My Brüel & Kjær 4006 mics have alternative black and silver capsules. The box says: “The black grid gives flat frequency response in a diffuse soundfield, the bright grid gives flat frequency response with 0 degree incidence in a free‑field.” What does this mean in practice? For the last 20 years I’ve been using the silver (bright) grid; under what conditions should I consider swapping them out?
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: One aspect of omnidirectional mics, which surprisingly few seem to be aware of, is that they are fundamentally designed to be used either in the free‑field (which means close to the sound source) or the diffuse‑field, beyond the Critical Distance (where the direct and reverberated sounds are equal in level) in a reverberant space. Some omni mics can be reconfigured, either electrically or mechanically, to work in either condition, but most have a fixed characteristic and using the wrong one in the wrong situation will deliver a noticeably bright or dull sound.
The Brüel & Kjær (now DPA) 4006 microphone achieves this mode change via replaceable ‘grids’ (not capsules) which are screwed in front of the diaphragm, and the patterns of holes in these different grids determines the microphone’s on‑axis frequency response. Basically, the silver (free‑field) grid has a hole at the centre, whereas the black (diffuse‑field) grid doesn’t!
So why is this capsule modification needed? Well, the frequency response of an omnidirectional mic is affected by where the sound source is relative to the diaphragm. When a sound source is close to the mic and directly on‑axis — that is to say, in the ‘free‑field’ — soundwaves strike the diaphragm from straight ahead. At low and mid frequencies, the wavelength is much larger than the diameter of the diaphragm, so the soundwaves simply diffract around the microphone as if it weren’t there. However, at high frequencies, where the wavelength is similar to the size of the diaphragm, some of the sound energy is reflected back into the incoming sound waves, creating a build‑up of pressure in front of the diaphragm. This acoustic interaction increases the microphone’s sensitivity to high frequencies, typically resulting in an HF boost of the order of 10dB at 16kHz (for a conventional small‑diaphragm omni mic) — it is quite a significant effect!
Importantly, if the sound source is off to the side of the mic, at 90 degrees of incidence, there is no pressure build‑up since the soundwaves are travelling across the diaphragm instead of straight into it, so there’s no HF boost! Similarly, in a wholly reverberant (diffuse‑field) environment, where sound approaches from myriad directions, there is no pressure build‑up either.
The microphone designer faces a dilemma: should an omni mic be designed to give a flat frequency response for close, direct, on‑axis free‑field sources, or for more distant, reverberant diffuse‑field situations?
So the microphone designer faces a dilemma: should an omni microphone be designed to give a flat frequency response for close, direct, on‑axis free‑field sources, or for more distant, reverberant diffuse‑field situations? Both are valid recording applications for omni mics in different situations. In some cases, the dilemma is dealt with by including switchable electronic EQ, allowing the mic to be used in either situation. Since this is an acoustic problem, though, the solution B&K chose was an acoustic one: they designed different grids to sit in front of the diaphragm which correct the HF boost by controlling the pressure build‑up itself.
The black grid supplied with your 4006 is designed for use when the mic is placed in the diffuse‑field, ensuring it delivers a flat overall frequency response for sounds arriving at random angles of incidence. So, in practice you would install the black grid when using the mic relatively far from the sound source in a reverberant space beyond the Critical Distance, where the ambient sound level equals or exceeds the direct sound — such as when used as a ‘space mic’ or perhaps in an A‑B stereo array.
However, if you moved the mic with a black grid close to a sound source on axis — such as when used as a spot mic — the inevitable pressure build‑up would deliver an unnaturally bright sound character. So in this situation the silver grid would be fitted instead to acoustically correct the on‑axis frequency response in this free‑field environment, where the direct sound level is substantially greater than reverberant sound. And conversely, if the silver grid is used when the mic is employed as a ‘space mic’ in the diffuse field the resulting sound would be rather dull as the grid is compensating for a pressure build‑up that no longer exists!
DPA took over B&K’s studio microphone business in the early 1990s but they still make the 4006, along with a variety of different grids, cones, and spheres which can be fitted for different applications to modify the mic’s frequency response and polar pattern. Finally, a word of warning: these grids and other attachments simply screw on and off easily, and while that has good practical benefits, you must be very careful not to touch the exposed diaphragm or let dust, dirt, or moisture settle on it!