, giving no output. When a sound arrives from the 0° direction a phase difference arises between the front and rear of the ribbon, due to the small additional distance travelled by the wave. The resulting difference in pressure produces movement of the diaphragm and an output results.

Figure 3.3   Idealised polar diagram of a figure-eight microphone

At very low frequencies, wavelengths are very long and therefore the phase difference between front and rear of the mic is very small, causing a gradual reduction in output as the frequency gets lower. In ribbon microphones this is compensated for by putting the low-frequency resonance of the ribbon to good use, using it to prop up the bass response. Single-diaphragm capacitor mic designs which have a figure-eight polar response do not have this option, since the diaphragm resonance is at a very high frequency, and a gradual roll-off in the bass can be expected unless other means such as electronic frequency correction in the microphone design have been employed. Double-diaphragm switchable types which have a figure-eight capability achieve this by combining a pair of back-to-back cardioids (see next section) that are mutually out of phase.

Like the omni, the figure-eight can give very clear uncoloured reproduction. The polar response tends to be very uniform at all frequencies, except for a slight narrowing above 10 kHz or so, but it is worth noting that a ribbon mic has a rather better polar response at high frequencies in the horizontal plane than in the vertical plane, due to the fact that the ribbon is long and thin. A high-frequency sound coming from a direction somewhat above the plane of the microphone will suffer partial cancellation, since at frequencies where the wavelength begins to be comparable with the length of the ribbon the wave arrives partially out of phase at the lower portion compared with the upper portion, therefore reducing the effective acoustical drive of the ribbon compared with mid frequencies. Ribbon figure-eight microphones should therefore be orientated either upright or upside-down with their stems vertical so as to obtain the best polar response in the horizontal plane, vertical polar response usually being less important.

Although the figure-eight picks up sound equally to the front and to the rear, it must be remembered that the rear pickup is out of phase with the front, and so correct orientation of the mic is required.

Cardioid or unidirectional pattern

The cardioid pattern is described mathematically as 1 + cosθ, where θ is the angle of incidence of the sound. Since the omni has a response of 1 (equal all round) and the figure-eight has a response represented by cosθ, the cardioid may be considered theoretically as a product of these two responses. Figure 3.4(a) illustrates its shape. Figure 3.4(b) shows an omni and a figure-eight superimposed, and one can see that adding the two produces the cardioid shape: at 0°, both polar responses are of equal amplitude and phase, and so they reinforce each other, giving a total output which is actually twice that of either separately. At 180°, however, the two are of equal amplitude but opposite phase, and so complete cancellation occurs and there is no output. At 90° there is no output from the figure-eight, but just the contribution from the omni, so the cardioid response is 6 dB down at 90°. It is 3 dB down at 65° off axis.

Figure 3.4   (a) Idealised polar diagram of a cardioid microphone

Figure 3.4   (b) A cardioid microphone can be seen to be the mathematical equivalent of an omni and a figure-eight response added together

One or two early microphone designs actually housed a figure-eight and an omni together in the same casing, electrically combining their outputs to give a resulting cardioid response. This gave a rather bulky mic, and also the two diaphragms could not be placed close enough together to produce a good cardioid response at higher frequencies due to the fact that at these frequencies the wavelength of sound became comparable with the distance between the diaphragms. The designs did, however, obtain a cardioid from first principles.

The cardioid response is now obtained by leaving the diaphragm open at the front, but introducing various acoustic labyrinths at the rear which cause sound to reach the back of the diaphragm in various combinations of phase and amplitude to produce a resultant cardioid response. This is difficult to achieve at all frequencies simultaneously, and Figure 3.5 illustrates the polar pattern of a typical cardioid mic with a three-quarter-inch diaphragm. As can be seen, at mid frequencies the polar response is very good. At low frequencies it tends to degenerate towards omni, and at very high frequencies it becomes rather more directional than is desirable. Sound arriving from, say, 45° off axis will be reproduced with treble loss, and sounds arriving from the rear will not be completely attenuated, the low frequencies being picked up quite uniformly.

Figure 3.5   Typical polar diagram of a cardioid microphone at low, middle and high frequencies

The above example is very typical of moving-coil cardioids, and they are in fact very useful for vocalists due to the narrow pickup at high frequencies helping to exclude off-axis sounds, and also the relative lack of pressure-gradient component at the bass end helping to combat bass tip-up. High-quality capacitor cardioids with half-inch diaphragms achieve a rather more ideal cardioid response. Owing to the presence of acoustic labyrinths, coloration of the sound is rather more likely, and it is not unusual to find that a relatively cheap electret omni will sound better than a fairly expensive cardioid.

Hypercardioid pattern

The hypercardioid, sometimes called ‘cottage loaf’ because of its shape, is shown in Figure 3.6. It is described mathematically by the formula 0.5 + cosθ, i.e.: it is a combination of an omni attenuated by 6 dB, and a figure-eight. Its response is in between the cardioid and figure-eight patterns, having a relatively small rear lobe which is out of phase with the front lobe. Its sensitivity is 3 dB down at 55° off axis. Like the cardioid, the polar response is obtained by introducing acoustic labyrinths to the rear of the diaphragm. Because of the large pressure-gradient component it too is fairly susceptible to bass tip-up. Practical examples of hypercardioid microphones tend to have polar responses which are tolerably close to the ideal. The hypercardioid has the highest direct-to-reverberant ratio of the patterns described, which means that the ratio between the level of on-axis sound and the level of reflected sounds picked up from other angles is very high, and so it is good for excluding unwanted sounds such as excessive room ambience or unwanted noise.

Figure 3.6   Idealised polar diagram of a hypercardioid microphone

Specialised microphone types

Rifle microphone

The rifle microphone is so called because it consists of a long tube of around three-quarters of an inch (1.9 cm) in diameter and perhaps 2 feet (61 cm) in length, and looks rather like a rifle barrel. The design is effectively an ordinary cardioid microphone to which has been attached a long barrel along which slots are cut in such a way that a sound arriving off axis enters the slots along the length of the tube and thus various versions of the sound arrive at the diaphragm at the bottom of the tube in relative phases which tend to result in cancellation. In this way, sounds arriving off axis are greatly attenuated compared with sounds arriving on axis. Figure 3.7 illustrates the characteristic club-shaped polar response. It is an extremely directional device, and is much used by news sound crews where it can be pointed directly at a speaking subject, excluding crowd noise. It is also used for wildlife recording, sports broadcasts, along the front of theatre stages in multiples, and in audience participation discussions where a particular speaker can be picked out. For outside use it is normally completely enclosed in a long, fat wind shield, looking like a very big cigar. Half-length versions are also available which have a polar response midway between a club shape and a hypercardioid. All versions, however, tend to have a rather wider pickup at low frequencies.

Figure 3.7   Typical polar diagram of a highly-directional microphone

Figure 3.8   A parabolic reflector is sometimes used to ‘focus’ the incoming sound wavefront at the microphone position, thus making it highly directional

Parabolic microphone

An alternative method of achieving high directionality is to use a parabolic dish, as shown in Figure 3.8. The dish has a diameter usually of between 0.5 and 1 metre, and a directional microphone is positioned at its focal point. A large ‘catchment area’ is therefore created in which the sound is concentrated at the head of the mic. An overall gain of around 15 dB is typical, but at the lower frequencies where the wavelength of sound becomes comparable with the diameter of the dish the response falls away. Because this device actually concentrates the sound rather than merely rejecting off-axis sounds, comparatively high outputs are achieved from distant sound sources. They are very useful for capturing bird song, and they are also sometimes employed around the boundaries of cricket pitches. They are, however, rather cumbersome in a crowd, and can also produce a rather coloured sound.

Boundary or ‘pressure-zone’ microphone

The so-called boundary or pressure-zone microphone (PZM) consists basically of an omnidirectional microphone capsule mounted on a plate usually of around 6 inches (15 cm) square or 6 inches in diameter such that the capsule points directly at the plate and is around 2 or 3 millimetres away from it. The plate is intended to be placed on a large flat surface such as a wall or floor, and it can also be placed on the underside of a piano lid for instance. Its polar response is hemispherical. Because the mic capsule is a simple omni, quite good-sounding versions are available with electret capsules fairly cheaply, and so if one wishes to experiment with this unusual type of microphone one can do so without parting with a great deal of money. It is important to remember though that despite its looks it is not a contact mic – the plate itself does not transduce surface vibrations – and it should be used with the awareness that it is equivalent to an ordinary omnidirectional microphone pointing at a flat surface, very close to it. The frequency response of such a microphone is rarely as flat as that of an ordinary omni, but it can be unobtrusive in use.

Switchable polar patterns

The double-diaphragm capacitor microphone, such as the commercial example shown in Figure 3.9, is a microphone in which two identical diaphragms are employed, placed each side of a central rigid plate in the manner of a sandwich. Perforations in the central plate give both diaphragms an essentially cardioid response. When the polarising voltage on both diaphragms is the same, the electrically combined output gives an omnidirectional response due to the combination of the back-to-back cardioids in phase. When the polarising voltage of one diaphragm is opposite to that of the other, and the potential of the rigid central plate is midway between the two, the combined output gives a figure-eight response (back-to-back cardioids mutually out of phase). Intermediate combinations give cardioid and hypercardioid polar responses. In this way the microphone is given a switchable polar response which can be adjusted either by switches on the microphone itself or via a remote control box. Some microphones with switchable polar patterns achieve this by employing a conventional single diaphragm around which is placed appropriate mechanical labyrinths which can be switched to give the various patterns.

Figure 3.9   A typical double-diaphragm condenser microphone with switchable polar pattern: the AKG C4141B-ULS. (Courtesy of AKG Acoustics GmbH)

Another method manufacturers have used is to make the capsule housing on the end of the microphone detachable, so that a cardioid capsule, say, can be unscrewed and removed to be replaced with, say, an omni. This also facilitates the use of extension tubes whereby a long thin pipe of around a metre or so in length with suitably threaded terminations is inserted between the main microphone body and the capsule. The body of the microphone is mounted on a short floor stand and the thin tube now brings the capsule up to the required height, giving a visually unobtrusive form of microphone stand.

Stereo microphones

Stereo microphones, such as the example shown in Figure 3.10, are available in which two microphones are built into a single casing, one capsule being rotatable with respect to the other so that the angle between the two can be adjusted. Also, each capsule can be switched to give any desired polar response. One can therefore adjust the mic to give a pair of figure-eight microphones angled at, say, 90°, or a pair of cardioids at 120°, and so on. Some stereo mics, such as that pictured in Figure 3.11, are configured in a sum-and-difference arrangement, instead of as a left–right pair, with a ‘sum’ capsule pointing forwards and a figure-eight ‘difference’ capsule facing sideways. The sum-and-difference or ‘middle and side’ (M and S) signals are combined in a matrix box to produce a left–right stereo signal by adding M and S to give the left channel and subtracting M and S to give the right channel. This is discussed in more detail in Fact File 3.6.

Figure 3.10   A typical stereo microphone: the Neumann SM69. (Courtesy of FWO Bauch Ltd)

Figure 3.11   A typical ‘sum-and-difference’ stereo microphone: the Shure VP88. (Courtesy of HW International)

A sophisticated stereo microphone is the Soundfield Research microphone. In this design, four ‘subcardioid’ capsules (i.e.: between omni and cardioid) are arranged in a tetrahedral array such that their outputs can be combined in various ways to give four outputs, termed ‘B format’. The raw output from the four capsules is termed ‘A format’. The four B-format signals consist of a forward-facing figure-eight (‘X’), a sideways-facing figure-eight (‘Y’), an up-and-down-facing figure-eight (‘Z’), and an omnidirectional output (‘W’). These are then appropriately combined to produce any configuration of stereo microphone output, each channel being fully adjustable from omni through cardioid to figure-eight, the angles between the capsules also being fully adjustable. The tilt angle of the microphone, and also the ‘dominance’ (the front-to-back pickup ratio) can also be controlled. All of this is achieved electronically by a remotely sited control unit. Additionally, the raw B-format signals can be recorded on a four-channel tape recorder, later to be replayed through the control unit where all of the above parameters can be chosen after the recording session.

The ST250 is a second generation stereo microphone based on soundfield principles, designed to be smaller and to be usable either ‘end-fire’ or ‘side-fire’ (see Figure 3.12). It can be electronically inverted and polar patterns and capsule angles are variable remotely.

Microphone performance

Professional microphones have a balanced low-impedance output usually via a three-pin XLR-type plug in their base. The impedance, which is usually around 200 ohms but sometimes rather lower, enables long microphone leads to be used. Also, the balanced configuration, discussed in ‘Balanced lines’, Chapter 12, gives considerable immunity from interference. Other parameters which must be considered are sensitivity (see Fact File 3.7) and noise (see Fact File 3.8).

Figure 3.12   The Soundfield ST250 microphone is based on soundfield principles, and can be operated either end- or side-fire, or upside-down, using electrical matrixing of the capsules within the control unit. (Courtesy of SoundField Ltd)

Microphone sensitivity in practice

The consequence of mics having different sensitivity values is that rather more amplification is needed to bring ribbons and moving coils up to line level than is the case with capacitors. For example, speech may yield, say, 0.15 mV from a ribbon. To amplify this up to line level (775 mV) requires a gain of around ×5160 or 74 dB. This is a lot, and it taxes the noise performance of the equipment and will also cause considerable amplification of any interference that manages to get into the microphone cables.

Consider now the same speech recording, made using a capacitor microphone of 1 mV μB⁻¹ sensitivity. Now only ×775 or 57 dB of gain is needed to bring this up to line level, which means that any interference will have a rather better chance of being unnoticed, and also the noise performance of the mixer will not be so severely taxed. This does not mean that high-output capacitor microphones should always be used, but it illustrates that high-quality mixers and microphone cabling are required to get the best out of low-output mics.

Microphone noise in practice

The noise coming from a capacitor microphone is mainly caused by the head amplifier. Since ribbons and moving coils are purely passive devices one might think that they would therefore be noiseless. This is not the case, since a 200 ohm passive resistance at room temperature generates a noise output between 20 Hz and 20 kHz of 0.26 μV (μV = microvolts). Noise in passive microphones is thus due to thermal excitation of the charge carriers in the microphone ribbon or voice coil, and the output transformer windings. To see what this means in equivalent self-noise terms so that ribbons and moving coils can be compared with capacitors, one must relate this to sensitivity.

Take a moving coil with a sensitivity of 0.2 mV μB⁻¹, which is 2 mV for 94 dB SPL. The noise is 0.26 μV or 0.000 26 mV. The signal-to-noise ratio is given by dividing the sensitivity by the noise:

2 ÷ 0.000 26 ≈ 7600

and then expressing this in decibels:

dB = 20 log 7600 = 77 dB

This is an unweighted figure, and ‘A’ weighting will usually improve it by a couple of decibels. However, the microphone amplifier into which the mic needs to be plugged will add a bit of noise, so it is a good idea to leave this figure as it is to give a fairly good comparison with the capacitor example. (Because the output level of capacitor mics is so much higher than that of moving coils, the noise of a mixer’s microphone amplifier does not figure in the noise discussion as far as these are concerned. The noise generated by a capacitor mic is far higher than noise generated by good microphone amplifiers and other types of microphone.)

A 200 ohm moving-coil mic with a sensitivity of 0.2 mV μB⁻¹ thus has a signal-to-noise ratio of about 77 dB, and therefore an equivalent self-noise of 94−77 = 17 dB which is comparable with high-quality capacitor types, providing that high-quality microphone amplifiers are also used. A low-output 200 ohm ribbon microphone could have a sensitivity of 0.1 mV μB⁻¹, i.e.: 6 dB less than the above moving-coil example. Because its 200 ohm thermal noise is roughly the same, its equivalent self noise is therefore 6 dB worse, i.e.: 23 dB. This would probably be just acceptable for recording speech and classical music if an ultra-low-noise microphone amplifier were to be used which did not add significantly to this figure.

The discussion of a few decibels here and there may seem a bit pedantic, but in fact self-noises in the low twenties are just on the borderline of being acceptable if one wishes to record speech or the quieter types of classical music. Loud music, and mic positions close to the sound sources such as is the practice with rock music, generate rather higher outputs from the microphones and here noise is rarely a problem. But the high output levels generated by close micing of drums, guitar amps and the like can lead to overload in the microphone amplifiers. For example, if a high-output capacitor microphone is used to pick up a guitarist’s amplifier, outputs as high as 150 mV or more can be generated. This would overload some fixed-gain microphone input stages, and an in-line attenuator which reduces the level by an appropriate amount such as 10–20 dB would have to be inserted at the mixer or tape recorder end of the microphone line. Attenuators are available built into a short cylindrical tube which carries an XLR-type plug at one end and a socket at the other end. It is simply inserted between the mixer or tape recorder input and the mic lead connector. It should not be connected at the microphone end because it is best to leave the level of signal along the length of the mic lead high to give it greater immunity from interference.

Microphone powering options

Phantom power

Consideration of capacitor microphones reveals the need for supplying power to the electronics which are built into the casing, and also the need for a polarising voltage across the diaphragm of many capacitor types. It would obviously be inconvenient and potentially troublesome to incorporate extra wires in the microphone cable to supply this power, and so an ingenious method was devised whereby the existing wires in the cable which carry the audio signal could also be used to carry the DC voltage necessary for the operation of capacitor mics – hence the term ‘phantom power’, since it is invisibly carried over the audio wires. Furthermore, this system does not preclude the connection of a microphone not requiring power to a powered circuit. The principle is outlined in Fact File 3.9.

It will be appreciated that if, for instance, a ribbon microphone is connected to the line in place of a capacitor mic, no current will flow into the microphone because there will be no centre tap provided on the microphone’s output transformer. Therefore, it is perfectly safe to connect other types of balanced microphone to this line. The two 6k8 resistors are necessary for the system because if they were replaced simply by two wires directly connected to the audio lines, these wires would short-circuit the lines together and so no audio signal would be able to pass. The phantom power could be applied to a centre tap of the input transformer, but if a short circuit were to develop along the cabling between one of the audio wires and the screen, potentially large currents could be drawn through the transformer windings and the phantom power supply, blowing fuses or burning out components. Two 6k8 resistors limit the current to around 14 mA, which should not cause serious problems. The 6k8 value was chosen so as to be high enough not to load the microphone unduly, but low enough for there to be only a small DC voltage drop across them so that the microphone still receives nearly the full 48 volts. Two real-life examples will be chosen to investigate exactly how much voltage drop occurs due to the resistors.

Firstly, the current flows through both resistors equally and so the resistors are effectively ‘in parallel’. Two equal-value resistors in parallel behave like a single resistor of half the value, so the two 6k8 resistors can be regarded as a single 3k4 resistor as far as the 48 V phantom power is concerned. Ohm’s law (see Fact File 1.1) states that the voltage drop across a resistor is equal to its resistance multiplied by the current passing through it. Now a Calrec 1050C microphone draws 0.5 milliamps (= 0.0005 amps) through the resistors, so the voltage drop is 3400 × 0.0005 = 1.7 volts. Therefore the microphone receives 48−1.7 volts, i.e.: 46.3 volts. The Schoeps CMC-5 microphone draws 4 mA so the voltage drop is 3400 × 0.004 =13.6 volts. Therefore the microphone receives 48−13.6 volts, i.e.: 34.4 volts. The manufacturer normally takes this voltage drop into account in the design of the microphone, although examples exist of mics which draw so much current that they load down the phantom voltage of a mixer to a point where it is no longer adequate to power the mics. In such a case some mics become very noisy, some will not work at all, and yet others may produce unusual noises or oscillation. A stand-alone dedicated power supply or internal battery supply may be the solution in difficult cases.

The universal standard is 48 volts, but some capacitor microphones are designed to operate on a range of voltages down to 9 volts, and this can be advantageous for instance when using battery-powered equipment on location, or out of doors away from a convenient source of mains power.

Figure 3.13 illustrates the situation with phantom powering when electronically balanced circuits are used, as opposed to transformers. Capacitors are used to block the DC voltage from the power supply, but they present a very low impedance to the audio signal.

A–B powering

Another form of powering for capacitor microphones which is sometimes encountered is A−B powering. Figure 3.14 illustrates this system schematically. Here, the power is applied to one of the audio lines via a resistor and is taken to the microphone electronics via another resistor at the microphone end. The return path is provided by the other audio line as the arrows show. The screen is not used for carrying any current. There is a capacitor at the centre of the winding of each transformer. A capacitor does not allow DC to pass, and so these capacitors prevent the current from short-circuiting via the transformer windings. The capacitors have a very low impedance at audio frequencies, so as far as the audio signal is concerned they are not there. The usual voltage used in this system is 12 volts.

Figure 3.13   A typical 48 volt phantom powering arrangement in an electronically balanced circuit

Although, like phantom power, the existing microphone lines are used to carry the current, it is dangerous to connect another type of microphone in place of the one illustrated. If, say, a ribbon microphone were to be connected, its output transformer would short-circuit the applied current. Therefore 12 volt A–B powering should be switched off before connecting any other type of microphone, and this is clearly a disadvantage compared with the phantom powering approach. It is encountered most commonly in location film sound recording equipment.

Radio microphones

Radio microphones are widely used in film, broadcasting, theatre and other industries, and it is not difficult to think of circumstances in which freedom from trailing microphone cables can be a considerable advantage in all of the above.

Figure 3.14   A typical 12 volt A–B powering arrangement

Principles

The radio microphone system consists of a microphone front end (which is no different from an ordinary microphone); an FM (Frequency Modulation) transmitter, either built into the housing of the mic or housed in a separate case into which the mic plugs; a short aerial via which the signal is transmitted; and a receiver which is designed to receive the signal from a particular transmitter. Only one specified transmission frequency is picked up by a given receiver. The audio output of the receiver then feeds a mixer or tape machine in the same manner as any orthodox microphone or line level source would. The principle is illustrated in Figure 3.15.

The transmitter can be built into the stem of the microphone, or it can be housed in a separate case, typically the size of a packet of cigarettes, into which the microphone or other signal source is plugged. A small battery which fits inside the casing of the transmitter provides the power, and this can also supply power to those capacitor mics which are designed to operate at the typical 9 volts of the transmitter battery. The transmitter is of the FM type (see Fact File 3.10), as this offers high-quality audio performance.

Frequently, two or more radio microphones need to be used. Each transmitter must transmit at a different frequency, and the spacing between each adjacent frequency must not be too close otherwise they will interfere with each other. In practice, channels with a minimum spacing of 0.2 MHz are used. Although only one transmitter can be used at a given frequency, any number of receivers can of course be used, as is the case with ordinary radio reception.

Facilities

Transmitters are often fitted with facilities which enable the operator to set the equipment up for optimum performance. A 1 kHz line-up tone is sometimes encountered which sends a continuous tone to the receiver to check continuity. Input gain controls are useful, with an indication of peak input level, so that the transmitter can be used with mics and line level sources of widely different output levels. It is important that the optimum setting is found, as too great an input level may cause a limiter (see ‘The compressor/limiter’, Chapter 13) to come into action much of the time, which can cause compression and ‘pumping’ noises as the limiter operates. Too weak a signal gives insufficient drive, and poor signal-to-noise ratios can result.

Figure 3.15 A radio microphone incorporates an FM transmitter, resulting in no fixed link between microphone and mixer

The receiver will have a signal strength indicator. This can be very useful for locating ‘dead spots’; transmitter positions which cause unacceptably low meter readings should be avoided, or the receiving aerial should be moved to a position which gives better results. Another useful facility is an indicator which tells the condition of the battery in the transmitter. When the battery voltage falls below a certain level, the transmitter sends out an inaudible warning signal to the receiver which will then indicate this condition. The operator then has a warning that the battery will soon fail, which is often within 15 minutes of the indication.

Licences

Transmitting equipment usually requires a licence for its operation, and governments normally rigidly control the frequency bands over which a given user can operate. This ensures that local and network radio transmitters do not interfere with police, ambulance and fire brigade equipment, etc. In the UK the frequency band for which radio mics do not have to be licensed is between 173.8 MHz and 175 MHz. Each radio mic transmitter needs to be spaced at least 0.2 MHz apart, and commonly used frequencies are 173.8, 174.1, 174.5, 174.8, and 175.0 MHz. An additional requirement is that the frequencies must be crystal controlled, which ensures that they cannot drift outside tightly specified limits. Maximum transmitter power is limited to 10 milliwatts, which gives an effective radiated power (ERP) at the aerial of 2 milliwatts which is very low, but adequate for the short ranges over which radio mics are operated.

In recent years radio mics in the UHF band have become available operating across the 800 MHz band of frequencies, for which licences are required. Aerials are correspondingly smaller than those for the VHF models giving greater convenience for the wearer. The UHF band has proved very reliable, and notably free from interference problems.

Aerials

The dimensions of the transmitting aerial are related to the wavelength of the transmitted frequency. The wavelength (λ) in an electrical conductor at a frequency of 174.5 MHz is approximately 64 inches (160 cm). To translate this into a suitable aerial length, it is necessary to discuss the way in which a signal resonates in a conductor. It is convenient to consider a simple dipole aerial, as shown in Figure 3.16. This consists of two conducting rods, each a quarter of a wavelength long, fed by the transmitting signal as shown. The centre of the pair is the nodal point and exhibits a characteristic impedance of about 70 ohms. For a radio mic, we need a total length of λ/2, i.e.: 64/2 = 32 inches (80 cm).

A 32 inch dipole will therefore allow the standard range of radio mic frequencies to resonate along its length to give efficient radiation, the precise length not being too critical. Consideration also has to be given to the radiated polar response (this is not the same as the microphone’s polar response). Figure 3.17 shows the polar response for a dipole. As can be seen, it is a figure-eight with no radiation in the directions the two halves are pointing in. Another factor is polarisation of the signal. Electromagnetic waves consist of an electric wave plus a magnetic wave radiating at right angles to each other, and so if a transmitting aerial is orientated vertically, the receiving aerial should also be orientated vertically. This is termed vertical polarisation.

The radio mic transmitter therefore has a transmitting aerial of about 16 inches long: half of a dipole. The other half is provided by the earth screen of the audio input lead, and will be in practice rather longer than 16 inches. The first-mentioned half is therefore looked upon as being the aerial proper, and it typically hangs vertically downwards. The screened signal input cable will generally be led upwards, but other practical requirements tend to override its function as part of the aerial system.

Figure 3.16   A simple dipole aerial configuration

Figure 3.17   The dipole has a figure-eight radiation pattern

Another type which is often used for hand-held radio mics is the helical aerial. This is typically rather less than half the length of the 16 inch aerial, and has a diameter of a centimetre or so. It protrudes from the base of the microphone. It consists of a tight coil of springy wire housed in a plastic insulator, and has the advantage of being both smaller and reasonably tolerant of physical abuse. Its radiating efficiency is, however, less good than the 16 inch length of wire. At the receiver, a similar aerial is required. The helical aerial is very common here, and its short stubby form is very convenient for outside broadcast and film crews. A 16 inch length of metal tubing, rather like a short car aerial, can be a bit unwieldy although it is a more efficient receiver.

Other aerial configurations exist, offering higher gain and directionality. In the two-element aerial shown in Figure 3.18 the reflector is slightly larger than the dipole, and is spaced behind it at a distance which causes reflection of signal back on to it. It increases the gain, or strength of signal output, by 3 dB. It also attenuates signals approaching from the rear and sides. The three-element ‘Yagi’, named after its Japanese inventor and shown in Figure 3.19, uses the presence of a director and reflector to increase the gain of a conventional dipole, and a greatly elongated rectangle called a folded dipole is used, which itself has a characteristic impedance of about 300 ohms. The other elements are positioned such that the final impedance is reduced to the standard 50 ohms. The three-element Yagi is even more directional than the dipole, and has increased gain. It can be useful in very difficult reception conditions, or where longer distances are involved such as receiving the signal from a transmitter carried by a rock climber for running commentary! The multi-element, high-gain, highly directional UHF television aerial is of course a familiar sight on our roof-tops.

These aerials can also be used for transmitting, the principles being exactly the same. Their increased directionality also helps to combat multipath problems. The elements should be vertically orientated, because the transmitting aerial will normally be vertical, and the ‘direction of maximum sensitivity’ arrows on the figures show the direction the aerials should be pointed in.

Figure 3.18   A simple two-element aerial incorporates a dipole and a reflector for greater directionality than a dipole

Another technique for improving the signal-to-noise ratio under difficult reception conditions is noise reduction, which operates as follows. Inside the transmitter there is an additional circuit which compresses the incoming audio signal, thus reducing its overall dynamic range. At the receiver, a reciprocal circuit expands the audio signal, after reception and demodulation, and as it pushes the lower-level audio signals back down to their correct level it also therefore pushes the residual noise level down. Previously unacceptable reception conditions will often yield usable results when such transmitters and receivers are employed. It should be noted though that the system does not increase signal strength, and all the problems of transmission and reception still apply. (Noise reduction systems are covered further in Chapter 7.)

Figure 3.19   The three-element ‘Yagi’ configuration

Aerial siting and connection

It is frequently desirable to place the receiving aerial itself closer to the transmitter than the receiver, in order to pick up a strong signal. To do this an aerial is rigged at a convenient position close to the transmitter, for example in the wings of a theatre stage, or on the front of a balcony, and then an aerial lead is run back to the receiver. A helical dipole aerial is frequently employed. In such a situation, characteristic impedance must be considered. As discussed in ‘Principles’, Chapter 12, when the wavelength of the electrical signal in a conductor is similar to the length of the conductor, reflections can be set up at the receiving end unless the cable is properly terminated. Therefore, impedance matching must be employed between the aerial and the transmitter or receiver, and additionally the connecting lead needs to have the correct characteristic impedance.

The standard value for radio microphone equipment is 50 ohms, and so the aerial, the transmitter, the receiver, the aerial lead and the connectors must all be rated at this value. This cannot be measured using a simple test meter, but an aerial and cable can be tuned using an SWR (Standing Wave Ratio) meter to detect the level of the reflected signal. The aerial lead should be a good-quality, low-loss type, otherwise the advantage of siting the aerial closer to the transmitter will be wasted by signal loss along the cable. Poor signal reception causes noisy performance, because the receiver has a built-in automatic gain control (AGC), which sets the amplification of the carrier frequency to an appropriate value. Weak signals simply require higher amplification and therefore higher noise levels result.

The use of several radio microphones calls for a complementary number of receivers which all need an aerial feed. It is common practice to use just one aerial which is plugged into the input of an aerial distribution amplifier. This distribution unit has several outputs which can be fed into each receiver. It is not possible simply to connect an aerial to all the inputs in parallel due to the impedance mismatch that this would cause.

Apart from obvious difficulties such as metallic structures between transmitter and receiver, there are two phenomena which cause the reception of the radio signal to be less than perfect. The first phenomenon is known as multi-path (see Figure 3.20). When the aerial transmits, the signal reaches the receiving aerial by a number of routes. Firstly, there is the direct path from aerial to aerial. Additionally, signals bounce off the walls of the building and reach the receiving aerial via a longer route. So the receiving aerial is faced with a number of signals of more or less random phase and strength, and these will sometimes combine to cause severe signal cancellation and consequently very poor reception. The movement of the transmitter along with the person wearing it will alter the relationship between these multipath signals, and so ‘dead spots’ are sometimes encountered where particular combinations of multipath signals cause signal ‘drop-out’. The solution is to find out where these dead spots are by trial and error, re-siting the receiving aerial until they are minimised or eliminated. It is generally good practice to site the aerial close to the transmitter so that the direct signal will be correspondingly stronger than many of the signals arriving from the walls. Metal structures should be kept clear of wherever possible due to their ability to reflect and screen RF signals. Aerials can be rigged on metal bars, but at right angles to them, not parallel.

Figure 3.20   Multipath distortion can arise between source and receiver due to reflections

The other phenomenon is signal cancellation from other transmitters when a number of channels are in use simultaneously. Because the transmitting frequencies of the radio mics will be quite close together, partial cancellation of all the signals takes place. The received signals are therefore weaker than for a single transmitter on its own. Again, siting the receiving aerial close to the transmitters is a good idea. The ‘sharpness’ or ‘Q’ of the frequency tuning of the receivers plays a considerable part in obtaining good reception in the presence of a number of signals. A receiver may give a good performance when only one transmitter is in use, but a poor Q will vastly reduce the reception quality when several are used. This should be checked for when systems are being evaluated, and the testing of one channel on its own will not of course show up these kinds of problems.

Diversity reception

A technique known as ‘spaced diversity’ goes a good way towards combatting the above problems. In this system, two aerials feed two identical receivers for each radio channel. A circuit continuously monitors the signal strength being received by each receiver and automatically selects the one which is receiving the best signal (see Figure 3.21). When they are both receiving a good signal, the outputs of the two are mixed together. A crossfade is performed between the two as one RF signal fades and the other becomes strong.

The two aerials are placed some distance apart, in practice several metres gives good results, so that the multipath relationships between a given transmitter position and each aerial will be somewhat different. A dead spot for one aerial is therefore unlikely to coincide with a dead spot for the other one. A good diversity system overcomes many reception problems, and the considerable increase in reliability of performance is well worth the extra cost. The point at which diversity becomes desirable is when more than two radio microphones are to be used, although good performance from four channels in a non-diversity installation is by no means out of the question. Good radio microphones are very expensive, a single channel of a quality example costing over a thousand pounds today. Cheaper ones exist, but experience suggests that no radio microphone at all is vastly preferable to a cheap one.

Figure 3.21   A diversity receiver incorporates two aerials spaced apart and two receivers. The signal strength from each aerial is used to determine which output will have the higher quality

Recommended further reading

AES (1979) Microphones: An Anthology. Audio Engineering Society

Bartlett, B. (1991) Stereo Microphone Techniques. Focal Press

Eargle, J. (2004) The Microphone Book, 2nd edn. Focal Press

Gayford, M. (1994) ed. Microphone Engineering Handbook. Focal Press

See also General further reading at the end of this book.