I remember a time in my college years when I was studying about microphones. I studied really hard for the midterm exam, and even got the highest score in the class. Later that day, I was scheduled to work on the crew to record a live concert. At one point during the setup, I turned to my teacher and told him that I had been handling microphones for better than twelve years of playing in rock bands and so on but that moment was the first time I had ever held a microphone and knew what I was doing with it. What a shock!
After signal flow logic, understanding microphones is one of the most important things you can learn about audio. A good understanding about microphone design and construction, along with the knowledge of what is available in the marketplace will be indispensable to your job as a sound technician. This bit of knowledge combined with a little experience and experimenting will allow you to deliver repeatable results.
A microphone is a transducer. A transducer converts energy from one medium into another. The job of the mic is to convert acoustical energy (sound) into mechanical energy, and then into electrical energy. A loudspeaker is also a transducer because it converts electrical energy into mechanical energy and then into acoustical energy. The better job of conversion that either of these transducers do, the more natural the reproduced sound will be.
Microphones are available in a variety of designs and price ranges. As with so many things, the better-sounding mics typically carry a higher price tab, right? Careful. Your warning light should have gone off with that comment. Who’s to say what sounds better? While it is true that most engineers will use a higher quality mic in most applications, a good engineer will not succumb to price, perceived value, or tradition in favor of the right sound. In fact, he/she may be found using a very common, relatively inexpensive mic to deliver the sound they’re after. It’s the sound and what is right for your ears- that is important.
There are a few terms that we should be familiar with which will help us to understand how microphones work and how to tell the difference between them just by looking. You should also compare the sound of the mics that you have at your disposal with what the accompanying literature has to say about them. The terms we’ll discuss are type, pickup pattern, frequency response, off-axis coloration, sensitivity, proximity effect, impedance and handling noise.
There are two basic types of microphones used in professional audio today – dynamic and condenser. A dynamic mic is constructed of a coil of wire attached to a dome-shaped plastic (e.g., Mylar) diaphragm. The coil of wire is suspended in a magnetic air gap. As sound strikes the diaphragm, it wiggles back and forth, and the coil of wire goes along for the ride. This action causes the coil of wire to cut the lines of magnetic flux, which thanks to God’s laws of physics, induces a flow of current in the wire. This very low strength electrical signal is then fed to the sound mixing console.
A condenser mic works in a similar fashion. It is constructed of a thin, metallicized plastic (e.g., Mylar) diaphragm, placed very close to a fixed, electrically polarized, perforated back plate. As sound strikes the diaphragm, it moves back and forth causing a corresponding change in the capacitance created between the two plates. This action results in a flow of current, which is then fed to the console.
The choice of using a dynamic mic or a condenser mic is very much a matter of application and personal taste. A few general comments can be made about the difference between the two types. Typically, a dynamic mic is considered to be more physically rugged or durable, and able to take a louder signal without distorting. By contrast, a condenser mic is a poor choice for a hammer, but is much more sensitive and reproduces transient peaks better than a dynamic mic. Transients are found in the attack of a percussive sound, such as a piano, drum, or guitar. The ability of a mic to reproduce those transients has a great deal to do with the faithful reproduction of the original sound.
Most sound engineers prefer the sound of condenser mics for recording orchestras, especially strings. The same could be said for miking drums or acoustic piano. With the same breath though, some engineers prefer to use dynamic mics for miking drums, especially for touring sound reinforcement systems since traveling can be hard on all audio gear. Another point of choice between a condenser mic and a dynamic mic is that of cost. A dynamic mic typically costs less than a condenser mic of similar quality.
A polar pattern is a graphic representation of the directional characteristics of a microphone. The most common polar patterns are the omnidirectional, unidirectional, and bi-directional. The unidirectional pattern includes three specific types – cardioid, supercardioid and hypercardioid. The Crown PZM mic has a hemispherical pickup pattern.
Why do so many pickup patterns exist? Each pattern provides a unique solution to a specific set of problems in a given application. An omnidirectional mic hears sounds from all around it. A unidirectional mic hears sounds from one specific location. A bi-directional mic actually hears sounds from two specific directions – directly in front of it, and directly behind it. And a hemispherical mic hears sounds above a certain plane.
An omni mic is one that is equally sensitive to sounds arriving from all directions – left, right, above, below, front and back. Be aware that a polar pattern diagram is intended to depict a three dimensional area centered at the mic’s diaphragm. Try an experiment to illustrate this characteristic. Hold an omni mic about a foot in front of you and aim it at your mouth. Now talk into it and listen to the sound. Next, try rotating the mic in front of you in all directions while you talk. As long as you maintain the distance from the diaphragm to your mouth as you rotate it, you should hear virtually no difference in either the loudness or character of the sound. You will hear a slight drop of high frequencies as you speak toward the rear of the mic because the housing of the mic tends to get in the way.
A unidirectional mic is one that is most sensitive to sounds arriving on-axis. That is, sounds arriving at the front of the mic are reproduced the loudest. Sounds that arrive from the sides and rear of the mic are attenuated, or not reproduced as loud as sounds arriving on-axis.
With an omni mic, the sound wave hits the diaphragm only from the front of the mic. You can often tell that a mic is an omni by noticing that there are no holes on the side of the mic that would allow sound to reach the back of the diaphragm. The cardioid pattern is most commonly achieved by cleverly combining the sound arriving at the front of the diaphragm with that arriving at the rear of the diaphragm. The sound that reaches the rear of the diaphragm must first go through the rear entry ports on the side of the mic, and then travel through an acoustical delay circuit.
If the sound originates from the front of the microphone, the portion of that energy arriving at the front of the diaphragm and that arriving at the rear of the diaphragm strengthen each other when they combine at the diaphragm. If the sound originates from somewhere else, like from the back of the mic, the energy from the rear will be somewhere between 0 and 180 degrees out of phase, and will therefore cause some amount of cancellation.
One point to remember here is to teach anyone holding a typical microphone not to choke the mic. That is to say, if they hold the mic close to the capsule, they will likely close off the rear entry ports, which will in turn degrade the sound quality of the mic and essentially turn it into an omni mic. Instead, they should hold the mic further down the handle, away from the screen mesh that protects the capsule.
The basic difference between cardioid, supercardioid and hypercardioid patterns is their acceptance angle and their maximum rejection points. The acceptance angle, or pickup arc, describes the area around the on-axis point at which there is no more than 3 dB of attenuation of the sound. For a cardioid mic this figure is 131 degrees. This means that within 65 degrees off-axis at any point around the mic, there will be no more than 3 dB of attenuation. The maximum rejection point is the angle (in three dimensions) at which the mic presents its greatest attenuation. The maximum rejection point for a cardioid mic is at 180 degrees – typically where the mic cable plugs in.
How do you use that information? We all know to talk into the front of the mic – the 0 degrees on-axis point of the polar pattern. For the best gain-before-feedback, aim the maximum rejection point of the mic at the sound source you don’t want to pickup – in this case, the stage monitors.
A bi-directional mic has two equally sensitive pickup areas (called lobes) at opposite sides of the mic. A sound arriving at the 0 degrees on-axis side of the mic will produce a positive-going voltage at the output of the mic, while the same sound arriving at 180 degrees off-axis will produce a negative output. The acceptance angle for a bi-directional mic is 90 degrees, with the maximum rejection point at 90 degrees off-axis. How can that possibly be a good characteristic? You may have seen two singers being recorded in a studio, positioned on opposite sides of the same mic. Now, the recording engineer could choose an omni mic for that application, and pick up both singers equally. The sound of the room they were singing in would also be picked up very nicely. If you simply switch to a bi-directional pattern, you’ll pick up both singers equally, but the room sound will cancel. The room sound is heard equally by both the positive side and the negative side of the capsule. The combination of those two signals – one in-polarity and one out-of-polarity – causes the room sound to cancel very effectively.
A hemispherical pickup pattern is a characteristic of pressure zone microphones. A PZM employs a condenser element placed in a unique housing which positions the element facing down, very closely over a flat surface called the boundary plate. The design ensures that all sounds reaching the element must first be reflected off of the boundary beneath it, and therefore arrive at the element in phase. If the mic is mounted on a floor, for example, then its inherent hemispherical pickup will hear sounds from all directions above the floor. The PZM is noted for its startling clarity, and the pickup pattern must be experienced to be fully appreciated. In a church setting, it makes a wonderful audience response mic.
Observing product literature for the frequency response of a microphone can give the sound technician a feel for its sound. It is generally preferable that the frequency response be uniform and peak free. Any serious dips in the response could result in an unnatural sound. Any serious peaks in the response could result in not only an unnatural sound but, if the mic is being used in a sound reinforcement application, it could be a major contributor to feedback problems as well.
One of the common rules for microphone choice is to choose the mic and placement that will deliver the sound that you want. For example, sometimes a peak in the response of a mic can be a real help. A nice smooth 3 dB peak around 5 kHz is a great benefit for vocals.
The patterns that we have considered so far have been theoretical design goals. One practical reality is that not all mics are created equally. Our preferred uniform and peak-free frequency response is quoted for the on-axis response. But what about the sounds arriving from other directions? Yes – you DO need to consider the effect of sounds arriving from all around the mic!
Theoretically a cardioid mic will attenuate sounds arriving off-axis, with the greatest attenuation at 180 degrees off-axis. In practice, this off-axis attenuation may or may not be uniform for all frequencies. Should the off-axis response be erratic, filled with dips and peaks at many different frequencies, then the sound that spills into the sides and back of that mic will be colored and then added with the original sound.
For example, say that a piano and an acoustic guitar are being played on stage together. You are miking each instrument separately. Each mic will pick up the instrument it is placed at, plus a certain amount of the other instrument will spill into its off-axis side. If the mic on the guitar has poor off-axis coloration, it will reproduce the piano leakage with that erratic response, in turn causing the sound of the piano to be colored or unnatural. You may have spent a great deal of time miking each instrument separately to get just the right sound, only to be short-changed by off-axis coloration when both sounds are combined.
Proximity effect is a characteristic of directional microphones resulting in an increase in the response at low frequencies when the mic is used at a close working distance. To illustrate this point, try another experiment – this time comparing the sound of a cardioid mic with an omni mic. Speak into each one from about two feet away and adjust the volume of each so they sound equal. Now, shut off the cardioid mic. Face the omni mic, start speaking and slowly move the mic closer to your mouth. Notice that the volume increases as you get closer to the mic, but that there is no change in the character of the sound. Now try the same experiment with the cardioid mic. Note that as you get closer to the cardioid mic, not only does the volume increase, but your voice sounds fuller and warmer. That’s because the low frequencies of your voice get boosted more than higher frequencies, almost as though someone had turned up the low frequency EQ control on the console.
Many singers and speakers like this effect because it makes their voice sound bigger. Proximity effect can also work as a disadvantage by making the overall sound too thick. Simply be aware that anytime you use a directional mic, its proximity effect should be taken into account.
The sensitivity rating of a mic describes its output level relative to a calibrated standard sound level. Not all mics are rated under exactly the same conditions, but many are. When the test conditions are the same, the results between mics can be compared, giving the engineer a feel for how hot one mic is relative to others he/she may be familiar with.
The value of knowing this rating for the mics you have at your disposal is that it allows you to make informed decisions about their use. If you place a mic with a very high output level on a sound source capable of very high acoustic sound pressure levels (e.g., a timpani), you may run the risk of overloading the input to your console.
Mics have a rated output impedance (Z). Mics with an output impedance of 50 ohms to around 200 ohms are considered low impedance mics. Those with an output impedance in the several thousand ohms range are considered high-Z.
Low impedance mics are connected to a console with a balanced mic cable. That cable may be several hundred feet long without causing any loss in frequency response to the signal that it carries. High impedance mics are connected with unbalanced cables, and you may remember from other studies that unbalanced cables are restricted in length to fifteen feet before the frequency response starts to degrade.
This is one characteristic that can immediately ruin your day if it’s working against you. When you pick up a mic, toss it around in your hand, or even place it on a mic stand, you don’t want to hear any extraneous sounds. If you tap your thumb on the handle of the microphone and hear a strong thumping sound, you’ve got a mic with poor handling noise. If you pull on the mic cable, scraping your fingers along the cable itself and hear that mechanical noise amplified through your sound system, you have a mic with poor handling noise.
Just try placing that mic on a boom stand and using it to mic a piano. You’ll pick up the piano of course, but along with it you’ll hear every vibration created on the stage, traveling mechanically up the mic stand and out to the mic, vibrating the case of the mic and being heard – quite clearly – over your sound system, or on your recording. Effective, integral shock mounting is one thing that sets apart the construction of cheap mics from the construction of high quality mics.
A microphone is one of the most important tools that a sound technician has. His/her choice of which of these tools to use in a given situation will have a major bearing on the end result – the sound that is captured on tape or that the congregation hears over the sound system. The variety of mics available on the market seems overwhelming, yet each one provides a unique solution to a given problem. As one audio scholar once put it, the availability of a certain piece of equipment is inversely proportional to its need.
Learn all you can about these tools. Experiment!!! Purchase the best quality mics that you can afford, but never overlook that cheap old mic that someone threw in the drawer. Someday, it may just turn out to be the perfect mic for the job.