Mics: Does Size Really Matter?

by Steve La Cerra
in Theory and Practice
The piano is one instrument where either large or small-diaphragm mics can be used, depending on the desired sound. Photo courtesy Sennheiser
The piano is one instrument where either large or small-diaphragm mics can be used, depending on the desired sound. Photo courtesy Sennheiser

Anyone reading FRONT of HOUSE is probably familiar with a variety of microphone designs using moving coil, ribbon or condenser capsules. I often joke that having a lot of mics is like having a lot of crayons: you don’t want the eight-pack, you want the super-variety pack with 152 colors and the sharpener built into the box. You never know when you’ll need that ever-so-subtle shade of color. If only mics were as inexpensive as crayons…

One of the varying aspects of these wonderful tools is diaphragm size. As you’d expect, the size of a microphone’s diaphragm has a profound influence on the way it sounds — but maybe not in the way you think. As a rough guide, “small” diaphragm means roughly a half-inch in diameter (or less), and “large” diaphragm means 1-inch in diameter (or more); I guess anything in between qualifies as “medium.”
Let’s start with small-diaphragm condenser microphones (a.k.a. “SDCs”). We’ve all used small-diaphragm condensers for instruments like high-hat, drum overheads and acoustic guitar. One of the characteristics that makes a small-diaphragm mic excel at such instruments is transient response, which refers to the manner in which the diaphragm reacts to fast sounds.
All things being equal (and we know they never are), a small diaphragm will have a faster transient response than a large diaphragm because a smaller membrane has less mass to move. Less mass = faster transient response and (probably) less color. Translation: a small-diaphragm condenser can sound more “detailed” and natural, while a large-diaphragm condenser mic might sound more smooth or “warm.” Score one for Small D.
A condenser microphone’s output level is related to the size of the capsule’s “plates” (the stationary plate is called the backplate, and the movable plate is the diaphragm). Generally speaking, larger diaphragms generate higher output levels because their surface area captures more air displacement (sound). The result is that small-diaphragm mics often have higher signal-to-noise ratios.
Lucky for us, amplification technology has come a long way since ye olden days when the noise added by a condenser mic’s amplifier was an issue (see sidebar). Using a small-diaphragm condenser mic for an instrument with soft dynamics (let’s say a fingerpicked acoustic guitar) in a live situation is probably not going to present a noise issue, because the noise floor of the environment will likely mask any noise generated by the microphone. But when you’re in the studio recording that same guitar, the microphone’s self-noise could become audible. Large diaphragms tend to be more compliant (elastic), making them easier to move, and that translates into increased sensitivity to lower level sounds. Score two for Big D.

‡‡    Frequency Response
When it comes to frequency response, SDCs have the advantage at the top end, while LDCs have the advantage at the bottom — sort of. First, the HF response: as diaphragm diameter is increased, the diaphragm tends to act less like a true piston and the vibrational modes break up. Sound familiar? That’s why we use different drivers for low, mid- and high-frequency reproduction in loudspeakers. Some SDCs have a frequency response reaching far beyond that of human hearing (e.g., the Earthworks QTC50 has a response extending to 50,000 Hz). At the bottom end, an LDC may have the edge over a SDC, but that’s more perception than reality (“the diaphragm is larger therefore it must be capable of capturing a lower frequency range…”). Beware of the fact that — due to the nature of the capsule design — omnidirectional microphones often possess great LF response regardless of diaphragm size. Directional large-diaphragm condenser mics (cardioid, supercardioid, etc.) may have the edge over directional SDCs in the bottom end. Call it a tie.

‡‡    Get Off My Axis
By nature, large-diaphragm mics create an acoustic shadow upon sound waves that approach the capsule. This affects high frequencies in particular, as the size of their wavelengths relates to the size of the capsule. Specifically, as frequency increases, wavelength becomes shorter. When wavelength is smaller than the diameter of the diaphragm, interference occurs between on- and off-axis sounds, producing phase cancellation that limits the high-frequency response. The result is that off-axis sounds tend to be more colored when captured by a large diaphragm. An infinitely small diaphragm would reduce this effect, but then the output would be so low that the microphone would not be usable.
Related to this phenomenon is the issue of pattern integrity. The directional pattern of a larger diaphragm breaks down as frequency decreases. For example, a SDC can maintain a fairly consistent cardioid pattern across a frequency range from 150 to 15,000 Hz, yet a cardioid LDC tends to become omnidirectional at frequencies below about 200 Hz. Also, at very high frequencies, the cardioid pattern of a large-diaphragm condenser starts to look like a super- or hypercardioid pattern. That consistency of pattern is one of the reasons a small diaphragm condenser might be a better choice when recording sounds at a distance such as choir or orchestra. Plus one for Small D.
Small-diaphragm condenser mics typically have the ability to handle higher SPLs than LDCs, which is something to consider for live sound applications — not so much due to sheer stage volume, but because of the proximity of the mic relative to the sound source. Your guitar player might not have an amp that’s blaring across the stage, but when you place a microphone three inches from the grill, that mic is “hearing” a higher SPL than you are when you’re standing ten feet away from the amp. You certainly don’t want to add distortion via the microphone, so it’s possible that an SDC might help you out here. And small-diaphragm condenser mics are usually easier to place because (duh) they’re smaller.
So which is “better?” The only real absolute here is that you’ll fare better with a large-diaphragm condenser on a quiet source. Other than that, buy the box of crayons with 152 colors. One of them is bound to make you happy.  

Condenser Microphones and Phantom Power

You already know that most condenser microphones require phantom power, a battery or — in the case of a tube microphone — a dedicated power supply. Do you know why? There are two distinct reasons:

Reason 1: The metal plates in a condenser capsule must be “biased” or charged with a small amount of electricity in order to operate. Phantom power is used to supply this voltage. Electret condenser capsules are permanently charged and do not require bias.

Reason 2: The output of a condenser capsule is exceedingly low. If audio were sent directly from the capsule through the mic cable, by the time the audio signal reached the mic preamp, it would be unacceptably noisy. To avoid this issue, condenser mics have built in amplification via a “head” amp or pre-preamp. This circuitry requires power, which is also derived from phantom power.

Note: Not all phantom power is equal. The phantom power “standard” is 48 VDC, yet on occasion you might run into a preamp or mixer that provides 18 VDC or 24 VDC phantom. Whether or not this is an issue depends upon the microphone you plan to use. Here are a few examples: The Audix SCX25A runs on phantom from 48 to 52 volts. It will not power up when phantom is below that level. The Neumann KM184 requires 48 DVC ±4 volts. On the other hand, a Shure SM81 will run on phantom ranging from 12 to 48 VDC. At lower supply voltages, the noise floor of the microphone may be slightly higher.

Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.