Microphones represent the interface between the acoustical energy in the air that surrounds us to the audio systems that we use to amplify, record, or otherwise manipulate sound. There are many different types of microphones designed for many different applications, though they all do the same basic task of converting the energy in sound waves into an electrical representation of these patterns. Though there are a number of different types of microphones, we will limit our discussion to the two main categories of dynamic and condenser types.
By far the most common type of microphone used today is the dynamic. The dynamic microphone shares the main structures of a loudspeaker in that it has a moving element or diaphragm, a voice coil, a permanent magnet, and usually works in a resonant chamber. Impinging sound waves move the element and the attached voice coil within the field of the permanent magnet. This back and forth motion creates an alternating-current signal to be produced in the voice coil, in frequencies mimicking those of the sound waves that produced them. This very small signal is then transferred to the input of a highly sensitive amplifier intended specifically for this application.
Condenser microphones are most often found in recording studios, but are still used in certain sound reinforcement applications. "Condenser" is a somewhat archaic word that has been mostly replaced by "capacitor". Condenser microphones use a capacitive element to create the electrical equivalent of the impinging sound waves. A capacitor consists of two conductive "plates" separated by an insulative material, which in this case is air. Condenser microphones require a potential difference (voltage) between the plates to function. This is a DC voltage commonly ranging from +9 volts up to +48 volts depending upon the design of the mic. This power supply is common in mixing consoles and is referred to as "phantom power". It must be highly regulated and as completely free of noise as possible. Stand alone phantom power supplies are available so a condenser mic can be used with a device that does not have phantom power.
One plate of the condenser element is able to move. The impinging sound waves cause the moveable plate to vibrate. Due to its opposite charge as compared to the other plate, it causes electrons to be forced on and off of itself and the other plate as it moves back and forth. This creates a very small AC current flow that is the electrical representation of the impinging sound waves. Coupling capacitors are used to filter the audio signal from the DC phantom power voltage.
Condenser mics are most commonly used in radio broadcast booths and recording studios. They can offer extreme sensitivity and clarity, with some even working with a built-in preamplifier section. Some modern condensers have dual pickups, and offer several selectable pick up patterns and filters to accommodate various recording or broadcasting needs.
Low impedance microphones are the most commonly used mics in the professional sound industry. They are immediately identified by the three pin XLR connector in the base of the mic, and are used with a "balanced" line input. In a balanced line, the audio signal is carried on two conductors. Being of low impedance, there is almost no practical limitation on the length of cable run between mic and input device. Using literally hundreds of feet of mic cable will have little effect on the performance of a mic or other types of interconnected devices. Balanced lines also offer excellent noise rejection properties not found in unbalanced lines. These benefits are the main reasons that balanced lines are used almost exclusively in pro sound.
High Z mics commonly use a ¼" phone plug on an unbalanced line, which places the audio signal on a single conductor. Though most work just fine, extending the mic cable will cause significant line loss and signal attenuation, and offer a greater chance of noise interference.
Microphones are classified not only in their type and impedance, but also in how the element responds to sounds coming in from different directions. An "omnidirectional" mic picks up sound equally (or nearly so) from all directions. Such microphones might be used to mic a choir or orchestra, and will also pick up other sounds such as echoes and reverberations. This can be good or bad, depending upon what you want in a recording.
Unidirectional microphones do not pick up equally from all directions, and there are a couple common types. The cardioid is absolutely the most popular mic used in pro sound. The pick up pattern is roughly heart-shaped, therefore the term cardioid. These microphones are commonly used by speakers and vocalists, and need to be held fairly close to the mouth so you can speak or sing directly into them. This part of the pick up pattern is referred to as the "forward lobe". Sounds coming from the side of the microphone are not picked up as well, or are "rejected" by the mic. This can be extremely useful for a couple reasons. During a live music performance, amplified musical instruments, drums, and monitor systems can create a high ambient sound level on stage. A unidirectional mic rejects much of the sounds coming in from "off axis", reproducing the sounds coming in through the forward lobe with much greater gain. Also, stage monitor speaker systems can offer an immediate path for feedback in the system. Ensuring that all monitors are located off-axis to open microphones is the first step in keeping the system stable and manageable.
The supercardioid mic is similar to the cardioid. The most sensitive area of pick up of a supercardioid is more tightly concentrated in a smaller front lobe. Though they generally pick up from the "rear lobe" a bit more than a cardioid does, off axis or side rejection performance is better. Supercardioid mics are employed in situations where increased side rejection is needed. Because of the concentrated front lobe, supercardioids are sometimes used where the sound source is at a greater distance from microphone than a cardioid can pick up well.
Applications and Specifications
Easily as important as a cardioid's off-axis rejection is the off-axis frequency response. A quality microphone will provide the same tonal quality to sounds coming on-axis as off-axis. Some amount of impinging off-axis sounds are certainly still picked up by the microphone. But if the tonal quality is different than what is picked up by the front lobe, the overall performance of the mic will suffer greatly. This is especially true in a handheld mic, as when two vocalists are sharing a mic.
Vocal mics generally limit low frequency response to about 100 Hz. There is little component below this in the human voice, though most instruments and drums do. So rejecting these frequencies can help a vocal mic reproduce just the voice.
Cardioid microphones intended for "mic-ing" a musical instrument, guitar or keyboard amplifier, will generally need frequency response extending down to 50 Hz so as cover the full range and timbre of the instrument. As guitar amplifiers and drums often play at high volume levels, a mic used for this application must be able to withstand the high sound pressure level without distortion or experiencing other problems.
Money Buys Tone
The Shure SM58 cardioid microphone is probably the most widely used and recognized vocal mic in the world, followed by the SM57 which is intended for mic-ing instruments and amplifiers. There are certainly many other companies selling quality microphones at competitive prices. These microphones retail in the $130 to $150 range, which is average for this type product. While dynamic cardioid mics can be found at a fraction of this price, the axiom holds true that you get what you pay for. Poor tone and performance from an inexpensive microphone cannot be overcome with even the best electronics.
Wireless Microphone Systems
Most popular cardioid mics are available in a wireless version. These mic/transmitters operate from a 9V alkaline battery, and offer the advantage of not having the microphone cord attached which limits mobility. Most systems today are UHF, though in the past were generally VHF. VHF does perform better, penetrating walls and overcoming other obstacles more easily. But recent problems with interference from cellular and other communications has required that these mic systems be used in the UHF band. As this is written, the future holds many changes for RF bandwidth allocations by the FCC. These analog wireless microphone systems will most likely all be converted to digital systems to guarantee their reliable performance for professional applications.
Wireless mics are available in handheld, lavalier (lapel), and head-worn types. The latter two require a belt pack-type transmitter. They offer the performer excellent mobility, leave his hands free, and can usually transmit several hundred feet in line of sight. The performance of these wireless systems is very close to their wired counterparts. They are generally available as single and dual microphone systems. Dual systems are actually two complete receivers built into one chassis. Due to the nature of FM, more than one transmitting signal on the same frequency can cause severe distortion or no signal to be received at all. This is why adding a second transmitter to a system for simultaneous use does not work.
Wireless system receivers generally operate from a wall adaptor, and will often have both balanced and unbalanced outputs for connection to a mixing console or other input device. Gain and squelch controls are also common features. Some have mult-channel tuners, and can automatically check for open channels. True-diversity receivers have two receiver sections and two antennas through which the system chooses the best signal.
Wireless System Drawbacks
There are several drawbacks in wireless mic systems. Battery life is always of concern, and a failing battery will of course cause the mic to stop working. Modern transmitters do provide excellent battery life, though the mic should be turned off when not actually in use.
Non-diversity receivers, including those with two antennas, are much more susceptible to drop out and interference than true-diversity types. Signal drop out can cause the receiver to roar with static if the gain and squelch controls are not optimally set, or if the transmitter is shut off.
Wireless mic systems, and especially those with true diversity, be quite expensive as compared to a wired mic. The benefits and drawbacks of a wireless mic should be carefully weighed before making the purchase of an expensive wireless system.
Moving around stage monitors and PA speakers with an open microphone is a invitation to feedback, especially when handled by those not accustomed to using a microphone. Feedback whine is a function of wavelength, not frequency. So if a significant amount of sound from a speaker is allowed back into the microphone where it originated and the distance is right, feedback is likely to happen even when the system is reasonably adjusted.
Handheld wireless mics are more susceptible to drop damage than a wired mic, and often require their own microphone stand clip. Having extra batteries on hand is a must. Many transmitters do have a low battery warning indicator.