Microphone Selection and Placement

July 22, 2009 11:14:11 AM PDT
Various Microphones

This month we’re shifting gears a bit and adding a new column that offers tips, tricks, and information for new users, home recording hobbyists, and prospective project studio owners. Perhaps you already have a nice arsenal of equipment at your disposal, or you just installed new recording software that you can’t wait to test drive. Maybe you just took advantage of the Promotions we’re currently offering. Whichever the case, capturing the best source material will always help make the mixing and editing process easier, more enjoyable, and most importantly, sound the way you want. With this in mind, let’s start with something near and dear to every engineer: microphones.

Selecting a Microphone

Because the microphone is the piece of gear closest to the source, it stands to reason that selecting the right tool for the job can make all the difference. There are two basic types of microphones to consider when choosing your weapon.

1) Dynamic Microphone – If you attend a live music performance, or ask a stranger to draw a microphone, a dynamic mic is most likely what you would see. The advantages of dynamic microphones are their relatively low cost, high durability, and lack of need for a power supply. The main element commonly consists of a plastic-film diaphragm connected to a coil of wire that is suspended in a permanent magnetic field. Once sound hits the diaphragm, the whole assembly moves to create a small electrical current. This inefficient design leads to a lack of high frequency detail. With quieter sources, these mics will require a lot of amplification, which can add noise to your signal.

NOTE: Dynamic mics are useful in the studio for recording sources with high SPL (Sound Pressure Level), like bass/guitar/keyboard amps and kick drums that aren’t producing a lot of high frequency detail. They are a wise choice when recording things within drumstick striking range.

Another type of dynamic microphone is called a ribbon microphone. Unlike traditional dynamic mics, these use a thin metal ribbon suspended in a magnetic field instead of a voice coil. This gives ribbon mics a smoother, more detailed sound, but adds significantly to the cost and fragility.

2) Condenser Microphone – You know that studio vocal that gives you chicken skin when you hear it, and you can almost feel the vocalist’s breath across your ear? Yeah, that was probably done with a condenser mic, whose main element consists of two small conducting plates. One is fixed, and the other is a moving diaphragm—commonly a lightweight, gold-coated plastic film. As sound hits the diaphragm causing it to vibrate, the space between the two plates changes, which varies the capacitance. When a fixed electrical charge is sent to the capacitor, these vibrations in the diaphragm can be accurately reproduced as an electrical signal. This system is much more efficient than that of dynamic microphones, resulting in a broader frequency response (near or beyond that of the human ear). Condenser mics also boast a faster transient response (reaction to sudden changes in level) than dynamic mics. One drawback of this type of microphone is they rely on a phantom power supply for operation. If your preamp, mixer, or interface doesn’t provide it (all Universal Audio preamps and channel strips do provide it), you will need an external phantom power source. The physical size of the diaphragm in a condenser mic will also dictate its tonal qualities.

NOTE: Small-diaphragm condenser mics provide a tighter, more focused pick up, which works well for high accuracy applications such as the 12th fret neck position of an acoustic guitar, or as a pair for stereo microphone setups and drum overheads. Large-diaphragm condenser mics are most often used for vocal work, capturing the body character of an acoustic guitar, or for single miking a variety of instruments. They are known for their warm, “flattering” sound.

Another type of condenser microphone is the electret condenser. These mics employ a permanently charged material fixed to the back plate instead of the diaphragm. This allows for a thinner diaphragm than a dynamic mic, and the ability to be powered by either internal batteries or phantom power. Electrets have improved greatly over the years and are close to matching the quality of some of their true condenser cousins.

Microphone Pick-up Patterns

Microphone Pick-Up Pattern

As this column evolves, we will cover in greater detail a few specialized miking techniques using many of the different patterns available (for example, using the figure-8 pattern in M/S Mid-Side technique). For now, we will focus on the three patterns most commonly used.

Cardioid – Aptly named for its heart shape, this is the most commonly used pattern in the studio. It provides good rejection from the back of the microphone while capturing the sweet spot "on-axis" sound from the front, and the "off-axis" sounds coming from the sides. Great all-around pattern for use in the studio.

Hypercardioid – More useful in live situations, hypercardioids are best used when bleed from surrounding sources and acoustic feedback need to be carefully avoided. The downside in studio use is that small movements by the performer can lead to audible changes in level.

Omnidirectional – As its name implies, omnidirectional mics pick up sound waves approaching the microphone from ALL directions. While this is the most basic pattern of the three, and the “purest” sound, it is the least commonly used in the studio. Consider this pattern when capturing choral groups choirs, and the like, or when natural room ambience is the goal.

Microphone Placement

The final topics of discussion in this introductory "Studio Basics" is are extremely important ones. Now that you have narrowed down the selection, and your microphone is ready for action, consider the following:

Proximity Effect – As you move a microphone closer to the source, bass frequencies increase as distance decreases. This is least apparent with omnidirectional mics, but must be considered carefully with directional mics (particularly directional condenser mics). Make sure you place a directional mic far enough away from the source to avoid “howling” or excessive bass response.

Mic-Stand Noise and Floor Rumble – Any vibrations traveling through a mic stand will transfer through the microphone and into your signal path. Keep cables attached or wrapped to prevent them rattling against the mic stand. Position your mic stand so it won’t transfer vibrations traveling through the floor (foot stomps, kick drums, bass amps, etc.). Due to their increased sensitivity, use shock mounts when recording with condenser mics. Low cut can be useful for these situations as well.

Phase Issues – This is a topic that will be covered more closely in following editions, but here is an important tip: When using multiple mics on the same source (acoustic guitars, drum kits), acknowledge the Rule of 3:1. Roughly translated, if Microphone A is 7 inches from the source, then Microphone B should be at least 3 times that distance, or 21 inches, away from Microphone A. This will help prevent phase issues created by the time delay between the microphones. If you’re using one of our UAD DSP Accelerator Cards in your studio, check out the Little Labs IBP plug-in. It’s great for all phase alignment needs.

That’s all for the first edition of "Studio Basics." Stay tuned for future columns, where we’ll dive in much deeper and provide more hints and information along with audio/video examples and images for you to use. Now that you know the basics, have fun breaking the rules.

— Mason Hicks