Returns on Magnetic Investment and the Measurement of Microphone Noise with a Fish Scale

By Cliff Henricksen

Permanent magnets are the fountain of performance for a lot of audio devices and for pretty much all loudspeakers as we know them, until someone invents something better (working on it…).

In professional high-frequency compression drivers, the “B-field” (magnetic flux density in Gauss or Tesla) translates directly to the transduction coefficient for that driver.  That would be how much force you get for a given amount of electrical current input.  Unfortunately, once you get near a flux density of 1Tesla (10,000 Gauss) at the driver’s voice coil, the efficiency or sensitivity gain is almost nonexistent, except for the very high end.  A higher flux density basically extends the break frequency of the high end, but not the low end or the (flat) region in the midrange.  So you get a little more output in the upper 1.5 octave.  But once you are in the >1T region (some compression drivers approach 2T!), you have to use a way-disproportionate amount of magnetic energy (thus way-disproportionately-large permanent magnet) to overcome this “saturation”, where the steel magnetic conductors look more like air than steel.  So in professional compression drivers, you don’t get a very good return in overall performance with a large investment in magnetic field strenth, especially above 1T in the gap.  Actually, it’s a poor investment, especially in that you can typically cut the costs of any driver’s magnetic strength and save a lot of money by just providing a little extra HF gain in the system.  Return on magnetic investment is pretty poor in compression drivers.  (Unfortunately for the pro audio business, little gains in measured high end are part of a sales mechanism that is largely driven by data and not by common sense.)

In contrast, the return-on-magnetic-investment in the RM1 is huge.  The strength of the B-field gives more level in all ranges.  This means that investing product cost in magnetic performance has dramatic returns in product performance.  And it’s not linear either;  the voltage developed in the RM1 is directly proportional to the strength of the B-field, so the output is not a 10 log effect; it’s a 20 log effect (voltage squared!).  For twice the B-field level, you get 6dB more output at the ribbon.  So our  investment in magnetic circuit cost (machined pure iron and pounds of NeO) gives RM1 customers a performance benefit with a big lever.  And more output means less gain needed for acheiving a useful signal level at the (relatively noisy) downstrem recording chain.  This means less noise at any sound level input. 

The fact that the RM1 weighs on the order of 8 pounds means that a lot product cost has been invested in magnetic performance.  Simply, more magnetic energy comes with more magnet volume.  And, to “contain” all that energy and focus it on where the ribbon “lives” (in its magnetic air gap), the magnetic circuit has to be commensurately large and heavy.  So the weight of the RM1 translates directly to our decision to dedicate product cost to magnetic efficiency, in order to combat noise, the age-old nemesis of ribbon microphones.  The magnetic circuit in the RM1 has been optimized with a modern FEA (finite element analysis) program, so it is actually as lightweight as it can be, making most efficient use of the permanent magnetic source energy of its two (expensive) Neodymium magnets. 

We affectionately refer to the RM1 as “the Shelby Cobra of Microphones” for this reason:  The two massive blocks of Neodymium in the RM1 are not unlike dropping a NASCAR V8 engine into a “polite” little aluminum-bodied English sporting motor car, resulting in world-beating performance.  It’s actually a frightening bit of magnetism, albeit all nice and politely-contained in its sculptured package.

You know how the weight of any amount of gold translates directly to how much it costs?  It’s pretty straightforward.  You pay more and you get proportionately more of this rare element.  The same is true of magnetic ribbon microphones.  If good magnetic design is done, the weight of the microphone translates to how much the designers invested in both magnetic performance as well as product cost.  So, in a way, you can almost measure the inherent or “native” noise any ribbon microphone will have by looking at how much it weighs.  Want a quiet ribbon mic?  Go to the microphone store with a fish scale.  Ignore comments from the sales guy.