Capacitor Microphones Explained

Exploration

Published in SOS February 1998
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Technique : Theory + Technical

PAUL WHITE looks at the workings of capacitor mics and discusses their advantages for studio recording.

At the heart of every studio microphone is a transducer system designed to convert sound into an equivalent electrical signal. This works by means of a moving coil suspended in a magnetic field, or by utilising the change in capacitance of an electrically polarised capsule where one of the capacitor elements is a lightweight, conductive diaphragm. So-called capacitor mics tend to be expensive when compared to dynamic models, but they have very real advantages that justify their cost.

CAPACITOR MICROPHONES

Capacitor microphones are able to respond to very high audio frequencies, and they are usually much more sensitive than their dynamic counterparts. In other words, they require less amplification than dynamic models to produce the same output level from the same signal source, making them more suitable for quieter or distant sound sources. The reason capacitor microphones have such a good f

"The majority of variable-pattern microphones are built around a dual-diaphragm design..."
requency response is that their diaphragms can be made much thinner and lighter than those of dynamic models, as they don't have to drag the mass of a voice coil along with them. A typical capacitor mic diaphragm is just a few microns thick, often fabricated from mylar, with a thin gold coating to make it electrically conductive. This diaphragm forms one plate of a capacitor; the other plate is fixed parallel to the diaphragm with a small air gap between them. This fixed backplate is generally perforated to allow air to pass through, and there may also be some holes drilled only part-way through, to form the mechanical damping system required to compensate for the natural resonance of the diaphragm. As you can imagine, any diaphragm acts like a drum skin and has a natural resonant frequency.

The electrical capacitance of the capsule changes whenever variations in air pressure cause the distance between the diaphragm and backplate to change, and if a fixed electrical charge is placed across the capsule, the voltage on the diaphragm is modulated by the sound pressure to produce a small electrical signal. This small signal voltage is amplified by circuitry within the microphone, so the phantom power source required by this type of microphone actually performs two separate functions: it charges the capsule and it drives the preamp circuitry. Figure 1 shows a typical capacitor microphone block diagram.

 
PROXIMITY EFFECT
 
  All cardioid mics exhibit a proximity effect, which results in a rising bass response when the mic is used close to the sound source. The reason this happens is that cardioid mics also capture sound from the rear of the capsule, which is then delayed in a labyrinth and then added to the sound energy arriving on-axis. The phase shift introduced by the labyrinth causes sounds arriving from the rear to be largely cancelled out, but this only works if the same level of sound arrives at the front and rear of the mic. When the sound source is distant, this is nominally the case, but for very close sound sources, the inverse square law conspires to ensure that there's more sound level at the front of the mic than at the rear. This reduces the efficiency of the port in cancelling low frequencies, the outcome of which is that singing very close to the mic produces a significant amount of bass boost.

In theory, dual-element capacitor mics set to a cardioid response should demonstrate exactly the same proximity effects as a fixed cardioid mic, but in practice the characteristics vary from model to model, depending on the porting arrangement used. Some manufacturers have managed to keep the proximity effect fairly well under control, whereas some mics generate huge amounts of bass boost when used close up. If this become a problem, it sometimes helps to use the low-cut filter on the mixing console or mic, in combination with a little conventional EQ. For studio work, though, it's better to move further away from the microphone. If you work very close, in addition to the risk of popping, you'll find that any small change in position causes a large change in both the level and tonal characteristics of the mic. A strategically placed pop shield can stop singers getting too close.

 
The polar pattern of the microphone depends on the design of the backplate and the acoustic chamber behind it, so it is possible to build single-diaphragm capacitor mics to produce any of the available polar patterns. However, the only way to adjust the polar pattern of a single-diaphragm capsule is to mechanically change the acoustic system at the rear of the capsule, and this is extremely difficult to do properly. Instead, if switchable polar-pattern microphones are needed, it's generally better either to use interchangeable capsules or a specially designed dual-diaphragm capsule that can recreate all the polar patterns via simple electrical switching.

VARIABLE-PATTERN CAPSULES

The majority of variable-pattern microphones are built around a dual-diaphragm design where two diaphragms are fitted either side of a common backplate. Porting, via perforations in the backplate, is used to give each side of the capsule a cardioid response, so in essence the capsule is really a pair of back-to-back cardioid mics occupying virtually the same point in space. By varying the signal level of one of the capsules, and by switching its phase, it is possible to sum its output with that of the fixed cardioid side, to produce all the commonly available polar patterns, as shown in Figure 2. Note that the right-hand diaphragm is polarised by a fixed voltage taken from the right-hand end of the resistor ladder, but the left-hand diaphragm is connected via a switch that can be set to any position along the ladder, allowing it to be set either positive or negative with respect to the backplate (the centre point of the ladder is the reference point and connected to the backplate). Varying the diaphragm's voltage is a simple way of controlling its sensitivity, and reversing the voltage also reverses the phase of its output. When both capsules are polarised with the same voltage (position E), the outputs combine to form an omni response, which is the result of having opposing cardioids electrically in phase with each other -- but as you can see from the accompanying polar-pattern plots in Figure 2, switching the polarising voltage produces a whole series of patterns. In theory, it is possible to move from a figure-of-eight pattern at one extreme, via the various widths of cardioid pattern, to omni at the other extreme, but most microphones provide a limited number of switched steps. Simpler models, for example, may offer just omni, cardioid or figure-of-eight patterns, whereas more comprehensive models may also include hypercardioid and wide cardioid -- it's all down to how many switch positions are provided.

OUTPUT STAGE

The output from a capacitor microphone must be balanced if it is to be operated from phantom power, and with the exception of valve microphones and certain specialist models, modern capacitor mics are invariably phantom

"...switching the polarising voltage produces a whole series of patterns."
powered. With transformerless models, the phantom power is isolated by means of blocking capacitors and summing resistors, whereas a transformer can be made to provide phantom power from a centre tap on the secondary (output) winding. There is much debate over whether transformerless or transformer-output mics are best, and whilst it is certainly possible to get a better transient response by dispensing with transformers, the benign saturation characteristics of transformers are thought by many to result in a warmer, more natural sound. This is rather like the valve versus transistor argument all over again, so the only advice I can give is to listen and make up your own mind!

DIAPHRAGM SIZE

Large-diaphragm capsules are currently popular, especially for vocal work, again because of that enigmatic word 'warmth'. In theory, small-diaphragm mics are more accurate because their small geometry produces a more accurate off-axis response, and with the diaphragm mass being less, the high-frequency response may also be better. However, large-diaphragm mics tend to be used mainly on-axis in close-miked recording situations, and their frequency response can still extend to 20kHz or even more, so there's no real practical limitation in choosing a large-diaphragm model unless you want to work with distant sound sources where off-axis sound is likely to make a major contribution.

"...these frequency anomalies can enhance certain parts of the human speech spectrum in a subjectively pleasing way."

There are various reasons why large-diaphragm capsules might sound different to small-diaphragm models, but the main one is that the natural resonance of the capsule is likely to be lower. A combination of damping and acoustic resonators will generally be used to flatten out the overall response to some extent, but despite the similarity of the paper specifications, there's often a distinct subjective difference between large- and small-diameter capsule models. The more distinctive sounding large-diaphragm models often have frequency responses displaying noticeable bumps and troughs, and though this is technically undesirable, in artistic terms these frequency anomalies can enhance certain parts of the human speech spectrum in a subjectively pleasing way. Because of this disparity between paper specifications and subjective performance, it is is essential to try microphones in a studio environment before deciding on their suitability for a particular purpose.

WHICH PATTERN?

So much recording is done with cardioid pattern mics that it seems we sometimes forget the other patterns exist at all, but each has its strengths -- otherwise there'd be no point in spending the extra money on a switchable-pattern model. Cardioid mics have the advantage that they exclude a reasonable amount of off-axis sound, especially from the rear of the mic, but the elaborate acoustic porting required to create the pattern often affects the subjective sound of the microphone in a negative way, resulting in a sound some users may describe as nasal or honky. On a well-designed mic, these artifacts are minor, but even so, direct comparison with an otherwise similar omni-response mic will usually show the omni to have a more 'open', natural sound.

 
CONDENSATION
 
  Conventional capacitor mics are very high-impedance devices, so any condensation on the capsule will affect electrical performance quite noticeably. This can be a problem when taking a mic from a cool locker and then having somebody sing into it at close range. What tends to happen is that the sensitivity of the mic falls off and irregular background noises such as pops or crackles start to appear. Using a pop shield will help, but it's important to make sure the mics are at room temperature before use, and to ensure that the room isn't cold enough to permit condensation. In extreme cases, the mic may have to be withdrawn from service for a short while and placed somewhere warm to dry out. Note that RF capacitor mics, such as the Sennheiser MKH series, don't tend to suffer from this problem.  
The problem with omni mics is that they pick up sound from all directions, but that needn't be too much of a disadvantage, as moving them slightly nearer to the sound source may be all that's required to reduce the spill level to around the same as you'd expect from a cardioid. What's more, room reflections (and spill from other instruments), will be captured more accurately because of the more uniform off-axis response. Indeed, if you're working with something like a folk ensemble, where spill between instruments is likely to be significant even with cardioid mics, you might find you get better results by using omni mics, even though you may have to accept a higher level of spill.

When miking more distant sounds, such as choral groups or acoustic instrument ensembles, or even when setting up drum overheads, omni-pattern mics benefit from their better off-axis response and their generally less congested sound.

The figure-of-eight pattern is rarely used these days except as the side-firing part of an MS (Middle and Side) stereo mic array, but there are studio applications where the unique polar response of the figure-of-eight is a definite strength. The great thing about a figure-of-eight is that, in a fairly dead room, it's almost completely deaf to sounds coming in from the side. When you're miking a singer who also plays guitar, for example, you can use this to your advantage by pointing the live end of the mic at the singer and the dead side of the mic towards the guitar. This will only be partly successful, as the guitar isn't a point source of sound, and the room is unlikely to be completely non-reflective, but even so, this technique can help you claw back a few precious dBs of separation.

SUMMARY

If you have a typical home studio and only need a good mic to record vocals or acoustic guitar close up, a decent cardioid capacitor mic is a good option, as it combines quality performance with affordability -- with low-cost models such as the AKG C3000 and Rode NT1 doing the rounds, most people can afford at least one good mic. On the other hand, a multi-pattern mic will provide more flexibility, especially if you're not limiting yourself to close-miking vocals and guitars, and even though you may find it's set to cardioid most of the time, those extra patterns may help you out of the occasional tight spot.

 
FURTHER READING
 
  If you're not familiar with how microphones work and what the various polar patterns mean, have a look at the 'Choosing a Microphone' article which appeared in the June 1995 issue of SOS for a bit more help.  
Whether or not to buy a valve mic is a different proposition altogether, as valve models start at around £1000 and go up as high as you like. The good ones do have a distinctive sound, but don't be seduced until you've done a head-to-head with a good solid-state model - you may be surprised at how little difference there is, and in some extreme cases, the tube mic may sound distinctly muddy by comparison. Note that a warm sound doesn't mean a dull sound. A good tube mic tends to sound clear and open, with a confident low end, but it shouldn't sound woolly at the bass end or spitty at the top. As a rule, tube mics are slightly more noisy than their solid-state counterparts, but in a close-miking situation this shouldn't be significant. On a more practical note, you do have to use an external power supply with a valve mic, as phantom power can't supply either the necessary current or voltage for valve circuitry, but this is no big disadvantage in a studio environment. As a rule, valve models require a little more care and attention than their solid-state counterparts, due to the fragile nature of valves. Unlike FETs, valves also deteriorate with age and must occasionally be replaced.

I hope the information in this article will help you decide on the right mic for your own requirements - or, at the very least, help to explain the apparently high cost of switchable-pattern capacitor mics. Microphones last a long time if they're looked after -- they can go on for decades with minimal maintenance, so even though it might seem like a lot of money now, a quality mic is a good long-term investment.

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