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.