I recently acquired a Blue [Bottle] Rocket Stage 2 with cardioid B8 capsule, and although I really like the sound it seems a bit ‘stiff’ — in the same way that a speaker can sound ‘stiff’ until it’s been ‘burned in’. So, do mic capsules ‘relax’ over time or will that stiffness persist? At the moment it sounds a little as though the signal is pushing against a rubber ceiling (sorry, that’s the best description I can give).
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Technical Editor Hugh Robjohns replies: No, capacitor capsules don’t generally ‘relax’ and I’ve never been aware of any kind of ‘burn-in’ period. Speaker ‘burn-in’ (if it exists at all!) is claimed to be due mainly to physical changes in the cone suspension material and adhesives as they start to move and flex. But the key here is that speaker cones move a relatively large distance, so the suspension undergoes a large movement range.
In contrast, the diaphragm in a capacitor microphone doesn’t have a separate suspension, as such, and doesn’t move any significant distance at all — we’re talking micrometres at best! So there’s nothing to relax or burn-in, really.
It sounds to me that you’re actually describing a dynamic compression effect, which is much more likely to be down to the impedance conversion circuitry in the mic, and the characteristics of the specific valve being used. However, it could also be related to the preamp or even the A-D converter you’re using, especially if you’re working with high peak levels.