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 center 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 center, 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 above, 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 microphone. A pressure increase also results for high-frequency sounds from the front. Coupled with this is the possibility for cancelations 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 above 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 3kHz its sensitivity at 180° will typically be 6 dB down compared with lower frequencies. Above 8kHz, the 180° response could be as much as 15 dB down, and the response at 90° and 270° could show perhaps a 10dB 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 450 either side of the front of the microphone.
High-quality omnidirectional microphones are characterized 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 most immune to noise Of all the polar patterns, since they are only sensitive to absolute sound pressure. Patterns such as figure-eight (especially ribbons) and cardioid, 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 Bi-directional 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° = O, 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 3dB down (cos 45° represents 0.707
or 1/√2 times the maximum output) compared with the on-axis output.
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 00 direction a phase difference arises between the front and rear of the ribbon, due to the small additional distance traveled by the wave. The resulting difference in pressure produces movement of the diaphragm and an output result.
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 that are mutually out of phase.
Like the omni, the figure-eight can give very clear uncolored reproduction. The polar response tends to be very uniform at all frequencies, except for a slight narrowing above 10kHz 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 cancelation, 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.
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 I (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 response
s. Figure (a) illustrates its shape. Figure (b) shows an omni and a figure eight
superimposed, and one can see that addi
-ng 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 separately. At 180°, however, the two are of equal amplitude but opposite phase, and so complete cancelation 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 6dB down at 90°. It is 3 dB down at 65° off axis.
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 below 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.
Hypercardioid Pattern
The hypercardioid, sometimes called 'cottage loaf' because of its shape is
shown in Figure below. It is described mathematically by the formula 0.5 + cosΘ,
i.e., it is a combination of an omni attenuated by 6dB, 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 3dB 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 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.
Specialized 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 cancelation. In this way, sounds arriving off axis are greatly attenuated compared with sounds arriving on axis. Figure above 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 theater 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.
Parabolic Microphone
An alternative method of achieving high directionality is to use a parabolic dish, as
shown in Figure below. The dish has a diameter usually of between 0.5 and 1 meter,
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 15dB 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 colored sound.
Boundary or Pressure zone Microphones
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 (15cm) square or 6 inches in diameter such that the capsule points directly at the plate and is around 2 or 3 millimeters 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 below, 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 polarizing 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 polarizing 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.
Stereo Microphones
Stereo microphones 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.
