Updated July 2024 for clarity and to remove some anachronisms.
In part one of this series I discussed portable audio recorders and the limitations of using their built-in microphones. This article will explain which microphone types are available and how to power them.
I will avoid excessive technical detail, instead emphasising practical concerns.
Types of microphones
A microphone is a device that converts a sound wave to an electrical signal, a process called transduction. There are quite a few ways of achieving this magical task and we can use these to define microphone types. Here are four of the most common.
Dynamic mics work on the simple principle of electromagnetic induction. This makes them inexpensive and easy to manufacture. They are robust, resist shock and water damage, and remain dependable over time. They can accept high sound pressures without a problem. For these reasons dynamic mics are popular for concerts or other contexts where the audio engineer has limited control over environmental factors.
Condenser is another word for capacitor, which defines the key component of the second type of microphone. Because a capacitor requires a charge to function, condenser mics (unlike dynamic) need to be powered. These mics vary from inexpensive to costly, but have sonic benefits compared with dynamic mics. For one, they have greater efficiency, so they produce a louder sound from a quieter signal. This translates into less noise. They also have a more nuanced sound, with greater fidelity to the source signal. On the down-side they are more prone to handling noise, shock, and damage. For this reason they often find their home in the recording studio, where the environment can be carefully controlled. Here their timbral superiority is likely to be a decisive advantage.
An electret microphone (sometimes called a pre-polarised condenser) incorporates a permanently charged material, so that it requires less external power than other condensers. Such mics are easy to make in quantity and hence are inexpensive, with the additional bonus of miniaturisation. The microphones in your computer, mobile phone, and other devices are all electrets. This is not to say that an electret must be cheap and nasty. The most expensive electrets can rival polarized condensers in quality.
In order to distinguish those condensers that are not electrets, the term DC-biased condenser is sometimes used.
A piezoelectric microphone is generally found in a contact mic, which you can easily make out of cheap parts from an electronics supplier. Such mics work well as triggers, since they are very sensitive to handling noise. They are also used as hydrophones (mics for underwater recording) since they can resist high pressures. But it's difficult to get a full frequency range out of them, due to their high impedance.
Other types of microphones (carbon, ribbon, etc.) have even more specialised usage, and you are unlikely to find them used on location.
One specialised configuration worth mentioning is the boundary microphone, which is comprised of a condenser and a flat panel situated in such a way that the panel acts as an amplifier, extending the reach of the mic out into the horizontal plane. These are useful when you wish to inconspicuously record from a flat surface. Examples include conference room tables, on stage during theatrical presentations, or at sporting events for capturing decisive events (e.g. on the back panel of a basketball hoop).
Pick-up patterns
Microphones are not equally sensitive to sounds from all directions. A polar plot imagines the microphone capsule at the centre and shows the volume attenuation at different angles. These can be used to define several key polar patterns.
An omnidirectional (or "omni") mic picks up sounds equally from all directions, at least at low frequencies. At higher frequencies the size of the microphone itself becomes a factor, and sounds from the axis of the body will be diminished. Hence it is important to point the microphone at your sound source, even when using an omni!
Omni mics are simple to design and generally have less self-noise than other patterns. Omni condensers are less sensitive to handling noise than other condensers. Furthermore, they tend to have greater frequency fidelity. These are all reasons to prefer omnis, but sometimes other patterns are simply more practical.
A figure eight (or bidirectional) microphone transduces sound pressure equally from front and back, but with a dead zone (node) in the middle. These are particularly useful in mid-side recording (an advanced topic that I won't cover in this series).
A cardioid mic gathers sounds from one side only, or, more precisely, in a pattern that looks rather like a heart (hence the name). These are useful on stage, where it is important to reject "bleed", that is, sounds from sources other than the desired subject. In the case of amplified gigs, cardioids also reduce the risk of feedback from on-stage monitors.
A hypercardioid has less pick-up at the sides but more from the rear. A supercardioid is slightly more directional again. A shotgun has by far the most directional pattern, but even here there are different degrees ("short", "long", etc.). These descriptive terms don't tell the whole story, so it's best to examine the published specification sheet to see the actual polar pattern.
Power requirements
A practical concern for field recording is how to power a microphone. Power requirements constrain what cabling and connectors are appropriate, restricting which devices can be linked together. Digital recorders might restrict your options. This is why it's important to know the microphones you wish to use before choosing a recorder.
These various interactions can be summarised under four headings.
No power
You will recall that dynamic microphones do not require power. So you can use any recorder and any compatible audio cable. This could be thin 3.5mm (eighth-inch) cabling used on consumer and semi-pro gear (AKA minijack) or the thicker quarter-inch instrument cable (jack). For each audio channel only a single strand of conductive wire is needed.
Phantom power
This is a formal specification that calls the delivery of 48V over two lines of a cable. Since we also need a line for the audio signal, a three-line cable terminated in XLR connectors has become the standard microphone cable. You can easily recognise the three pins or three sockets of XLR (picture below). Because the power is balanced over the two lines, it is not visible to microphones that do not need it... hence it is "phantom". This is a happy situation because it means that dynamic mics can easily use the same XLR cabling with no worries.
Unfortunately, not all devices that claim to supply 48V actually do so. Manufacturer save on costs by delivering less voltage. This might especially be the case with portable recorders running from batteries. It's important to determine exactly what the recorder delivers, though this information can be hard to find. A detailed technical review is the best source, since manufacturers rarely own up!
You also need to know exactly what your microphone requires. One mic might happily run off 10V while another needs the full 48V. Furthermore, a mic that prefers 48V might work with 10V but with increased self-noise, greater distortion, or lower sensitivity. You might well be using an expensive microphone but only reaping the audio quality of a lesser model.
The current must also be specified. This is where things get tricky. The original phantom specification called for only 2 mA, but the latest spec calls for 10 mA. An older device might deliver a trickle of current compared to what some contemporary mics require.
Many excellent mics are optimised for studio usage, where power delivery can be controlled. These models might be inappropriate for field recording.
Plug-In Power
Electrets require much less power than other condensers. This has been standardised as Plug-In Power (PIP) which is nominally 5 V and less than 1 mA. PIP can be delivered over a simple two line 3.5mm cable with tip-ring connectors (minijack). Since the cable is unbalanced it should only be used for relatively short runs, to prevent the signal from degrading.
Beware that PIP is sometimes wrongly described as phantom power. But there is a big difference in the microphones each method supports, not to mention the cabling that is required. If the device does not have XLR connectors then it can not supply phantom power. (Though it could well have XLR connectors and still not provide proper phantom.)
PIP is advantageous when you want a compact and inexpensive recording setup. It permits longer recording times since battery drain is far less.
Battery Power
Some condensers can be run off batteries in the microphones themselves. This completely frees them from the power capabilities of the recorder. The battery type and life will vary.
An example is the Sennheiser ME66 pictured at the top of this article. This is a modular microphone capsule system that must be paired with a K6 power supply in order to make up a functioning microphone. This configuration allows you to buy multiple capsules, each with different polar patterns, and switch them as required.
Since it is an electret, Sennheiser has cleverly designed the ME66 to work using either phantom power (12V or more), or a common 1.5V AA battery. A switch turns the battery on, and momentarily lights an LED to reassure us that the battery still has juice.
The microphone terminates in XLR, so that it can be connected to a phantom cable. In addition, an XLR to 3.5 mm adapter cable allows the mic to connect directly to a consumer recorder. You might wonder why Sennheiser didn't forgo the battery and simply design the mic to run off PIP. The answer is that the lower PIP current would not support this microphone's excellent specifications -- which are as good as most DC-biased shotguns.
According to user reports, running on battery power increases microphone noise above 8 kHz, but I consider that a fair trade-off for this sort of flexibility. I don't do a lot of recording that requires a directional mic, but when I do, it's nice to be able to run the ME66 from a pocket-sized recorder.
Conclusion
If you own a digital recorder with phantom power you can use any microphones you wish, providing the recorder conforms to the full specification of 48 V and 10 mA. If it does not, you will be limited in which DC-biased condenser microphones you can use. Do your research to avoid disappointment!
You can also buy an adapter to lower the phantom voltage to the PIP standard while converting XLR to minijack. In this way you can use electrets with a phantom power socket.
If your smaller recorder does not have phantom power, it should instead support PIP. You will be able to use dynamics, electrets, or condensers with their own battery power.
The third article in this series will help you choose the correct microphone for the task.
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