The C 542 BL ($320) is one of a handful of boundary-layer microphones made by AKG. Boundary-layer microphones are unique in their fundamental attributes: the evenness and low-end extension of the mic's frequency response depend on the size of the boundary on which the mic resides; the pickup pattern is normally hemispherical; and there is less differentiation between the direct and the reverberant sound (especially in frequency response) than with other kinds of mics. At the same time, room sound contributes largely to the audio that a boundary-layer mic captures.
The AKG C 542 BL is a hemispherical- picku p, boundary-layer mic in a 3.2-inch diameter disk with a 12 dB/octave, 150 Hz highpass filter in the attached phantom-power adapter.
PZM (a trademark of Crown) mics are similar but not identical to boundary-layer mics. (PZM stands for pressure-zone microphone.) In a PZM mic, the transducer faces downward toward the boundary's surface; in a boundary-layer mic, the transducer faces forward (or upward) toward the sound source. According to AKG, the C 542 BL's upward-facing mic element has several advantages over the downward-facing elements in some other pressure-recording microphones, including more-accurate and detailed sound and more consistent performance.
Those characteristics make boundary-layer mics popular for amplifying voices in theatrical productions, in conference settings, and in churches. Boundary-layer mics also have become popular in music recording applications such as miking pianos and large ensembles. I use a pair of boundary-layer mics for recording interviews, because the technique allows the subject to move around the space without sacrificing intelligibility on the recording.
The C 542 BL is a classic boundary-layer hemispherical-pickup mic in a 3.2-inch diameter disk. Although some boundary-layer mics are designed for specific applications, the C 542 BL is intended to cover the full range of boundary-layer use. The disk is only 0.2 inches thick, making the C 542 BL a small, low-profile package. The attached phantom-power adapter in the mic's XLR connector has a 12 dB/octave, 150 Hz highpass filter.
On the Surface
There are two methods of deploying boundary-layer mics: either attach them to a wall, a ceiling, or a floor or attach them to a sheet of Plexiglas or other portable flat surface. I performed my evaluations using the first method. Adhesive is included with the C 542 BL for attaching it to surfaces, but the disk has three holes drilled through it to allow permanent mounting to Plexiglas, walls, ceilings, or floors. If you aren't going to mount the mic permanently, the adhesive will come in handy, because boundary-layer mics don't lend themselves to or function optimally with direct-stand mounting. The C 542 BL has rubber feet for easier floor deployment and to isolate the mic from vibration.
I tried the C 542 BL on a number of instruments (vibraphone, drum overheads, acoustic guitar, djembe, and shakers) and a few voices (spoken and sung) in my personal studio. Unfortunately, I was unable to try a pair of the mics on piano. For sport, I compared the C 542 BL with one of the notorious Radio Shack boundary-layer mics, and the difference was clear: the C 542 BL captured a much more balanced spectrum of sound and exhibited greater extension and openness of high frequencies, especially above 8 kHz or so.
Placing a pair of C 542 BLs about 48 inches apart on the ceiling in a two-point stereo arrangement over the vibes rendered a nice, wide stereo image and achieved an even balance through the instrument's range, leaving no hole in the middle. The attack of my harder-than-usual mallets was quite audible, though not as sharp and peaky as it would have been with close-miking. The low-frequency pickup was pleasing in its range but did not seem smooth. Overall, it was an excellent sound on a notoriously difficult instrument.
Solo-sung vocals were my least favorite application for the C 542 BL — it had a bit of a “bark” in the midrange. It showed little change in sound quality, however, when I played rock star and threw myself around the room while singing. (Under those conditions a cardioid condenser would have captured audio that was all over the sonic map.) I could definitely see recording a vocal group in a nice room with this mic.
Next, I recorded a spoken-word passage while walking all around the room as I read, occasionally turning my back on the mic. Other than when I turned my back, the sound varied only slightly. This is the classic boundary-layer experience.
The C 542 BL did a fine job on other instruments as well, though on acoustic guitar the mic did not have the detail of a good small-diaphragm condenser. The contribution of the room sound, however, gave the guitar an immediacy that did not feel unnaturally dry, as close-miking can.
The C 542 BL is a good choice for applications in which boundary-layer pickup is advantageous. It has a clean, well-balanced sound that is free of noise problems and is contained in a compact, low-profile package. Overall, the mic is an excellent and affordable performer.
Overall Rating (1 through 5): 4