Size Matters: The Differences In Large- And Small-Diaphragm Condenser Microphones
September 28, 2012, by Bruce Bartlett
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.
Directional mics have a rise in low-frequency response when used close to a sound source. It’s called the proximity effect. An SDC tends to roll off more at low frequencies than an LDC.
So the proximity effect emphasizes the mid-lows in an SDC, but emphasizes the deep lows in an LDC. As a result, an up-close SDC may have a “puffy” midbass boost – a coloration less likely to be heard with an LDC.
Because its proximity effect tends to sound better, an LDC is the most popular choice for micing vocalists in the studio.
As a side note, the proximity effect is a low-Q phenomenon (about +3 to +6 dB per octave). When rolling off the lows to compensate for a mic’s up-close bass boost, use a broad, low-Q filter setting such as 0.5.
Because of its smaller mass, an SDC diaphragm responds more quickly to transient sounds than an equivalent LDC. This makes an SDC a good choice whenever for capturing fast transients cleanly, as with an acoustic guitar, metal percussion, or cymbals.
Figure 2: A large-diaphragm capsule is usually housed inside a large vertical cylinder. It is side-addressed. (click to enlarge)
Most SDC mics have less off-axis coloration than LDC mics. Here’s why.
When sound waves approach a mic diaphragm off-axis, they travel across the diaphragm. Each sound wave has a high-pressure peak and a low-pressure trough. High-frequency sounds with small wavelengths tend to partially cancel – due to phase interference – when the waves pass across the diaphragm.
The smaller the diaphragm, the less phase shift there is across the diaphragm from a side-arriving sound wave. Less phase shift means less cancellation of high frequencies. In other words, an SDC tends to have a flatter high-frequency response off-axis, while an LDC tends to roll off in the highs.
Also, most LDC designs mount the mic capsule inside a cylindrical housing or grille (Figure 2). That creates reflections and filtering inside the grille which affect the mic’s frequency response and polar pattern.
In contrast, in most SDC mics the capsule is mounted on the end of a stick- or pencil-shaped housing (Figure 3). The capsule has no grille around it to mess up its frequency response and polar pattern – another reason for the SDC’s lack of off-axis coloration.
Figure 3: A small-diaphragm capsule is usually mounted on the end of a narrow cylinder. It is end-addressed. (click to enlarge)
When is off-axis coloration a problem? Whenever sound approaches the mic at a wide angle away from the front. Examples are an orchestra, grand piano, or other large sound sources. Because SDC mics pick up highs well off-axis, they find use as a stereo pair to capture an orchestra or symphonic band.
Off-axis coloration is less of an issue when mic’ing studio singers because they tend to stay on-axis. However, leakage comes into the mic from all angles, so an LDC tends to color leakage more than an SDC.
Specs aside, consider the psychological effect of a larger mic on a singer’s performance. Let’s face it: an LDC in a shock mount just looks cooler than an SDC on a clip. Singers feel special when their microphone looks large and important.
An LDC side-addressed design looks more like a passive “ear,” saying “I’m listening.” An SDC “points” accusingly at the singer, saying “you’d better watch it.”
Choose an LDC when the application needs:
—Low noise and high sensitivity
—Deep low-frequency response
—Good-sounding proximity effect on singers
—A cool look
Some applications: Studio vocals, ambience, deep-sounding drums, mic’ing a drum kit overhead when the overhead mics are the main pickup for the toms, quiet or distant instruments or vocals, distant single-mic technique for “old-time” or bluegrass bands.
Choose an SDC when the application needs:
—Extended high-frequency response
—Flatter frequency response
—Lower handling noise
—Excellent transient response
—Low off-axis coloration (A wider pattern at high frequencies)
Some applications: Acoustic instruments, percussion, cymbals, orchestral stereo mic’ing, and spot mic’ing
The specifications of any particular mic, and its sound, are often more important than whether that mic is an LDC or SDC. You might come across an LDC with poor S/N or poor low-frequency response. A particular LDC might sound brighter than an SDC. It depends on the individual mic’s design.
For example, an Audio-Technica AT4041 is an SDC, but it has an excellent response down to low frequencies. Don’t be locked into using an LDC when an SDC might sound better, and vice versa.
The old adage, as always, still applies: use whatever mic that sounds best.
Bruce Bartlett is a microphone engineer (www.bartlettmics.com), recording and live sound engineer, and audio journalist. His latest book is Practical Recording Techniques 6th Edition.
Size Matters: The Differences In Large- And Small-Diaphragm Condenser Microphones