microphone feature
30 YEARS OF
MICROPHONE
DEVELOPMENT,
AND COUNTING
In a recent Line Up article
Patrick Morvlyth challenged a
statement by the retro
enthusiasts that the
microphone had reached the
peak of its development 30
years ago. CHRIS WOOLF MIBS
adds some flesh to the bones of
Morvlyth’s assertion.
he microphone always was a
precocious child. It was born some
decades before its partner in crime,
the electronic amplifier, and by
adolescence (1924) it was known in all the
guises we recognise today. Electrodynamics
(including ribbons – one of the later ideas),
electrostatics (the ‘condenser’
microphone), electroresistive (the carbon
microphone), and even the piezo electric
‘crystal’ microphone – all had been
demonstrated and were being employed
usefully.
A child prodigy might have run out of
steam at this point but the microphone has
had a succession of demanding masters to
spur its development on. Besides the
humdrum requirements of telephony
which provided the mass market then as
now, there was also the glitzy world of
entertainment. Radio, cinema and recorded
music also needed high quality, reliable
microphones. It is, perhaps, a salutary
reminder that it was the users just as much
as the manufacturers who paid for the
T
research laboratories that discovered and
documented the fundamentals of
microphone theory, and their translation
into the tools of the trade.
The next major spurt in development
came with the rise of television and the
expansion of the pop-music industry. These
phenomena, centred on the 1960s and 70s,
created a demand for smaller, more
cosmetically appealing and better specified
microphones. After the economic restraints
of a World War, a great many elegant and
innovative electronic and manufacturing
advances appeared. It was also a period
which, in hindsight, is seen as something of
a golden age in the modern music industry,
when cultural invention exploded but crass
commercial interests had not yet managed
to wrest complete control. What better
moment for the retro-mafioso to pick as the
pinnacle of microphone development?
Early Retirement?
So has our child prodigy really slipped into
premature old age? Given that microphone
transducer forms have been so thoroughly
explored, and that modern devices achieve
a better frequency response and dynamic
range than the signal chains they feed – can
there really be any more revolutionary
changes? Well, there has certainly been
continued evolutionary development –
and much of it brilliant in the restrained
fashion of the impressive academic.
Some microphone developments have
been peripheral, but by no means
unimportant. For a start, in the 1970s
neither the XLR nor phantom power had
become the de facto standards that they are
today. With proprietary connecting leads,
The original
AKG C414
swapping mics around
was significantly more
difficult and thus direct
comparison harder. The
lack of universal piped
phantom power often
persuaded many users to
work with dynamics,
even when they were not
an ideal choice. Moreover, some of the
most profound advances might have
entirely escaped the casual user because
they dealt with internal minutiae that meant
little in an advertising brochure and only
revealed their benefits after sustained
listening.
LINE UP Feb/Mar 2005
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New Transducers
The sine qua non of inventors is that they
are never satisfied with what currently
exists, so there is continuing research into
new forms of transducer – including onepiece semiconductor microphones with
silicon diaphragms – but so far only two
techniques have shown much promise.
The first is optical sensing of a
conventional diaphragm using a laser.
Already developed as a working,
commercially available microphone – and
of great use in environments that demand
intrinsically safe equipment – this form
doesn’t look likely to wander into the high
quality market. The wavelength of light
makes it rather a coarse tool for detecting
the minute diaphragm disturbances caused
by sound.
The other hopeful is the diaphragmless
‘Microflown’. In reality, this is a very small
hot-wire anemometer which detects the
heat-transferring flow of air particles caused
by pressure change. It is a true velocity
microphone and essentially bi-directional
with a frequency response that extends
down to extreme LF. Currently it is being
produced mainly as an industrial research
tool but the idea shows some promise . For
the time being, though, the moving coil,
the rare but not entirely forgotten ribbon
and, above all, the capacitor transducer
continue to reign supreme.
The classic capacitor – ‘condenser’, if
we must – design does have one important
new variant since the 1980s. As long ago as
1935 the Braunmuehl/Weber patent gave
details of the dual-diaphragm capsule form
widely used in studio microphones. It also
mentioned the option of the inverse
arrangement – the single diaphragm with
dual-backplates – a form we now call the
‘symmetrical’ capsule. It was almost 50
years later that Sennheiser produced the
MKH 20, 30,40, 50… range of RF capacitor
microphones using this single-diaphragm,
dual-backplate concept, the symmetrical
damping of which ensures exceptionally
low distortion.
Another major change in electrostatic
capsules has been the rise of the electret.
The principle of a permanently charged
electrostatic microphone was explored as
early as 1928 – that precocious child again
The ‘capsule’ of a Microflown
microphone
22 LINE UP Feb/Mar 2005
“Some of the
most profound
advances
might have
entirely
escaped
the casual
user.”
– but until recently it was deservedly
reviled. The early designs used unstable
materials with a short life span, and the
performance of the heavy diaphragms
made from electret films had few of the
attributes of the ‘true’ capacitor design.
However the modern formulation of much
more reliable charged films, and their use
on the backplate instead of the diaphragm,
has meant that electret capsules
can now be built in very similar ways to
conventional DC capacitor designs.
Indeed, even dual-diaphragm
variable pattern electret
mics are now available
(the AKG C4000,
for example),
and it can
come as a
shock to
discover
The sophisticated
internals of the latest
high-tech AKG C414 XLS
how many ‘condensers’ are actually
permanently charged – the widely revered
DPA 4006, for example. In fact, DPA and
AKG are both renowned for having moved
this technology forward.
The simplicity of the electret capsule,
which gets rid of all the paraphernalia of
DC polarisation or RF excitation, allows for
many new arrangements. Minute capsules
enable almost invisible personal
microphones, and low power requirements
permit battery-powered designs. Electret
microphones can also be made highly
resistant to moisture, and their robustness
(as well as their inexpensiveness) explains
why they are used in far greater numbers
than any other transducer – even if most of
them are in telephones.
The more detailed structure of
transducers has also changed as a result of
new manufacturing processes and
materials. Dynamic microphones now
typically use neodymium magnets to give
much higher field strengths and thus nearly
double the output voltage for a given sound
pressure. Nor have diaphragms remained
unchanged. The simple membrane needs
considerable help in controlling resonances
to give a flat frequency response without
ringing, and though the subject of damping
is rarely something that advertisers can get
excited about, it is central to the
microphone’s performance. New moulding
techniques allow extremely refined and
accurate ridges, corrugations and shaping
to be incorporated into diaphragms to give
a much more controlled response than was
previously feasible.
Improved Electronics
Behind everyelectrostatic transducer there
lies a head amp or impedance converter.
Thirty years ago it
might have been a hissy,
mechanically noisy thermionic
valve – although the relatively
new FET was already preferred.
Between then and now the FET has
become vastly more linear and far quieter.
If the valve is still used it is only for
nostalgic reasons but, amusingly, often in
conjunction with its bête noir, the bipolar
transistor. Although output transformers
can still be found, the transistor-driven
output – which provides much greater
peak voltages across the full frequency
range without distortion – has enabled
capacitor microphones to twin high
sensitivity with high SPL capability. Many
designs can drive levels of over +10dBu
while still maintaining less than 0.5%
distortion, and dynamic ranges in excess of
130dB are quite feasible. Transformers are
also sensitive to electromagnetic induction
and, since eletromagnetic interference is an
ever-increasing bane and the acceptable
level of noise floors is diminishing, use of
the vulnerable and expensive wound
component is only likely to decline further.
Not only are the electronic components
better quality these days, but they are also
considerably smaller. Surface mount
devices can be fitted closer to capsules
(thus reducing critical lead lengths) and
also allow smaller microphones. The
lavalier of 30 years ago was a giant
compared to 21st century versions –
althouth a modern personal, with its
unbalanced output, only needs to house a
single FET behind the capsule.
The compact microphone, epitomised
by the Schoeps CCM range, fits an
impedance buffer, a bipolar balanced line
driver, a DC-DC converter, and even a cable
connector into the barrel space of only
20x25mm. The value of this compact
design is that both the visual and the
acoustic footprint are minimised.
Now that we have amplifiers that do
not impose significant noise or overload
restrictions, it becomes logical to apply
their benefits to electrodynamic
microphones too. Royer is now selling a
ribbon microphone with a phantompowered impedance converter (working in
the reverse direction to the ones we are
used to – increasing the ribbon’s low
output impedance) in place of the
transformer, to give ‘ribbon’ qualities allied
to typical capacitor microphone output
voltages. Blue has also brought out some
moving coil microphones that use buffer
amplifiers to isolate the dynamic element
from the vagaries of long mic lines or
serious load impedance anomalies.
Digital Circuitry
Thirty years ago digital technology barely
existed but it has swept through the audio
world with stunning speed, slowing only at
the very margins of the acoustic
transducers – microphones and
loudspeakers – but even there it is making
inroads. Digitally interfaced microphones,
or ones that incorporate digital circuitry,
would have been impossible to build at all
until very recently, and only within the past
handful of years have designers been able
to add low power ADCs, DSP and PIC chips
(the ubiquitous embedded microcontrollers) that can run economically
enough to be used in microphones without
an AC mains power cable.
Although the digital microphone has
not taken the world by storm as yet, it is
well developed and has some significant
benefits. The enormous dynamic range that
a capsule is capable of transducing can
challenge the best of analogue cabling and
preamps. Using 24 bit digital words 140dB
ranges can be delivered reliably through a
signal chain. We may not need to use that
range in a mix, but there is a great
24 LINE UP Feb/Mar 2005
attraction in having it available as a source
without it ever having been compressed by
either automatic or manual gain control
systems. Nor are digital outputs restricted
to dedicated audio cabling – beyerdynamic
has just introduced a microphone that
connects directly to recording software in a
PC or Mac via a USB connection.
Digital technology is also now used in
apparently conventional microphones –
like the new AKG C414B XLS – in which
the increasingly complex switch functions
are run by a micro-controller. The
mechanical components in a microphone
and the open switch ports in the case are
areas that challenge reliability, and AKG’s
novel approach is far from gimmickry.
Digits play an even greater part in
developments such as the Audio Technica
AT895 microphone. This takes the
common principle of using acoustic delays
The
Microtech
Gefell
KEMı970
array
microphone
“The digital
microphone... has
some significant
benefits.”
with an array of capsules to make a
directional microphone, but substitutes
digital delays instead. These can have a flat,
tailored or even a dynamically variable
frequency response, which is impossible in
the acoustic domain. This technique is an
extremely powerful one and is destined to
be more widely used. The voice-tracker
array used for some computer microphones
mimics this idea, and a related one
(Trinnov) has already been employed to
produce high order directivity
microphones for surround sound from
omni arrays.
Mic Arrays
Indeed the use of arrays has been one of
the least commented-on developments in
recent years. The single capsule ‘first order’
configurations have all been known for a
very long time – the only variant that has
been ‘discovered’ in the last few decades is
the boundary layer (PZM) microphone with
its hemispherical pattern. But arrays, either
as discrete entities or integral to a single
body shell, have produced some very
interesting new microphones.
On the larger scale, the dummy head,
sphere and similar baffled ‘microphones’
used for stereo recording – and specifically
for binaural work – are easily noticed. They
mostly date from the late 70s and still have
some strong adherents, although the
technique doesn’t seem to be of universal
merit.
The sharp-eyed might also pick out the
various MS and XY stereo microphones,
almost all of which are younger than 30.
Much harder to discern are the designs
which use multiple capsules (often, but by
no means always, electrets) to produce
unusual mono polar patterns or frequency
responses.
The AKG D202 was an early forerunner
of this approach – with a separate HF and
LF dynamic capsules – but the idea has
been considerably refined by many
manufacturers. The previously mentioned
AT895 uses five capsules, and the
Microtech Gefell KEM970 uses eight in a
type of line array to give markedly different
vertical and horizontal polar patterns.
Sanken has also used arrays inventively in
An outline ‘timeline’ showing some evolutionary markers
many of its shotgun microphones – the CS-3e, for
instance, uses three capsules to combine tight
directivity with a much more even frequency response
than a short rifle can usually aspire to.
No Grey Hairs
Not all microphone developments are in the public
arena, but even this short list should be enough to
convince most people that while the microphone may
have a degree of maturity, it has become neither staid
nor grey-haired.
Thankfully, my task here has been to look
backwards over thirty years rather than forwards. That
would be a great deal harder. It would also require an
eye not only for technical invention and direction, but
also for fashion. Sales to the ‘Music Industry’ (MI)
market – primarily the home studio – will continue to
provide a great deal of the funding for the development
of new microphones, and even the most esoteric
research departments will keep one hand ready to
massage this sector’s particular foibles. Regardless, the
microphone is likely to continue to stride ahead, and in
2035 we can expect it to look – and very likely
sound – quite different to a 1975 model.
Thanks
The author would like to thank Martin Schneider of
Neumann and Jackie Green of Audio Technica for
adding their extremely valuable suggestions for this
article.
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