Inside Audio: David Moulton

How Microphones Have Changed

After the end of World War II, the best microphones were the ones used for live broadcasting. These were tube-amplified condenser microphone designs intended for generic recording of medium-to-large ensembles in reverberant spaces (recorded in mono – often just a single microphone). Except for announcers, close-micing was the exception rather than the rule. The microphones of choice (such as the Neumann U47 and AKG C-12) were distinguished by a combination of factors – a sweet and warm musicality to their timbre and a wonderful ability (as seen in retrospect) to resolve low-level details, essential for the making of live orchestral recordings.

By about 1975, these classic designs evolved into solid-state designs, such as the Neumann U87 and the AKG 414, which are still with us today. At the same time, the explosion of recorded popular music, rock-and-roll, as well as multitrack technology led to a variety of new design topologies. Some of these came from sound reinforcement, others from communications, and some simply arose as a function of new materials and technologies (the electret condenser, for instance).

The primary developmental constraint for these new microphones was the need to cope with extremely high sound pressure levels (150 to 160 dB SPL!) with consistency and reliability, arising from the almost universal abuses of close-miced high-intensity rock-and-roll performances.

THE GREAT DIGITAL DIVIDE

Beginning a few years after the advent of digital recording and the general adoption of the CD, microphones once again began to evolve. Analog recordings had a noise-floor (at best) of approximately -70 dBV, and so the self-noise of microphones had not been a big issue. Similarly, the move away from minimalist recording of acoustic ensembles had reduced our awareness of and desire for low-level resolution of complex multisource sounds (essentially, we stopped recording such sounds for a while, preferring to close-mic everything – even digeridoos!).

Beginning in the 1990s, the reduced noise-floors of digital recording media (all the way down to about -90 dBV for 20-bit recordings) led to an increased sensitivity to noise limitations of mics and mic preamps, and started to raise the bar, once again, for microphone performance. At the same time, the growing use of measurement microphones for music recording (the B&K 4000 Series microphones are directly descended from such mics) also illuminated the increasing possibilities afforded by an enlarged and scrupulously maintained audio window.

So, during the ’90s, we have seen a steady improvement in microphone self-noise. A microphone that would have been highly regarded 20 years ago with a self-noise equivalent to approximately 25 dBA SPL is now considered mediocre in that regard. Microphones with self-noise around 15 dBA SPL are OK, while mics with self-noise below 10 dBA SPL represent the current state-of-the-art.

At the same time, manufacturers have begun to notice that "flat" isn’t necessarily "great," and have begun once again to develop "microphones as paintbrushes." This has resulted in a plethora of modern "tube" and ribbon microphones, whose alleged virtues of warmth and musicality replace their sales arguments for accuracy and flat frequency response.

One fascinating tangent to all this is the arrival of retro microphones from the Iron Curtain nations emerging from their Cold War isolation. The Microtech Gefell is such a microphone, as are the Octava microphones from Russia. These mics are designed and manufactured, using techniques and materials held over from the 1960s, when the Cold War closed off the Western World (and multitrack pop recording markets) to them. Buying an Octava is a little like buying a 1957 Chevy made in 1999 after being in continuous production, but not much development, for the last 40 years.

WHERE ARE WE TODAY?

These days, we’ve seen a real drop in the noise-floors of microphones and mic preamps, in an effort to keep up (er, down) with the noise-floors now available from hi-res digital formats. The result? Source recordings are, at their technical best, far more transparent and free from obvious electronic artifacts than ever before, as well as more revealing of acoustical limitations and deficiencies.

Once again, an effort appears to be underway from manufacturers of condenser microphones to make their mics yield compelling detail at very low signal levels. The diversification of recording styles and genres has also fueled this.

And we haven’t given up our high sound pressure levels, so we’ve had to develop microphones with some really stellar dynamic ranges – finally, at 130 dB, almost equal to what the human ear is capable of perceiving.

Manufacturers have an increasing awareness that the way a microphone "sounds" is probably more important than any of its other virtues, and they seem to be paying increased attention to the nature of this often ineffable subjective quality.

The best directional microphones are getting a lot better at achieving decent frequency response off-axis. This results in greatly enhanced stereo and surround recordings and imaging, as well as performance stability.

WHAT DOES IT MEAN?

If we were all to start over today – buying all our microphones from scratch – we would find that there are a bunch of exciting new microphones out there, from both new and old (and new old) manufacturers. These microphones can yield remarkable sonic quality and an enhanced audio window. Also, happily, some of them begin to give us the kind of musical warmth and sweetness we got so hooked on in the ’50s and ’60s. It’s really pretty exciting and gratifying.

Thanks for listening.

Dave Moulton would like to thank Tom Bates for his insights and suggestions for this article. You can complain to Dave (not Tom) about anything, at his Web site: moultonlabs.com.