The Polyribbon Story
I've always admired ribbon microphones, having built them in my high school years from gum wrapper foil,
surplus magnetron magnets, and filament transformers.
Much Later after research and design engineering stints at Shure, Electrovoice, and other places I wanted
to add a ribbon mic to my new microphone company. But I wanted something a little different from a standard figure
I was fascinated with the multi pattern/non figure 8 ribbons of the past: The RCA 77 A, B , C, D , and DX....
the RCA KU and BK series, The RCA Starmaker pressure ribbons, and the Western Electric 639 and 670.
The multi pattern mics were pretty good for the day, but lacked the smooth response and full audio response
that I would want today. These designs were conceived with time domain and phasor analysis, but I sought
out to find ways to improve performance with modern network and computer methods. And listening.
thing...most of them were made for and had a 10kHz bandwidth. This made
sense at the time since there is a fundamental relationship between
output level, noise, and bandwidth. Since very few transcription and
means went past 5-10kHz and were relatively noisy, sacrificing the upper audio octave to gain more output
made perfect sense. It certainly doesn't today.
The first non figure 8 ribbon microphones had two motors...a mass controlled ribbon for pressure gradient,
a resistance controlled ribbon for pressure. Mass control means
that the largest reaction force is inertia, and resistance control
means it's energy dissipation or transport. Both those conditions
respectively are required for flat response in a system where the
electrical output is dependent on velocity of a conductor in a magnetic
As has long been known the sum of those two ribbons can make cardioid or any other first order gradient pattern.
there were some fundamental problems with this approach. The biggest
was this: the omnidirectional pressure section wasn't really omni at
all frequencies. Diffraction tended to make off axis response quickly
drop off above about 2 kHz, so the cardioid and similar settings "grew
a back Lobe"
or tended to go figure 8 above that frequency. An
ingenious solution was found though...realizing that the pressure
element became unidirectional
at high frequencies the pressure gradient ribbon was simply rolled off there...using the pressure ribbon only.
little thought shows that the sum of two transducers at low frequency
but only one at high frequency tended to make a bass shelf effect
of 6 dB in cardioid for example. This was dealt with either with a
crossover network or restricting the frequency range or both.
was done quite cleverly in the dynamic/ribbon Western electric 639A
using a series inductor and a capacitor on a tertiary transformer
to allow reasonable capacitor values:
This made much better pattern control, as long as the frequency range was restricted particularly on the figure 8 setting.
There was a compromise in vertical pattern, because the two elements are spaced apart.
RCA did a similar thing earlier on the 77 A, B, and C and KU2 by acoustical filter means.
Dr. Harry Olson later decided to eliminate the issue by using a variant
of Ben Bauer's Shure development of the single element
unidirectional microphone, the model 55 Unidyne.
This was commercialized as the RCA 77D and 77DX. In this case only a single ribbon was used, but it had a rear port
opening that formed a constant group delay low pass filter to sound
presented to the back of the ribbon.
A variable shutter was
fitted to the opening to allow mechanical selection of polar patterns.
Fully opened resulted in a figure 8 pattern
and fully closed an omni pattern, with cardioid and the like in between.
The result was an Iconic microphone.
general the patterns were better than the two ribbon approach, but the
labyrinth (we'll talk about that later) and rear port assembly did tend
to create a longer front to back path and thus reduced high frequency
response particularly in the figure eight setting. Again, not so much a
problem in a ten kHz world...and at lower frequencies it raised
sensitivity. At non figure 8 settings it was less of an issue....the
rear port low pass filter cuts off at high frequencies, resulting in a
pressure interference unidirectional microphone.
Dual ribbon... required crossover network and restricted frequency range. Fairly poor vertical patterns due to element spacing.
ribbon...no crossover. No need to restrict frequency range per
se, but the added hardware on the back of the ribbon increased front to
and did restrict range on the figure 8 setting. Good vertical polar pattern.
solution quickly became obvious to us. Use an open unencumbered ribbon
motor for the figure 8 setting, and use a inertance resistance phase
back port on a second ribbon that is electrically selected.
Close the port on this section for omni. Only one element is active at
a time, so better vertical patterns are achieved as well.
gives the advantages of both types, with none of the
disadvantages. It would require a more complicated system
consisting of electrical
switching along with mechanical valve opening and closing. But we knew this was the way to go, and proceeded with it.
What is it? the RCA 77s had it as well as the KU2 skunk, KU3, BK5B, and Starmaker.
It turns out that any velocity sensitive pressure microphone like a dynamic or ribbon has to be resistance controlled...that
is the largest reaction force needs to be a dissipative friction like
force. This was done early on at Bell Labs by having a dynamic mic
diaphragm pump air through a layer of silk fabric or the like between
it and a compliance chamber, resulting in the famous eight ball mic.
It's still done today on pressure microphones, and in the case of
variable D as developed by Electrovoice unidirectional mics as well.
works quite well for heavy dynamic mic diaphragms....their mass
resonating with the compliance chamber can easily be tuned to mid range
frequencies...in the range of the geometric mean of the desired frequency extremes.
low tuned ribbons a compliance chamber would have to be very large...a
patent of the day for such a thing mentioned a15 cubic inch volume!
Harry Olson seemed to realize this pretty much from the beginning and
chose another method for creating a stable, known resistance: a transmission line.
infinite lossless transmission line presents a purely resistive input
impedance. In the acoustic case the resistance is a simple inverse
of cross sectional area of a pipe. The right loading for the ribbon was found to result in a pipe of 6-9 mm diameter.
a fairly small diameter pipe could be coiled up inside a microphone to
provide resistance loading for the ribbon. To be the predominant
reaction force of the ribbon it had to be greater than the mass or
spring force reactance of the ribbon and it's air load, but not so much
that it reduced overall output
by "driving with the brakes on".
it was called the labyrinth. In the 77 it was created either as stacked
spiral plates or a phenolic block drilled with many interconnected
In others, like this probe microphone (the Starmaker), it was part of the body casting:
it of course was not infinite in length. In a reasonable size about a
meter was the maximum practical length. But as we know an open pipe
and the impedance is neither constant nor resistive. Organs, horns, and
other musical instruments rely on this. Dr. Olson had to suppress
this resonance, and he did so by stuffing the tube with fibrous sound absorbing media resulting in a lossy transmission line.
This interestingly gives the line a reactive impedance but does supress reflections. We did find that corners where the labyrinth turned around abruptly
caused some pretty bad resonances, particularly in the RCA 77 series,
that created a very large midrange response hump as shown.
We were able to correct this very effectively with geometry changes and some acoustic networks.
we have great advantage today with modern neodymium magnets. Fairly
large flux density is required to get good output from a ribbon
microphone. In past times high flux densities were possible only with
large heavy magnetic structures. We were able to get high flux with
much smaller magnets and a FEA optimised magnet structure.
Voicing and frequency tailoring
We used several methods to get the desired wide smooth frequency response.
carefully controlled the diffraction peak created by the transducer
assembly. We used wave plates as RCA and BBC/Coles did to tune the
extreme high end. This is particularly effective as the boost provided
gradually diminishes off axis, where the pressure gradient ribbon
naturally has better HF response due to the decreased front to back
distance. This maintains good off axis response/
Body reflections are always an issue. We use carefully placed resonant traps to eliminate them.
controlled ribbons need some acoustic damping to prevent large peaks at
harmonics of the ribbon fundamental resonance.
Doing so tends
to reduce bass response and sensitivity however. To recover that we use
a low resistance baffle around the ribbon.
It's transparent at
high frequencies where the reactance of the predominately inertive near
field dominates, but at low frequencies
it becomes a significant portion of the near field path impedance and acts as an obstacle, increasing output.
ribbons must be formed accurately and consistently for predictable
performance. To achieve the performance we want the ribbon to pole
piece gap is only 0.1mm. We developed a precision preloaded ball
bearing corrugating machine to insure this. Several other ribbon
microphone firms bought these machines from us for their
The ribbons are then tuned off microphone as they are on field replacable cassettes. A special fixture allows precise alignment
and tuning testing with an electrical impedance meter.
The labyrinth stuffing operation is monitored with an acoustic impedance meter.
Finally each Polyribbon Has individual response testing with NIST certified Bruel &Kjaer labaratory reference microphones
and the results are printed and included with each microphone.
What we've tried to achieve is a modern manefestation of the classical variable pattern ribbon microphones, with condenser like
frequency range and very smooth response but retaining the warm quality ribbons always had.
L M Watts Technology