The professional audio market tend to publish specifications which (as often as not) look very ordinary. The classic case used to be open-reel audio tape machines (RCA, Ampex, EMI, the Australian Rola/CEI (see CEI MkIV at left), Stellavox, and so on) which typically had quoted frequency responses of around 50Hz to 10KHz at 7.5 inches per second. I measured some of these at various times, and they actually went from about 28Hz to around 21-23 KHz at the -3db points. Interestingly - they could record 30 Hz at full level (0 VU) and play it back cleanly. (Why "interesting"? Read on ...)

High-end "Hi Fi" open-reel recorders, on the other hand, had typical quoted responses of 30Hz to 22 KHz (a much wider specification than the 50Hz - 10KHz of the professional gear). I also measured some of these at various times, and they were generally close to these specs (when new). But when I tried recording full level down at 30Hz on these - it not only sounded horrible, it also looked horrible on an oscilloscope. So this "30Hz low end response" really wasn't much use in reality (unless of course you were happy with 40% distortion!)

When I discussed this particular point with various people, the general concensus was that this difference in low frequency performance stemmed from the substantial differences in the tape heads. The professional machines used heads with a far larger cross-section on the pole-pieces, ie: they were simply much bigger (presumably to handle this bottom end cleanly).

Similar comments applied to their noise figures. "Hi Fi" machines and professional alike both quoted around 55db signal/noise ratio. But no prizes for guessing which one was really the quieter. (Noise measurement is of course one of those areas where one can push the interpretation of the measurement method to extremes in order to "fiddle" the results.)

So even specifications are tricky to compare when one piece of equipment is "Hi Fi" and the other is professional. It becomes a real "apples and oranges" situation.

This sort of difference is also evident with microphones. Some mid-range (semi-pro) brands will quote a response down to 20 or 30 Hz. In the professional market, Neumann condensor, RCA, Western Electric and others also quote similar low-frequency roll-off figures. But the professional makes (such as the Neumann U 89 i shown on the left) actually respond, accurately in this region and below. Many condensors have a usable sub-sonic response and require a high-pass filter (typically with a 20Hz or higher roll-off) to be switched in to prevent sub-sonic building rumble in the 1-2 Hz region from overloading the pre-amplifiers (from nearby traffic or local air-conditioning). The high-frequency limit is typically around 20KHz. These professional studio microphones (Neumann and AKG condensors) also have exceptionally good noise performance (typically around 70db) and very high overload margins (SPL of 134db for 0.5% THD), so they aren't at all fazed by a trombone blasting away right next to them - the signal stays clean. Of equal importance, these professional pressure gradient microphones achieve their frequency response figures at distances of a metre or more from the sound source. The "Hi Fi" brands usually only manage to achieve the stated frequency response with the sound source at close range.

These sorts of important differences won't necessarily be obvious from just perusing the main specifications. As one obvious example - the frequency response of a typical good quality "Electret" unit is around -3db at 40Hz and 12KHz. But even more importantly, the S/N ratio of such units rarely exceeds 30db or 40db, and this alone makes them virtually useless for serious studio work.

Incidentally, good quality dynamic microphones can achieve far better noise figures than Electrets, typically being between 50db and 60db at moderate (normal speech) sound-pressure levels. But condensors still eclipse even dynamics by (typically) 20db or 30db!

One of these days, I'll get myself a real Neumann ... heavy sigh ... (well, I can dream, anyway ... :-)

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Last update: Tuesday 20th March, 2001