Choosing a Microphone – Part 2
Written by Alex Cloud, Audio Engineer
In the last blog you learned about some aspects of a microphone’s sound to help you decide on which mic to use. Next, we’ll discuss some practical factors that also play a role when choosing a mic.
The amount of sound pressure a microphone can withstand is important when recording loud sources like guitar amps, kick drums, brass instruments, and loud singers. If you try to record a source that’s too loud, you might distort the microphone. Engaging a pad on your preamp or console channel strip will not fix this problem, because the distortion happens in the microphone itself. Related to SPL handling is saturation. Sometimes with sounds that are just a little bit too loud, certain microphones will not quite distort but will exhibit a saturated, retro kind of sound. Usually this rounds off some of the transients providing a thick, almost compressed character. Sometimes this is a desirable effect, sometimes not so much.
OUTPUT LEVEL, NOISE
Another thing to consider is the microphone’s output level, and how that interacts with whichever preamp you are using. Some mics are just quieter than others, which may require you to crank the preamp gain a little more than usual. This can be a problem if you have a noisy preamp. Conversely, some microphones are louder than normal. If your preamp does not have a lot of headroom, or does not have a switchable pad, you might drive the preamp into distortion. Related to a microphone’s output level is its self-noise. Some mics are inherently noisy, and if you have to use a lot of gain on the preamp you’ll be amplifying that noise as well. Sometime’s output level is affected by the interaction between the output impedance of the microphone and the input impedance of the preamp. If your preamp has a switchable or sweepable impedance control, this might affect the output level of the microphone. This can also have a subtle effect on the frequency response, transient response, and saturation of a microphone (usually this only affects dynamic or ribbon microphones, while condensers tend to be immune to changes in impedance.)
Another practical factor is a microphone’s durability. This is not a measurable, quantifiable characteristic that can be printed on a chart, it’s just a subjective judgement of how fragile or resilient a microphone is. You don’t want to place delicate microphones in places where they could fall or be struck and get damaged as a result. This is not just a matter of physical blows. Some ribbon mics are particularly susceptible to damage from plosives and loud low-frequency sounds. Some condenser microphones can short out from the humidity in a singer’s breath on a hot day.
SIZE AND PLACEMENT
Just as important, especially when close-mic’ing in crowded spaces, are the physical dimensions of the mic itself. Can you fit the microphone into the space it needs to be in? And if it is a directional microphone, is its on-axis point at the end of the microphone body or its side? Does that affect the ways that mic could be placed? There are plenty of times when I’ve wanted to use a particular microphone on a source because of all of the sonic characteristics described previously, but I had to use a different one because the preferred choice was just too bulky or cumbersome and would have inhibited the artist’s performance. Take for example the task of mic’ing up a busy drum kit, with lots of cymbals.
So between these two blog posts, you’ve learned to consider a microphone’s:
- Frequency response
- Polar pattern
- Off-axis frequency response
- Transient response
- SPL handling
- Output level and noise
- Physical dimensions
There are many aspects of microphone design that can affect the properties listed above. Too many, in fact, to list in a simple blog series. But one crucially important aspect of microphone design that will affect all of those characteristics is the method the microphone uses to turn sound into electricity. For all of their different traits and idiosyncrasies, every microphone has the same job – turn sound into electricity. There are three main methods used by different microphones to achieve this.
Some microphones have a thin diaphragm flecked or “sputtered” with bits of metal that, along with an electrically charged metal plate, forms a type of electrical component called a capacitor inside the microphone. As the sound pressure vibrates the thin diaphragm, it moves back and forth relative to the metal plate. That movement changes the capacitance of this makeshift capacitor, and the changes in capacitance mirror the changes in sound pressure level.
Microphones that use this principle to generate electricity are called condenser microphones (“condenser” is what they call capacitors in some countries. Interestingly, in those countries that call capacitors “condensers”, the name is reversed – the microphones are known as “capacitor” microphones. Language is bizarre.) Making overly broad generalizations, condenser microphones tend to be associated with extended frequency response especially in the high frequency range, quick transient response, and high output level. Condenser microphones are often separated into the subcategories of large-diaphragm condensers and small-diaphragm condensers. The cutoff between these two categories is usually placed at a diaphragm size of around 1 inch. Still speaking generally, large-diaphragm condensers tend to have a more forward, intimate or hyped quality with a higher output level. Small-diaphragm condensers tend to sound more natural (or maybe a touch brighter depending on the mic) with an output level lower than large-diaphragm condensers though still higher than for other types of microphones, and generally even off-axis frequency response.
Some microphones contain a diaphragm that has a coil of wire attached to it. This coil of wire is suspended in a magnetic field in the center of a circular or U-shaped magnet. The diaphragm moves back and forth along with changes in sound pressure, which causes the coil of wire to move back and forth in the magnetic field. Similar to an electromagnet, this movement causes an electrical current to be induced into the coil of wire, which varies along with the changes in sound pressure.
Microphones that generate electricity this way are called dynamic microphones, or sometimes dynamic moving coil microphones. Making overly broad generalizations, dynamic microphones tend to be associated with limited frequency response with emphasis on the midrange frequencies, slow transient response, high SPL handling, and ruggedness or durability.
There is another type of microphone that uses a different implementation of the same electromagnetic principle that drives dynamic moving coil mics. In this type of microphone, an extremely thin corrugated metal ribbon is suspended in the “U” of a horseshoe-shaped magnet. Changes in sound pressure cause the ribbon to move back and forth in the magnet, and this movement induces a current in the ribbon.
Microphones that generate electricity this way are called ribbon microphones or dynamic ribbon microphones. Making overly broad generalizations, ribbon microphones tend to be associated with a “darker” or “warmer” frequency response that is somewhat lacking in the highest frequencies (although as many engineers will tell you, the highs that are there tend to sound natural and smooth, and ribbon microphones have a reputation for “taking EQ well”, so the treble content can be boosted for a pleasing result) and a neutral transient response somewhere between that of condensers and dynamics. It should also be noted that the vast majority of ribbon microphones exhibit a figure-eight polar pattern, as sound entering from the sides does not move the ribbon back and forth.
While it can be helpful to remember characteristics commonly associated with condenser mics, dynamic mics, and ribbon mics, it is also important to know that those are merely generalizations about these categories of microphone – often the variances amongst microphones of the same type are greater than the supposed differences between the different types of microphones. So if I’m specifically looking for, say, a microphone with extended high-frequency response, off-axis sound that largely matches the on-axis sound, quick transients, and a high output level, I am likely to reach for a small diaphragm condenser – but not every small diaphragm condenser exhibits all of those characteristics or too the same degree as every other small diaphragm condenser. Thinking in terms of categories of microphone can be helpful to point you in the right direction, but you still need to think of each microphone as its own unique entity and listen to the sounds it produces. Experience, along with careful analysis, will lead you to develop your own preferences, habits, and techniques. At Omega, our staff engineers collectively have over 300 years of experience. In the next blog, I’ll lead you through some insights and advice we’ve learned regarding microphone choice and technique.