The aluminium foil element of the ribbon microphone is only lightly tensioned, causing resonance at very low frequencies. When digital recording became the order of the day, ribbon microphones made a major comeback because high-frequency transfer loss was no longer an issue. Its ability to record fast transients accurately without adding upper-range resonances became again a very positive attribute.
Articles about the ribbon microphone dates back as far as 1931. Most of the early publications were authored by Harry F. Olson who patented the ribbon microphone. Few publications besides those by Olson can be found about the ribbon microphone, but plenty of patents relating to the ribbon microphone are freely available. A selection of those patents is discussed in the paragraphs to follow.
Olson 1932
Harry Olson filed the first patent for the ribbon microphone in 1931 and the patent was awarded on 25 October 1932 with the title “Apparatus for converting sound vibrations into electrical variations”.
The patent illustrations are of very sturdy mechanical designs as illustrated in picture below:
It describes the ribbon as a relatively small item that is supported in such a way that it resembles
the motion of a particle in free air. A device of this nature is classified as a velocity microphone.
The combination of mechanical parts surrounding the ribbon is called a baffle. The size of the
baffle around the ribbon is calculated according to the highest frequency that the microphone is
designed for. The baffle should be designed so that the path length from the front of the ribbon to
the rear of the ribbon is half the wavelength of the required frequency. Picture below provides a graph
from the patent to illustrate the effect of the baffle size on the frequency response of the
microphone.
The ribbon is made of conducting material that is light in weight and that has low elasticity, for
example aluminium foil. The ribbon must not be stretched tightly between its supports. It is crimped
in order to suspend it rather loosely between its supports to promote flexibility along its whole
length. The supports are made of non-ferro-magnetic conducting material, but it is electrically
isolated from the ribbon by non-conductive material. The two signal wire leads are electrically
connected to the two ends of the ribbon. The light weight and small restoring force of the ribbon
causes its natural vibration frequency to be below the audible range. Tests that were done before
the patent submission had shown that a natural vibration frequency of approximately 10Hz
produced the most desirable results. The patent states that “When a diaphragm of small mass is
suspended in this manner its mechanical reactance is small compared to the impedance of the air.
In other words its mass reactance is negligible over a large frequency range compared to the
acoustic resistance of the air which it displaces.”
The ribbon is suspended in the air gap between two poles of a magnet in such an orientation that
its surfaces are parallel with the magnetic force lines. The magnet can be a permanent magnet or
an electromagnet. The gap between the ribbon and the magnet poles are kept to a minimum to
prevent the leakage of air around the ribbon, but the gaps must still be sufficient to prevent
frictional contact between the ribbon and the magnet. The patent suggests an air gap of 5.6mm
with the ribbon slightly narrower. The ribbon cuts the magnetic field lines while moving in the air
gap between the poles because of the sound pressure variations across it. This causes an electromotive force that is proportional to its movement. The electromotive force can be amplified
with suitable electronic equipment.
The magnetic pole pieces with its supporting structure forms the baffle. The baffle increases the
path length from the front to the back of the ribbon. The length of this path has an influence on the
response of the microphone. The paths around the top and bottom supports of the ribbon are
shorter than the paths around the baffle, but its effect is relatively small because it influences only
a small part of the ribbon. The clamping structure that secures each end of the ribbon is made of
non-magnetic material (ex. copper or brass if it is made from metal). Although it is desirable to
make the ribbon as light as possible, it is sometimes necessary to vary its thickness in order to
increase its efficiency within the particular baffle setup. The movement of the ribbon is caused by
the phase difference of the sound wave between the front and back of the ribbon. The phase
difference is determined by the distance that the sound wave has to travel around the baffle from
the front to the back of the ribbon. The greatest phase difference occurs when the path length
around the baffle is half a wavelength of the sound wave under question. Olson provides a helpful
visual representation of this effect in his patent. Picture below shows a horizontal cut through the
microphone and a sinusoidal representation of a sound wave. If the path length A-B around the
baffle is plotted as E-F on the axis of the sound wave, then the pressure difference between points
A and B on the ribbon will be equal to the sum of C-E plus D-F.
The sound intensity at the opposite sides of the baffle is virtually the same for all wavelengths that
are longer than twice the distance around the baffle. At wavelengths shorter than this, the intensity
on the approaching side of the ribbon increases and the intensity at the retreating side decreases.
The reasoning follows that the pressure difference across the ribbon is proportional to the
frequency as long as the distance around the baffle is less than half the wave length. Due to the
nature of the design the ribbon microphone exhibits very directional characteristics. Sound waves coming from an angle will produce less of a pressure difference across the ribbon. A sound wave
directly from the side will produce virtually zero pressure difference. The pressure difference can
be calculated with simple trigonometry rules.
An alternative design is also provided in the patent. The alternative design does not make use of a
ribbon, but of a lightweight diaphragm. This design is however overly complicated and will not be
discussed further.
Olson & Weinberger 1933
In 1933 Olson, in cooperation with Julius Weinberger, filed a patent that made use of a
combination of the pressure gradient (velocity) ribbon microphone and a pressure component
microphone to achieve unidirectional operation. Unidirectional operation is desirable in order to
improve the ratio of the sound source relative to the sound reflections in the room. The patent
achieved its purpose through the use of a normal ribbon (as in the 1931 patent) assembled in
series with a modified ribbon. The modified ribbon was adapted to act as a pressure microphone
by enclosing the back of the ribbon. The normal and modified ribbons are working in phase for
sounds generated in front of the microphone, but are out of phase for sounds originating from the
rear side of the microphone. This patent illustrates the influence of the baffle in the extreme case of
making the baffle infinitely large by enclosing the rear side of the ribbon completely.
Anderson 1937
Leslie Anderson added electronics to Olson and Weinberger‟s unidirectional microphone to file a
patent in 1937. The electronics made it possible to adjust the phase differences between the
two ribbons, thereby making it possible to adjust the directionality of the microphone from the
mixing desk.
Ruttenberg 1938
In 1938 Samuel Ruttenberg filed a patent to address one of the imperfections of the ribbon
microphone. When speaking close to the microphone, the low frequencies are over
emphasised because the higher frequencies are attenuated. The high frequency attenuation
happens because different sections of the ribbon move out of phase. Thus one section cancels out
the electrical current that is generated during vibration of another section. The sections move out of
phase due to the fact that the ribbon is longer than the wavelength of the higher frequency sound
waves. Ruttenberg addressed this problem by designing a special housing for the microphone
which closes up the rear of the microphone with an adjustable shutter. When closing up the rear of
the microphone, its characteristics are changed from that of a velocity microphone to that of a
pressure microphone because the sound wave does not have access to the rear of the ribbon. This
idea clearly borrows from the same principles than Olson and Weinberger‟s patent on the unidirectional microphone. The microphone‟s operation in pressure mode tends to minimise the
effect of high frequencies being attenuated. This patent clearly illustrates that tampering with the
physical surroundings of the ribbon microphone, does have a distinct influence on its operation.
Bostwick 1938
Telephone conferencing has been around for longer than one would expect. In 1938 Lee Bostwick
already addressed the problem of feedback between speaker and microphone during
teleconferencing. He filed a patent for a device making use of two ribbon microphones and a
loudspeaker. The device is constructed with a loudspeaker facing downwards onto a deflector
underneath that reflects the sound waves in a horizontal direction toward the conference
attendees. Two ribbon microphones are fitted perpendicular on top of each other, both on top of
the loudspeaker. Because of its directionality, the ribbon microphones are insensitive to the sound
waves emanating from below it, but are sensitive to the voices of the conference attendees around
the table.
Bostwick‟s teleconference solution
Olson 1940
In 1940, Harry Olson filed a patent for an improved version of the 1933 unidirectional microphone . Olson discovered that he can design a unidirectional microphone with a single ribbon instead
of the dual-ribbon method used previously. Furthermore, this microphone could be easily changed
from unidirectional to bidirectional, to non-directional operation. The method of achieving this was
to place a pipe structure (resemblance of a smoking pipe) behind a single ribbon. The pipe ends in
a labyrinth that is filled with a soft fabric acting as acoustic resistance. The pipe has shutters that can be opened or closed to achieve the desired results, i.e. shutters open allows bidirectional
operation, shutters partly closed enables unidirectional operation and shutters completely closed
constrains it to non-directional operation. This patent illustrates once again the influence of the
surroundings of the ribbon on its operation.
Anderson 1942
Leslie Anderson built upon Olson‟s 1933 patent by filing a patent in 1942 about the magnetic
equalization of sensitivity in a unidirectional microphone. It is fundamentally an improvement
on Olson and Weinberger‟s design by adding a mechanism to adjust the magnetic fields for the two
ribbons and by changing the shape of the pole pieces in such a way that it is possible to vary the
flux density of one air gap relative to the other. This patent indicates support for Olson and
Weinberger‟s idea of modifying the baffle in order to change the operation of the microphone for a
specific purpose.
Rogers 1942
Ernest Rogers filed a patent in 1942 for a microphone with selective discrimination between sound
sources . He claimed that the microphone can be used to determine the direction of a sound
source. The microphone receives sound waves approaching the microphone straight-on, but
attenuates sound waves approaching the microphone from an angular displaced direction. The
construction of the microphone is basically four ribbon microphones assembled in an X-pattern.
The ribbons are connected to a mixing circuit in such a way that the phase relationship between
the ribbons can be adjusted. By adjusting the phase relationship, the microphone can be tuned so
that the phases of sound waves from a certain direction cancel each other out, while the phases of
sounds waves from another direction will add to each other.
Olson 1946
The ribbon microphone inherently has a directivity pattern. Olson filed a patent in 1946 to
combine two ribbon microphones in a perpendicularly fashion in order to get a microphone that has
a 360° pattern on the horizontal plane. This microphone would be ideal for use in orchestras
for example, recording all the instruments around it, but attenuating sound waves reflecting from
the ceiling and from the floor. Part of the idea is borrowed from Bostwick‟s teleconferencing
microphone discussed earlier.
Anderson 1947
Anderson‟s patent application in 1947 was filed as an improvement on Olson‟s 1931 design.
According to Anderson, Olson‟s design displayed a considerable drop in output when the distance
around baffle approaches one fourth of the sound wave‟s wavelength. Anderson‟s design claimed
to have a more uniform response over the operating range of the microphone and also to have an enhanced high frequency response compared to the conventional design. This was accomplished
by mounting one or more semi-circular bands behind the ribbon. The bands provide cavities that
are resonant at the frequencies where the enhancements are deemed necessary.
Olson 1950
Olson and Preston applied for a patent in 1950 that introduced a new magnet design for the ribbon
microphone. The new design brought the magnet closer to the ribbon by doing away with the
pole pieces and using a magnet structure consisting of two large magnets, one on each side of the
ribbon, with an oval hole in each magnet. The holes in the magnets are there to reduce the path
length from the front to the back of the ribbon, thereby extending its upper frequency response.
Olson‟s motivation for the design is that the efficiency of the magnetic structure increases as the
magnetic source is placed closer to the air gap because the amount of leakage flux decreases. It is
also an attempt to minimise the effect of the pole pieces on the difference in sound pressure
between the front and the back of the ribbon.
Anderson 1954
In 1954 Anderson filed a patent concerning the magnetic circuit of the ribbon microphone. This
patent addressed the problem of the air gap between the ribbon and the magnetic structure. If the
gap is too small, the ribbon might touch the structure as it moves. If the gap is made larger, then
the sensitivity of the microphone is adversely affected. Therefore the gap must be adjusted to a
very specific distance. The patent provides no recommendations about the optimum size of the air
gap or anything about the magnetics itself. It is purely an assembly method to mount the magnetic
yoke pieces accurately and reliably in a repeatable manner.
Fisher 1965
Charles Fisher addressed the upper frequency limit of the ribbon microphone in his 1965 patent. The patent illustrates the design of a ribbon microphone with an exceptionally large
horseshoe magnet at the base with conical pole pieces attached to the magnet. The conical pole
pieces are constructed such that it does not only taper off to the top, but also makes the opening
for the ribbon narrower as it protrudes away from the magnet. The pole pieces and the ribbon are
thus broad at the end closest to the magnet and narrow at the end furthest from the magnet. The
front to rear path around the baffle at the furthest end is thus much shorter than that of the
conventional design. The patent claims good response and directivity up to 20 kHz, but no
frequency response graphs are provided to support the claim.
Royer 1999
For three and a half decades (mainly the period that the condenser microphone dominated) the US
patent office did not receive any submissions concerning the ribbon microphone. David Royer and Richard Perrotta broke the silence in 1999 with their patent to modify the original ribbon
microphone. The digital conversion of sound recordings was the motivation behind their
invention. Non-ribbon microphones produce large frequency dips and phase distortions that get
falsely interpreted by the digital equipment as valid data. Ribbon microphones do not have this
problem. Royer and Perrotta deemed it necessary to modify the ribbon microphone so that it could
be used at higher volume levels. To achieve the desired effect they designed a ribbon microphone
with magnetic pole pieces which are much wider from front to back and with the ribbon positioned
not in the centre, but one quarter of the pole width from the front of the microphone. In the original
ribbon microphone the pole pieces are tapered to become narrower towards the ribbon. In Royer
and Perrotta‟s design the pole pieces are purposely not tapered. The frequency response graph in
the patent application shows a very flat response from 40 Hz up to 15 kHz. Frequencies lower than
40 Hz are attenuated by as much as 5 dB (at 20 Hz) and frequencies above 15 kHz are attenuated
by as much as 7 dB (at 19 kHz). No graph is provided to compare the volume levels of the original
(prior art) with the new design. This design increases the baffle size slightly with the increased
thickness of the magnetic poles. Olson‟s 1931 patent only shows the frequency response between
100 Hz and 7 kHz, so no direct comparison can be made between the original and the new
designs concerning the effect of the baffle size on the frequency response.
Royer 2004
In 2004 Royer and Perrotta filed two more patents. They filed two patents for the same
microphone on the same day, but each application emphasised a different aspect of their
invention; thereby slightly changing the classification of each patent. One patent emphasises an
angled magnet structure to improve the sensitivity and frequency response of the microphone.
The other patent emphasises the specially designed transformer for said microphone. The
applicants incorporated some of the principals of their previous patent, but this microphone was
designed with pole pieces that taper narrower towards the ribbon (unlike the thicker magnetic poles
of the 1999 patent). The angled magnet design effectively reduces the front-to-rear distance
around the baffle. This is in accordance with Olson‟s rule that a shorter distance around the baffle
will produce a higher frequency range. The angled magnet design aids in the sensitivity of the
microphone because it maximises the amount of magnetic flux lines running perpendicular to the
ribbon. Totally perpendicular magnets would have the highest impact on the sensitivity, but that
would increase the path length around the baffle. The patent‟s design with magnets at an angle
less than 80º, provides a good compromise between the baffle length and flux direction in order to
get good sensitivity while also improving the frequency response. Royer and Perrotta made use of
Neodymium magnets in this design. The Neodymium magnets are much more powerful than the
magnets used in previous designs. The alloy for the pole pieces in this patent is Permendur or
Hyperco 90 which has a much higher magnetic permeability than iron. The magnetic structure has thus been improved in more than one way. Detailed dimensions for the air gap and the ribbon are
provided in the patent. This particular design also helped to reduce the size and weight of the
microphone. Much other detail is also provided, but no graphs are provided to support the claims of
the improved sensitivity and frequency response.
Royer‟s modern version of the ribbon microphone
The special purpose transformer is a complex toroidal transformer design. It has four primary coils,
is a tape-wound-core transformer, has turn ratios of 100:1 and 200:1, includes at least three
interleaved secondary coils, and/or includes at least three interleaved primary coils. The primary
and secondary windings are also interleaved with each other. A buffer circuit is also enclosed
within the outer casing and is electrically coupled to one of the secondary coils. The input
impedance of the buffer circuit is at least 10 MΩ including a field-effect transistor. The core of the
transformer consists of a “tape wound” configuration instead of compressed ferrite or composite
materials. The “tape wound” core is constructed from thin M-6 nickel iron strips that are rolled up
very tightly into the shape of a doughnut.
These two patents are both noted for their thorough description of the workings of a ribbon
microphone and its illustration of the improvements that are possible with modern technology.
Akino 2005
Hiroshi Akino addressed the design of the ribbon (not the microphone motor) in his patent
application of 2005. He patented the idea of depositing a thin layer of gold over the traditional
aluminium ribbon. Gold is also deposited to the ends of the electrode plates. The reason for the
gold plating onto the aluminium ribbon is to prevent oxidation of the ribbon. The electrode plates
are also gold plated to prevent corrosion due to intermetallic electrolytic reactions. Preventing corrosion at the ribbon-electrode-junction ensures that the impedance of the junction remains as
low as possible, thereby preventing the loss in sensitivity that would usually occur over time.
Crowley 2005
During the same year Robert Crowley filed a lengthy patent with twenty different claims. The
patent aims to enhance quality and repeatability during the manufacturing process of the ribbon
microphone. Crowley stresses the importance of the ribbon in his patent. He addresses the tradeoff between mass and resistance of the ribbon by using composite materials. The ribbon in this
patent is claimed to consist of carbon fibre nanotube filaments attached to a layer of conductive
material.
Tripp 2007
With the main focus still on the ribbon, Tripp and Crowley filed a patent in 2007 for the invention of
a polymer ribbon. Their patent makes us of a polymer ribbon that exhibits high toughness, high
conductivity and good shape-memory. The ribbon‟s corrugated structure is formed by compressing
the ribbon between two dies and then heating and cooling it to set it permanently into this shape.
The polymer ribbon is coated with a conductive coating. The polyethylene terephthalate (PET) that
it consists of does not become brittle with age because it contains no plasticisers. The invention
claims that the ribbon will not be damaged by phantom power (48VDC applied by mixing boards)
because the PET that is used is about eight times stronger than a normal aluminium ribbon.
Another advantage of the high strength of the PET is the fact that the ribbon can be made much
thinner and thus lighter, making it more responsive to low intensity sound waves.
In 2009 Akino patented another ribbon design. In any ribbon microphone design the ribbon
must be set to a resonance frequency that is below the lowest frequency to be recorded. To
achieve this, the tension on the ribbon must be very low. The corrugated form of the ribbon helps
to realise this low tension. The corrugated ribbon is manufactured by feeding the ribbon through a
set of spur gears. According to the patent the flat areas between the corrugations cause a high
mechanical resonance within the required frequency range to be recorded. To solve this problem a
ribbon was invented onto which a special pattern is transferred by means of a roller with a transfer
mould. The selective embossing of the ribbon produces a more rigid ribbon that suppresses the
sharp resonances. On the frequency response graphs in the patent it can be seen that the
specially formed ribbon suffers less from high frequency resonances (noticeably in the region
above 10 kHz), but it suffers from lower sensitivity above 5kHz.
Horng 2009
The National Chung-Hsing University in Taiwan recognised an opportunity to address the design of
the ribbon with the aid of micro electro-mechanical systems (MEMS) technology. The patent
claims that the frequency response of the microphone will be better if its sensitivity is improved and
that the sensitivity can be improved by using a lighter ribbon. The weight and the size of the ribbon
are however restricted by conventional manufacturing processes. According to this patent it is the
size and weight restrictions of the ribbon that causes the attenuation of low and high frequencies.
By applying the techniques that are used in semiconductor fabrication and in MEMS, the patent
describes the manufacturing of a smaller diaphragm and voice coil. No dimensions are provided in
the patent to judge the reduction in size and no frequency response characteristics are given to
illustrate any improvements.
Cloud 2010
With new designs for the ribbon itself being thoroughly covered since 2005, Cloud and Sank‟s
patent focussed on other aspects of the ribbon microphone, namely the magnet motor assembly, a
back-wave chamber and electronics. The patent introduces round magnet poles to the design.
The rounded poles cause sound waves to reflect of it in a diverging pattern, thereby diffusing the
reflected sound waves. Therefore the ribbon reacts to the sound with minimal interference from
sound waves reflecting off the pole pieces. The result is a flatter frequency response compared to
conventional flat magnets. No frequency response graphs are provided in the patent to support this
claim. Another part of the patent illustrates a different design with a horizontal ribbon and a blast
filter in front of the ribbon to protect it against high pressure sound waves. The same configuration
also has a chamber at the back of the ribbon which prevents reflected sounds from entering the
microphone from the rear. This design is completely different form the normal upright configuration
of the ribbon microphone, looking more like the ice-cream cone design of an ordinary condenser
microphone. The design is complemented by the addition of a differential cascade JFET amplifier
that is powered by phantom power from the recording desk. The patent claims a signal boost of
+20 dB with its JFET amplifier.
Akino 2012
In his 2012 patent, Akino claims that the ribbon can be protected against impact by short-circuiting
the ribbon‟s electric circuit when it is not in use. He claims that the back-emf generated in the
ribbon will suppress its movement significantly due to electromagnetic damping. No calculations or
data is provided to quantify his claim. Although the back-emf effect is a scientific fact, the author is
of the opinion that the electromagnetic damping will not be significant enough to qualify the patent
as anything more than a marketing exercise. This patent presents a good simulation challenge for
students to determine how substantial Akino‟s claim is.
Other Publications
Most literature that can be found on the ribbon microphone is merely concerned with patents and
advertising. The few publications that could be found with academic content on the ribbon
microphone are summarised below.
Ke 2009
Ke, et al. designed a symmetrical voice coil using MEMS technology. The aim of the MEMS
design was to increase the effective length of the coil while reducing its physical dimensions. They
succeeded to manufacture a voice coil with a 77.5 mm effective length while limiting the overall
length of the ribbon to 17 mm. Compared to a standard 50 mm ribbon, it is a gain of 55% in
effective length with a 66% size reduction.Picture below provides a drawing from Ke‟s paper to illustrate
the design. The voice coil is designed in such a way that half of the windings run through the
magnetic field (all in the same direction) while the returning half of the windings run outside of the
magnetic field (in the opposite direction). If the returning conductors were also inside the magnetic
field, they would have generated an emf in the opposite direction. By having the return path outside
of the magnetic field, the total length of the electric conductor in the magnetic field is increased
without the returning conductors cancelling the emf. The paper includes an overview of the design
and a COMSOL®
simulation of the magnetic flux density between the magnets. No frequency
response information is provided. The frequency repose of this novel design would provide
valuable information since the voice coil is fixed to the magnets and not allowed to move freely in
the magnetic field like a conventional ribbon.
MEMS ribbon
Horng 2011
Horng presented a conference paper in 2011 after further study on the miniature ribbon
microphone design that Ke published in 2009 under Horng‟s guidance. The conference paper
expands on the previous publication by illustrating more detail on the MEMS manufacturing
process and providing test results on the displacement of the voice coil, the velocity of the
diaphragm, and the microphone‟s frequency response. It was shown that the displacement of the
bare diaphragm (without coil) is slightly larger than the displacement of the diaphragm after printing
of the coil. The reason is the additional weight of the coil. The displacement of the voice coil is
almost linear with increasing sound pressure over the measurement range of 10 Pa to 100 Pa. It is
not clear what equipment was used for the displacement measurements. The velocity
measurements were done with a laser Doppler vibration system. Although the displacement tests
showed a minor difference, the velocity tests showed a more prominent difference between the
bare diaphragm and the diaphragm with coil. The measurement results of displacement and
velocity cannot be directly compared since the displacement measurements were performed from
10 Pa upwards and the velocity measurements were done at a sound pressure level of 1 Pa. A
displacement of 0.5 µm was measured for the diaphragm (including coil) at 10 Pa. At a pressure of
100 Pa a displacement of 6.0 um was measured. The velocity at 1 kHz with a sound pressure of
1 Pa is only 1.3 µm/s. This provides the reader with an understanding of just how small these
measurements are (and thus prone to measurement errors). Horng‟s frequency response
measurements show an increasing sensitivity from 100 Hz (-64 dBV/Pa) up to 20 kHz (-
50 dBV/Pa). It is worth noting that the microphone‟s sensitivity is not dropping at high frequencies
(> 10 kHz) as one would expect from a closed baffle design.
Kwon 2016
Kwon and Honorato performed out of the ordinary tests with a ribbon microphone at the Naval
Postgraduate School in Monterey, California. They tested the ribbon microphone as an underwater
transducer. They observed that its directional characteristics approach that of an ideal dipole with
its two lobes becoming narrower with increasing frequency.
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