Matching microphones with portable recorders
Choosing the right microphone for your recorder is crucial for obtaining reliable signals. However, the audio recording market is full of proprietary, idiosyncratic designs that significantly complicate the task of matching a recorder with a microphone. I am going to focus of three issues here: (1) sensitivity and impedance, (2) power requirements, and (3) the input/output (I/O) interface. You can find lots of useful information regarding these issues at the Microphone Data website.
Input/output sensitivity and impedance
Microphone output sensitivity
The output sensitivity value indicates the level of electrical signal that will be produced by a specific microphone in the presence of a standard acoustic stimulus. This measurement is normally specified as the number of millivolts generated by the microphone when subjected to a sound pressure of 1 Pascal (mV/Pa). Some manufacturers will also express output sensitivity in dB with respect to a reference voltage (Vref), according to the following formula:
Typically, Vref will refer to 1 V into a load of 1000 Ω (dBV), but some manufacturers will use the reference of 0.775 V into a load of 600 Ω (dBm). Note that the unit of dBu will be used instead of dBm when termination impedance is disregarded (see 2.5). If we assume that a typical lavalier condenser microphone has the sensitivity of 7.75 mV/Pa, then an acoustic stimulus of 1 Pa (or 94 dB SPL) will produce an electrical output of 7.75 mV (or -40 dBu), while a typical dynamic microphone (0.775 mV/Pa) will produce an output of 0.775 mV (or -60 dBu). Thus, one might offer a generalization that dynamic microphones are typically about 20 dB (or ten times) less sensitive than condenser microphones and will require higher gain to achieve comparable signal levels, though these numbers will vary depending on microphone design.
Output sensitivity is, therefore, a crucial specification, as it determines whether a particular microphone/preamplifier combination will produce a loud enough signal without adding excessive amounts of noise and distortion. Conversely, a high-sensitivity microphone may easily overload when coupled with a preamplifier incapable of handling sensitive microphones and high signal levels. To counter that, some preamplifiers are equipped with a 20 dB pad or signal attenuation to help deal with particularly "hot" or sensitive microphones. Figure 1 shows waveform and spectrogram images of a recording of the Polish word "czarna" with the Marantz PMD660 digital recorder with a high-sensitivity condenser microphone (Countryman B3; 10 mV/Pa) and a low-sensitivity dynamic microphone (Sennheiser HMD25-1; 1 mV/Pa). The PDM660 preamplifier is not able to handle either microphone type, thus causing significant clipping (left panel) or excessive noise (right panel).
Figure 1. Examples of signal degradation resulting from an incorrect use of a microphone preamplifier
The notion of output sensitivity is directly related to that of self-noise. Self-noise (a.k.a., inherent noise or noise floor) is noise generated by electrostatic activity of the device itself. Each electrically powered element of the recording chain produces some level of self-noise. In theory, it should be easy to simply compare the self-noise figures of the microphone and recorder and see if the recorder's noise floor is sufficiently lower than that of the microphone (the rule of thumb value is about 10 dB) so that it does not degrate the microphone's performance, especially at higher gain settings. However, not all manufacturers publish these figures, and even if they do, they are likely to use different nomenclature and standards. I have put together a set of simple guidelines that will help you determine and compare the self-noise figures of your microphone and recorder. You can find the article here. You can see the effects of increased self-noise due to incorrect microphone-recorder matching in Figure 1 (right panel).
Every microphone has an output impedance and every microphone preamplifier has an input impedance. These characteristics describe the electrical resistance to current flow out of the microphone circuitry and into the preamplifier. Microphone impedance is usually quite low (e.g., 200 Ω) to allow a reasonable current to flow and the amplifier impedance is made large enough (e.g., 2 kΩ, or 10 times larger) for that current to develop a reasonable voltage across its input. If one selects a microphone whose impedance is greater than that of the preamplifier, the microphone will not deliver its full signal level and, as a result, the audio signal will be noisy and low in level. As a quick fix, you can use an in-line impedance matching transformer, such as that in Figure 2. Every major microphone manufacturer (e.g., Shure A96F) should have one available.
Figure 2. An impedance-matching transformer to connect low-impedance dynamic microphones to portable recorders with an 1/8 (3.5 mm) stereo input
Microphone and recorder power matching
The bad news is that microphone power requirements are unnecessarily complicated because of the multitude of designs and standards available. The good news is that dynamic microphones do not require any power supply at all. So if you have a solid microphone pre-amplifier (e.g., the MixPre) that can drive your dynamic microphone, you don't need to be concerned with powering your microphone at all. So why would one even consider a condenser microphone? Well, the main reason for us linguists is that condenser microphones are more sensitive, can be more easily miniaturized, and so those few that are perfect for field speech research, happen to be condensers.
Self-powered condenser microphones
Your best (i.e., easiest) option is to buy a microphone that is self-powered. Such a microphone comes with a built-in battery option. The battery compartment is either built in the microphone shaft (e.g., Audio-Technica AT8010 with a 1.5 V AA battery), or comes with a battery pack (e.g., a belt back available with the Audio-Technica AT831b). The AT831b is one of my favorite microphones as it works with most field recorders, precisely because it has its own power supply. Another example is the inexpensive (but quite good, actually) Audio-Technica ATR3350 (a.k.a. Radio Shack 33-3003) which comes with a small, in-line power supply that uses the LR44 button-type battery (Figure 3).
Figure 3. Audio-Technica ATR3350 microphone, powered by a single LR44 battery, photo courtesy Audio-Technica
Many (if not most) professional condenser microphones require so-called "phantom power." Phantom power is a DC voltage (11-48 V) that supplies power to the preamplifier of a condenser microphone. Phantom power is available on most professional field microphone preamplifiers (e.g., Sound Devices MixPre,) and field recorder itself (e.g., Fostex FR-2LE). Phantom power requires a balanced circuit in which XLR pins 2 and 3 carry the same DC voltage relative to pin 1. A balanced dynamic microphone is not affected by phantom power, though phantom power should be turned off while using a dynamic microphone. There are three potentially complicating factors:
- some microphones require full 48 V, while others need 12 V or 15 V,
- some small, inexpensive recorders have inferior phantom power supplies, and
- phantom power supply can drain your batteries rather quickly.
Plug-in power (bias) is a DC voltage (1.5-9 V) that is provided on a single conductor (typically the same conductor as audio), and, unlike phantom power, plug-in power does not require a balanced connection. Plug-in power microphone inputs are commonly found on many consumer-grade audio and video recorders. If a dynamic microphone is connected to a plug-in power interface, the audio signal will degrade considerably. Special in-line adapters, such as Shure A96F, must be used to block the bias voltage from the microphone (see Figure 2 above). Phantom power and plug-in power are not interchangeable. Sony, Sharp, Edirol, Samson, and others have had a line of decent plug-in power devices (including DAT recorders, MiniDisc recorders, digital dictaphones, camcorders, and microphones) and you can still find some good stereo microphones, such as the ECM-MS907. Such microphones tend to be inexpensive and offer surprisingly pleasing sound. They are designed primarily for consumer use and I would not recommend them for critical speech research. Plug-in power devices typically use the stereo 1/8-inch interface (see below).
Your microphone output connector will have to match your recorder's (or pre-amplifiers') input connector. Simple as it may sound, the I/O interface matching is often a problem with field recorders. Most professional-grade devices use the balanced XRL interface, but this type of interface is often simply too large to fit on small recorders.
The balanced XLR interface is the industry standard for most professional microphones, pre-amplifiers, mixing consoles, and reorders. If possible, I would encourage you to use a microphone and recorder that both use the XLR interface.
Some smaller devices (e.g., M-Audio MicroTrack II) use a balanced 1/4-inch (TRS) interface, primarily to save space. The electrical properties of this type of input are very similar to the standard XLR interface. In addition, the 1/4-inch interface may perform double duty by switching between microphone and line-level signals. You should try to find a microphone cable with a balanced 1/4-inch male connector on one end (into the recorder) and a female XLR connector on the other (into the microphone).
Unbalanced 1/8-inch (3.5 mm) stereo
As I mentioned above, the 1/8-inch stereo interface is typically used with plug-in power devices. It is the most ubiquitous interface on consumer-grade recorders (e.g., Samson Zoom H2, Edirol R-09HR, and many small digital recorders). If you plan to use an external microphone with such a device, you need to use a compatible microphone for best results. One of my favorite such microphones of this type is the Audio-Technica AT822 stereo microphone, which has been specially designed for use with DAT recorders and camcorders.
Mono or stereo?
Most professional microphone interfaces have a separate input for each channel. You can use a mixer to route and pan such signals appropriately. Some two-channel recorders (e.g., Fostex FR-LE2) are able to automatically combine the separate signals into a stereo file, with one microphone in the left and the other in the right channel, without an option for panning.
Consumer-grade field recorders will typically have a stereo 1/8-inch input, so you should use a stereo microphone (e.g., Sony ECM-MS907) to be able to record in stereo. If you connect a mono microphone into such an interface (e.g., Audio-Technica ATR3350) you will end up with a stereo file with the signal in the left channel, and the right channel empty.
Because the business of matching microphones with field recorders is rather messy, let us look at a couple of scenarios to help you with the process.
Bigger is better
If you want to use a high-end solution, your job is simple because most high-end devices use the industry standard XLR interface, levels and impedance, phantom power, etc.
If you want to use a dynamic microphone (e.g., Sennheiser HMD25-1) all you need to worry about is a pre-amplifier (either stand-alone or built-in) that will offer high gain and low noise. Easier said than done! Still, you have some options, such as the Sound Devices 702, Sound Devices MixPre, Sound Devices USBPre, and even the Fostex FR-2LE.
If you want to use a condenser microphone, you can use exactly the same devices mentioned in the preceding paragraph. For example, my Beyerdynamic Opus 55 Mk II works admirably with all of them.
Small is beautiful
Sometimes, it is necessary to use a very small, unobtrusive setup. However, as mentioned above, small recorders are often not equipped with professional features. Say you wan to match the Shure Beta 53 condenser microphone (an excellent choice for speech research) and a small all-in-one recorder. Your choices are very limited because the Beta 53 requires a decent phantom power supply and a clean pre-amplifier to achieve optimal results. Fortunately, Shure supplies battery packs (or in-line pre-amps) for many of their condenser microphones (including the Beta 53). You can use either the MX1BP belt pack or the RPM626 in-line pre-amp (Figure 4). The MX1BP is an extremely useful tool, as it provides a phantom power supply (with a 9 V battery) and a low-cut filter in a compact and lightweight belt pack. Shure is likely to discontinue it in the near future, but I would encourage you to try to find it either new or used. The RPM626 is an in-line pre-amplifier, but it still requires 11-52 volts of phantom power for operation. It does, however, make the Shure Beta 53 compatible with a variety of phantom power supplies, including those found on portable digital recorders.
Figure 4. An in-line microphone pre-amplifier (e.g., Shure RPM626, Beyerdynamic CV 18, etc.), which makes miniature condenser microphones, such as Beta 53 compatible with standard phantom power supplies.
(So the power problem is now solved. But what about the recorder? In this case, I would recommend a recorder that has balanced microphone inputs (e.g., 1/4-inch). One such recorder is the M-Audio MicroTrack II, which can be paired with the Shure Beta 53 well. I wish it had a better (cleaner) gain, but it should do just fine when the smallest possible setup is required. Note that the MicroTrack II has on-board phantom power supply, but you need to keep in mind that using it will reduce your battery life considerably.
There are many low-end digital recorders available. Too many to list here. You are likely to have to use a stereo 1/8-inch plug-in power interface and a weak, noisy preamplifier. Not all is lost, however. If you want to record in stereo, the Audio-Technica AT822 is a great microphone, but it is not exactly low-end or cheap. If you don't care about stereo, you can use the Audio-Technica ATR3350 microphone (see Figure 3 above) or, if you do, add a signal splitting adapter (see Figure 4 below) and plug two ATR3350 microphones in it It is not ideal, but it will work. Also, check out my post on how to convert a cheap lavalier into a headset.
Figure 4. A stereo 1/8-imch adapter to 1/8-inch female inputs