There are times with the choice of a microphone to best meet a particular application comes down to a large-diaphragm and small-diaphragm condenser model (LDC and SDC, respectively).
But what do these terms even mean, and what are the pros and cons of each?
An LDC has a diaphragm diameter of 1 inch or larger, while an SDC diaphragm is under 1 inch. That simple spec has a wide range of effects on the mic’s performance.
Sensitivity & Noise
All else being equal, an LDC is more sensitive than an SDC, and also tends to generate a higher output voltage, given the same input SPL. Typical LDC sensitivity is about 22 mV/Pa, while typical SDC sensitivity is about 10 mV/Pa. (One Pa or pascal is 94 dB SPL.)
Why is a larger diaphragm more sensitive? Remember that a condenser mic is made of a conductive diaphragm next to a conductive backplate. Those parts are charged with a bias voltage across them, forming a capacitor.
When sound waves vibrate the diaphragm in and out, the capacitance varies in step with the sound waves, which in turn generates a signal voltage that varies in step with the sound waves.
The changes in capacitance due to the vibration are bigger for a large, high-capacitance diaphragm than for a small diaphragm, so the output signal voltage is higher for a large diaphragm. Also, just as a large sail moves a boat with more force than a small sail, sound waves force a large diaphragm to move more than a small one. The greater diaphragm displacement in the LDC results in a higher signal voltage.
Because an LDC provides a stronger signal above the noise floor of the mic’s electronics, the signal-to-noise ratio (S/N) tends to be higher. So an LDC is often a good choice when amplifying (or recording) a quiet instrument or ensemble from a distance without adding noise from the mic or mic preamp. That’s why an LDC excels as an ambience or room mic
Again, all else being equal, most LDC directional mics have a deeper low-frequency response than SDC directional mics (Figure 1). That’s because the resonance frequency of the diaphragm is lower in the LDC due to the diaphragm’s higher mass.
Figure 1: The published frequency responses of a Neumann U 87 Ai, an LDC (above), and a Neumann KM 184 A, an SDC. (click to enlarge)
The response difference also could be an intentional design decision. So if you want to capture a deep, authoritative tone from tom toms or a vocal, you might make an LDC your first choice. On the other hand, the smaller mass of an SDC’s diaphragm helps it respond better to extreme high frequencies as with cymbals.
Suppose you’re using overhead mics on a drum set. If you want those mics to pick up mainly the cymbals, use a pair of SDCs because they have less low end. If you want to pick up the entire set – including toms – with overhead mics, use a pair of LDCs because they tend to have more low end.
Note: an omnidirectional condenser mic of any size tends to have an excellent low-frequency response. Because of the physics of an omni condenser design, the mic responds well to very low frequencies, independent of the diaphragm size.
Most LDCs have a grille structure surrounding the diaphragm. That grille causes internal reflections and filtering, which usually results in a rougher frequency response than with an SDC, which lacks that grille.