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Why is there a silvery coating inside a vacuum tube?
We listen to a lot of recordings here at Audio Masterclass and I for one am a long way from being bored by the rich variety of sound textures available from different microphones, and microphones used differently. But sometimes we receive a recording that is 'off the scale' in the richness of its texture.
Take a look at the waveform above. What do you notice? Nothing special? Then look again...
Notice how the negative-going side of the waveform extends further than the positive-going side. This is evidence of asymmetry between positive and negative somewhere in the signal chain. (Other than the highly unlikely possibility that the person speaking has an asymmetrical voice!)
The microphone is a Bock 251, which is a vacuum tube emulation of microphones of a bygone era. Take a listen...
I'm guessing that the speaker actually does have quite a rich voice, but the extra layer of fruitiness caused by the microphone takes it into a realm of sound texture that we don't often hear. This kind of texture might be suitable for radio, or for a film trailer. It might become annoying during the longer duration of an audio book.
One likely cause of such asymmetry is quite easy to explain. Some audio circuits are inherently symmetric between the positive-going and negative-going sides of the waveform. But an amplifier with a single active device is inherently asymmetric. The incoming signal must be 'biased' to the mid-point of the device's linear operating range so that it can handle negative-going and positive-going peaks as similarly as possible. If the bias is shifted too much towards the negative or positive, then either the negative-going or positive-going peaks, respectively, may encroach into the device's non-linear region.
If the cause is in the mic, then there are three possibilities. Firstly, perhaps the designer wanted the asymmetric sound. That's fine, it gives us another toy to play with in the studio. Or perhaps the individual mic is not in correct calibration. Or perhaps the individual tube inside this mic isn't performing to specification.
So if you like this sound, and the mic was designed this way, it would definitely be a good tool to have to hand. Or if it's just a one-off occurrence, the Bock 251 that you buy might be less fruity than you expected.
Of course, it is entirely possible that the asymmetry arose somewhere else in the signal chain. My feeling is however that the microphone is the most likely cause.