Understanding Microphones

Microphones can generally be classified according to their transducer type, diaphragm size, polar patterns and preamp circuitry.

Transducer Types

A transducer is a device that converts energy from one type to another. In microphones, this transducer is known as a “capsule” and is responsible for turning sound waves into an electrical signal.

Microphones can employ a number of different types of capsules. The most commonly used capsules for recording applications are dynamic, condenser and ribbon.

Dynamic microphones are typically robust, inexpensive and less sensitive to ambient sound, making them ideal for use in settings where they may be subject to intense handling and/or damage or where rejection of nearby sound sources is important.

Condenser microphones are the most commonly used type for recording applications due to their sensitivity to sound. This sensitivity is created by charging the condenser element with an electrical current either via a battery or phantom power. 48 volts of phantom power is typically available on a mixer’s microphone preamps as well as most standalone microphone preamps.

Ribbon microphones are the most fragile of the three capsule types mentioned here due to the suspension of a very thin ribbon element within the capsule. The delicacy of this design makes ribbon microphones exceptionally well suited for picking up the nuances of intricate sound sources. Recent technological advancements have helped to improve the sturdiness of the ribbon design significantly and it is now much lauded by contemporary engineers for recording guitar amplifiers and other high sound pressure level (SPL) sources.

Diaphragm Size

The diaphragm is the component of the microphone capsule that vibrates in response to sound waves. Diaphragm material, design, thickness and diameter—or size—all help to determine a microphone’s frequency, transient and polar responsiveness.

Diaphragms can be categorized into three sizes—large, medium and small. Larger diaphragm microphones are typically more sensitive due to their increased surface area, but also have a more limited frequency response since sound waves have to move more mass.

Small diaphragm microphones are capable of handling higher sound pressure levels due to their stiffer diaphragms. They also have an increased frequency response, particularly in the higher end of the frequency spectrum. Their decreased sensitivity relative to large diaphragm microphones makes them less susceptible to proximity effect and ambient noise due to their directional characteristics.

Preamp Circuitry

Sound that has been converted into an electric signal by the microphone capsule is of a very low level and requires pre-amplification before it is again amplified by a microphone preamp. The design of this preamp circuitry is one of the defining characteristics of any microphone.

There are two basic types of microphone preamps, field effect transistors (FETs) and vacuum tubes. As solid state devices, FET circuits are typically rugged, efficient and low noise, resulting in clean, responsive and predictable performance. These qualities make FET microphones versatile for a variety of recording applications.

Vacuum tubes, while less efficient, are popular for their ability to produce harmonic content as a result of saturation of the tube circuit, which results in a “warm” sound characterized by fullness and depth. This attribute makes them a favorite for recording vocals. Tubes perform at their best after they have been in use for an hour or more and the tube has had sufficient time to warm up.

All microphones should be handled with care, but tube microphones are particularly fragile and extra care should be exercised in setup, use and storage. Tubes also wear out with time and need to be replaced. Fortunately, this can generally be done by the user.

Polar Patterns

A polar or “pickup” pattern is the shape of the area that a given microphone is able to “pick up” sound. A microphone capsule’s polar response area is different across all frequencies.

Most microphones are single pattern, in that they will pick up sound from one area. Some microphones are multi-pattern, in that the user can select the polar pattern that is most useful for a particular application. The most common polar patterns are cardioids, supercardioid, hypercardioid, omnidirectional and bi-directional or “Figure 8”.

The cardioid polar pattern is the most common because it is effective at capturing sound only in front of the diaphragm, reducing unwanted external reflections, leakage or “bleeding” from other sound sources such as other vocalists, instruments or equipment. In general, directional polar patters are used to reject unwanted ambient sound, and less directional patters are used to capture more ambient sound.

Professional recording studios often employ isolation booths to eliminate this problem, but awareness of your surroundings and patience, forethought and experimentation in microphone positioning can still yield excellent results.

To determine which microphone/polar pattern to use, it is important to have an understanding of your recording environment and its inherent and adaptable acoustic properties.

Room Acoustics

The orientation of the microphone relevant to the acoustic space is an important variable in microphone positioning. “Room sound” can add a unique and desirable character to recorded tracks, particularly when acoustic elements such as absorption, diffusion and reflection are accounted for in the architectural design. This is the case in many recording studios, houses of worship, concert halls and performing arts centers.

However, not all engineers have the luxury of such recording environments and must adjust their microphone positioning techniques accordingly. It is nevertheless possible to minimize the effect of less optimal environments through a variety of methods. One particular variable to consider is unwanted reflections. Here are some suggestions to minimize its effect:

  • Move the sound source (e.g. a vocalist) back from hard surfaces in order to allow strong sonic reflections to fall off in intensity before reaching the microphone
  • Use acoustic baffles to help isolate the vocalist
  • Use a mic-stand mountable reflection filter
  • Install acoustic treatment materials
  • Reduce the distance between the sound source and the microphone

This last suggestion above—reducing the distance between the vocalist and the microphone—can help to increase the ratio of direct sound to indirect sound, i.e. sound that has reflected off of another surface before arriving at the microphone, as well as increasing the signal to noise ratio—minimizing the amount of unwanted sound by maximizing the volume of the sound you want to record.

Acoustics is a complex science worthy of a series of articles. The important thing to recognize is that sound travels in all directions and is absorbed, diffused and reflected by every surface to varying degrees. Simple awareness of this phenomenon will help you manage the impact of your environment on your recordings.

For many engineers and recording artists, using microphones is the most interesting and rewarding part of the recording process. Microphones can be seen as instruments in themselves due to their widely varied designs, applications and their undeniable influence on the final sound of any recording. Knowing a little about why microphones are designed the way they are and how they have been used to capture some of your favorite music will give you valuable insight into using them most effectively. Microphones are also electronics. As transducers, they convert acoustic energy into electrical energy and are therefore subject to the same electrical properties that govern other electronics. A basic understanding of electricity can go a long way in helping you to get the most from your microphones and overcome the challenges that can be associated with acoustics and the transfer and processing of electrical energy.

Using microphones requires a unique blend of art and science to achieve the best results. Fortunately, generations of audio engineers have created a bank of useful knowledge upon which to draw. This information is a great place to start as you learn to develop your own style and technique for using microphones. Just remember that the some of the most influential techniques in use today were a result of continuous experimentation. Don’t be afraid to try something new. Like the engineers that came before you, you may discover a technique that influences the art and science of recording for generations to come!

Bob Rock, musician, engineer and producer for such legends as Metallica, Motley Crue, David Lee Roth, Bon Jovi, Michael Buble, Bush, The Cult, The Offspring, and 311. Music Hall of Fame inductees and Juno winner.

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