If you think polar patterns have to do with
the migratory habits of large, white bears,
you’d better read this!
Vocal quality, whether it’s on your platinum album or your first demo, is one of the most important things in any recording. While there are many factors involved in getting the best possible vocals on tape (see my article ‘Better Vocals’ Sept. 1991), your microphone is the first link in the hardware chain. As such, it must be chosen carefully.
Selecting the right mic can be difficult for the home studio buff on a tight budget. More often than not, it’s the last item purchased; people assume that any mic will do. Not so! Take the time to research and purchase the best that you can afford with the features and sound quality you need. Begin with a knowledge of microphone terminology, then use your ears and budget to determine your purchase. The process begins…
The three most common mic types are dynamic, ribbon and condenser. Dynamic mics (also known as moving coil and pressure-gradient) are very much like a speaker. A coil of wire is attached to the back of the diaphragm, which is positioned over a magnet. When sound pressure hits the diaphragm it vibrates, and the coil moves back and forth within the magnetic field, causing a small electrical current. Dynamic mics are typically used on stage and in live performance because they are extremely rugged and handle environmental factors well. Many of them tend to boost the middle tones, creating a solid sound that improves the tone of some singers. But even though technology has helped refine their sonic characteristics, dynamics usually lack the sensitivity and high-end response of ribbon and condenser mics.
Ribbon mics are similar to dynamics in their use of magnetic principles, but instead of a coil, they employ a thin, metal ribbon clamped at both ends between two magnets. The ribbon produces an output so low that it requires an on-board transformer to boost it to a usable level. Ribbon mics are the most fragile, which means they are best kept in the recording studio. They are noted for being very quiet and having good transient and high-frequency response.
Condenser mics don’t use magnetic principles. Instead, they use capacitors. There is a fixed rear plate, usually made of ceramic, and a moveable front plate (the diaphragm), made of a thin piece of coated mylar. The thin metal coating (gold, or more often silver) makes the diaphragm conductive.
The plates are separated by a small volume of air. As the diaphragm vibrates, the distance between it and the rear plate changes, causing a commensurate change in the electrical charge. Condensers also require a built-in preamplifier to boost their signal. Since their diaphragms are generally less massive than the ones found on dynamic mics, they respond to incident sounds more quickly. They also tend to have excellent high-end response, lending a brighter, more transparent sound. There are also electret condenser mics. Unlike standard condensers, they have a permanent charge, and therefore don’t require a polarizing voltage.
Condenser mics require power for their built-in preamps. This comes from a battery or from a so-called phantom power supply. If you have a choice between the two, use the power supply – it supplies a more constant voltage (usually 48V). Phantom power is built into many mixers, and separate units are available.
The polar pattern (more commonly known as ‘pickup pattern’) describes the way the mic picks up and rejects sounds from different directions. The standard patterns are cardioid, supercardioid (also known as ‘hypercardioid’), bidirectional, (‘figure-8’), and omnidirectional. Most of the more expensive condenser mics feature switchable pickup patterns, so you can choose the best one for the job. When reading polar patterns on spec sheets (see Fig.1), note that 0° refers to the front of the mic, 90° to the sides, and 180° to the back. Most charts plot the microphone’s sensitivity at several different frequency ranges.
Cardioid mics are named so for their heart-shaped pickup patterns. Being very directional, they are most sensitive to on-axis sound – ie., sounds directly in front of them. Because they reject most off-axis sounds, a cardiod might make an excellent choice for recording solo vocals, especially where there isn’t an isolation booth available (which is the case with many home studios.) If room noise or feedback causes problems and you need still better off-axis rejection than the cardioid offers, there are even more directional supercardioid mics available.
Bidirectional or figure-8 mics are equally sensitive at the front and the rear of the mic, rejecting sounds from the sides. They are ideal for recording two voices at the same time, especially if you are after a natural blending of the two. However, the mic records both people continually, so if one singer is laying out for verse, he/she must be quiet! In addition, both voices are stuck together on one track – they can’t be treated individually. If this becomes a problem, your best bet is to use two cardioid mics.
But even with a solo vocal, cardioid mics can pick up unwanted low frequencies from the sides. In this case, you might consider using a figure-8. This avoids the problem, while capturing some of the room sound from the back.
As the name implies, omnidirectional mics are equally sensitive to sounds from all directions. Omnis are well-suited for recording a group of singers, but they too can also be used with soloists to pick up room ambience. Again, you’re stuck with the ambience once it’s recorded!
A microphone’s frequency response curve describes its characteristic sound to a large degree (see Fig.2a). One would thing that the ideal mic would have a perfectly flat response throughout the 20Hz to 20kHz range of human hearing. Actually, some peaks and dips can be advantageous in particular applications. For example, a peak in the 5-10kHz range means brighter overtones or “sizzle.” Although response below 100Hz is useful for drums and low frequency instruments, most vocalists don’t produce sounds that low. A bass roll-off in the 50-100Hz area is usually preferable to rumble. Many pro mics offer a bass roll-off switch for that exact reason.
The proximity effect is the increase in low frequencies as the vocalist moves closer to the mic. An experienced singer usually knows to move closer to the mic at lower dynamic levels and vice versa. He or she also knows how to use the proximity effect to advantage. It can add body to a singer’s sound, and it can be used as a special effect. In the hands of a less experienced vocalist, proximity makes things sound “boofy” and destroys the intelligibility of lyrics.
Sometimes manufacturers will show separate frequency response curves for different proximities (see Fig.2b). By the same token, mics are subject to ‘off-axis coloration’. This refers to a change in tone and frequency response as you move off-axis with a directional mic. Even though the sensitivity to sound pressure may drop as you move away from the front of the mic, the frequency response should remain as flat as possible. The reason is that you don’t want the vocal sound to suffer as the singer rocks back and forth.
A microphone’s transient response is its ability to handle quick attacks and signal peaks. Condenser and ribbon mics tend to have better response than dynamics due to their lower mass diaphragms. Usually, the smaller the mic is, the smaller (and lower mass) its diaphragm is, so smaller mics tend to have better transient response. Normally, vocals don’t have very sharp transients (apart from anything else, they are usually run through a compressor that minimizes them), but if you are going to get double duty out of your mic – ie., if you’re also planning on using it for instruments like piano and other percussion – transient response is something to consider.
The sensitivity or output level measurements shows the amount of signal (or electrical output) you get at a specific input sound pressure level (SPL). The higher the output level, the less pre-amplification is required. This results in a lower noise floor.
Overload causes distortion at any point in the audio chain. Often it is not the microphone that is distorting. However, the ability to withstand SPLs somewhere in the vicinity of 130-140dB without breaking up is highly desirable. But, sometimes when a condenser mic distorts, it may just need new batteries (or a higher phantom voltage, if appropriate.)
Try Before You Buy…
Now that you have a better idea what to look and listen for, it’s time to actually test some mics before committing to one. The best way is to borrow a mic and try it out under your recording conditions. If you have a friend with the model you’re interested in, or if you know someone at a local music store who will loan it out at for moderate fee, put it to the test with your gear. If not, go to your preferred music store and set up the mics you’re considering side-by-side and run your tests. To hear what’s commonly used in pro studios, buy some studio time with an engineer during off-peak hours, when rates are usually discounted. Again, set up mics and compare. Use the engineer’s ears and experience to guide you.
Testing 1, 2 …
When testing, defeat all EQ and don’t go through any effects or reverb. This way you will be listening only to the microphone. The time to try running through your favorite effects units is after you have narrowed the field or made a decision.
People assume that any mic will do. Not so! Every microphone has it’s own characteristics, and my choice might not be yours.
Bear in mind that the most important consideration should be how the mic records vocals on tape. Listening through a P.A. system will not give you a true picture. Make a recording with each mic and listen to the tape results carefully, making notes and comments. Sing through your entire range at various distances from the mic, and be sure to include all styles, volumes, and tone qualities from the least to the most extreme. This is the only true way to know how the mic will respond. Remember that some things can be compensated for with additional processing, but why not get the best “raw” sound that you can? Imagine the final result when you do unleash that extra processing gear!
I can not stress enough how important it is to match the right mic to the right application. All of the above specs and considerations should help you understand why a mic responds the way it does, and why you do or don’t want to use it on any particular project. Use the specs, features and price comparisons to narrow down your initial choices, and then leave it up to your ears! Let out a few “wails” to test that sensitivity and distortion, “eat” the mic for a good show of the proximity effect and check for a good high-end “sizzle.”
Remember, the microphone is an extension of your voice – it has a high impact on your sound. Every microphone has it’s own sound characteristics, and my choice may not be yours. But don’t ignore the importance of the choice you make… It could be the difference between your first demo and that platinum album!