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Shure KSM8 Dualdyne

Dynamic Vocal Microphone
Published May 2016
By Hugh Robjohns

Shure KSM8 Dualdyne

With its unique dual-diaphragm design, is Shure’s latest offering the greatest advance in dynamic mic technology since the Unidyne?

There is a widespread popular belief that the microphone is a ‘mature technology’ that hasn’t seen any significant development since the studio heydays of the 1950s/60s/70s (delete according to your favourite musical decade!). This is, of course, complete nonsense: new developments are still appearing quite regularly, bringing substantial improvements in quality and accuracy, reliability or manufacturing costs. Occasionally we even see some completely new microphone technologies, with yet more hovering tantalisingly over the horizon.

One of the very latest advances was introduced by American microphone manufacturers Shure at the Winter NAMM show at the start of this year, in the form of “the world’s first dual-diaphragm dynamic handheld microphone”, the KSM8.

Obviously, handheld dynamic mics are nothing new, and neither are microphones with dual diaphragms: every switchable-pattern capacitor mic has used that particular technology since the mid-1930s! However, what is new — groundbreaking, in fact — is the application of the dual-diaphragm configuration in a moving-coil capsule. No one has been able to do that before now, and Shure proudly describe it as “The most significant dynamic microphone technology advancement the industry has seen in more than 50 years.”

OK, that’s a very bold marketing claim, but actually, I don’t think they’ve over-hyped it. Dual-diaphragm technology has important benefits for vocal work, which is why virtually every studio vocal is conventionally recorded with a dual-diaphragm (capacitor) mic. So having the benefits of that technology available on stage for the live-sound market is very appealing.

At this point I expect you’re desperate to know how the dual-diaphragm configuration works, but rather than get bogged down in the technicalities here, I’ve put it all in a side-box for later reference. In the meantime, I want to focus first on the development of Shure’s remarkable new microphone.

Dyne Be The Glory

The capsule technology inside the KSM8 has been christened ‘Dualdyne’, in keeping with their familiar ‘Unidyne’ product range terminology and in obvious reference to the use of two diaphragms. The diaphragm at the top of the capsule is a conventional ‘active diaphragm’ with an attached coil of wire suspended behind it and sitting within a strong magnetic field. So far, so conventional — except that the active diaphragm is also stabilised by a series of (patent-pending) sound-diffracting plates positioned directly above it. Called the ‘Diaphragm Stabilisation System’, or DSS, this smart technology protects the active diaphragm, preventing excessive excursions or rocking motions during plosives or physical knocks. It also helps to minimise high-frequency handling noise.

At the bottom of the KSM8’s capsule is a second ‘passive’ diaphragm, which is there to seal the rear chamber and its internal acoustic labyrinth. This labyrinth is designed to delay sound from the rear, and is critical in achieving a directional polar pattern. The passive diaphragm obviously moves in response to sound waves, and thus transmits sound waves into the rear chamber, but it affects low and high frequencies differently.

In simple terms, the direct result of that frequency-dependent behaviour is an almost complete removal of the mic’s proximity effect, as well as a better-controlled polar pattern, giving improved off-axis rejection and reduced off-axis coloration, too. From a practical perspective, with virtually no proximity effect the on-axis sound character doesn’t change with source distance, so the performer enjoys a much larger and more consistent ‘sweet spot’. This translates to greatly reduced level and tonality changes and less need to compress or EQ the mic in the FOH mixer, even if the user’s mic technique is less than perfect!

Given the absence, until now, of dual-diaphragm technology in moving-coil mics, it won’t surprise anyone to learn that constructing such a design is extremely difficult, not least because of the need to squeeze the magnetic assembly between the two diaphragms, the spacing of which is restricted by the requirements of the internal acoustic structure. Developments in powerful, rare-earth magnetic materials have helped a lot here, and the KSM8 uses a Neodymium magnet in a “micro-injection mould of aerospace SoftMag material” (whatever that is!) to maximise the magnetic field in a very small space and thus ensure an acceptable electrical output level.

The capsule assembly process apparently involves ultrasonic and internal laser welding to help ensure excellent mechanical reliability, and the practical realisation of the Dualdyne capsule took Shure’s engineers seven years of development and optimisation. Every single component is new — this is no evolution from previous Unidyne capsules: it came quite literally from a clean sheet of paper and a lot of innovative thinking!

One aspect of the KSM8 which does borrow from the past, though, is its internal pneumatic shockmount. Building on a technology that Shure first introduced in 1966 with the Unidyne III capsules used in SM57s and SM58s, the KSM8’s pneumatic shockmount sits below the Dualdyne capsule to decouple it from the microphone body, thereby minimising low-frequency handling noise. This pneumatic shockmount employs tuned and damped cavities which are effectively pressurised by the sound waves which pass through the rear of the capsule and are then guided down into the shockmount itself. Shure describe this arrangement as a “reverse airflow technology”, and although the pneumatic shockmount was patented over 50 years ago, they claim that no other manufacturer has implemented it successfully.

In The Box

The KSM8 mic is supplied in high-quality rigid protective case with zip closure, and comes with a ‘soft-flex’ stand adaptor clip and 5/8-inch thread adaptor. At first glance it looks just like any other conventional stage vocal mic — there’s nothing in its styling that signposts the clever technology within.

Picking the mic up reveals that it is pleasantly weighty, at 330g, and it feels well balanced in the hand. The nicely tapered die-cast aluminium body is available in a brushed nickel or black-painted finish, with overall dimensions of 188mm x 48mm. This makes it almost identical in size to Shure’s KSM9 capacitor stage vocal mic.

The dent-resistant mesh grille is constructed from a hardened carbon-steel wire and permanently lined with a hydrophobic fabric. In combination, these are designed to provide effective plosive and wind protection, as well as keeping breath moisture away from the capsule. In the event of some tragedy befalling the mic, replacement grilles and complete capsules are available, if required, so the KSM8 should endure the rigours of touring live sound over a very long life.

Book Of Numbers

Looking at the mic’s published specifications, the frequency response is given as 40Hz to 16kHz, but these appear to be the limits at which the response has fallen by 10dB relative to the level at 1kHz. Unusually for a stage vocal mic, the response is largely flat between 100Hz and 10kHz and the substantial ‘presence peak’ found in most stage vocal mics is absent in the KSM8. In fact there are only a couple of small (less than 3dB) ripples in the response (around 3 and 5 kHz), with a fairly steep roll-off above 12kHz. At the low end, a small amount of proximity effect is apparent if the source is closer than about 25mm (one inch), but between 25-75mm (one to three inches) the response is well balanced and very consistent. Moving significantly beyond that range causes the bass level to fall off gently, and by 600mm (two feet) there is an obvious but still gentle attenuation of frequencies below about 100Hz. In practical terms, working the microphone in the normal way exhibits a far more consistent and balanced sound than would be experienced with, say, an SM58, which can go from distinctly boomy to noticeably thin with only the smallest of positional changes. If the vocalist presses their lips against the grille there is a small but noticeable increase in the bass end, but it really is very moderate in comparison to most stage mics.

Its unique capsule gives the KSM8 a cardioid polar pattern that is unusually uniform across the frequency spectrum.Its unique capsule gives the KSM8 a cardioid polar pattern that is unusually uniform across the frequency spectrum.As polar patterns go the KSM8 boasts a remarkably consistent cardioid response, with an excellent rear null across most of the frequency range, and Shure claim it to be the purest cardioid polar pattern the company have developed to date. The amount of rear rejection degrades slightly around 6-8 kHz, but not enough to cause any significant issues. Importantly, the mic provides a pretty consistent 5dB of rejection of sources at 90 degrees right across the frequency range, so that whatever off-axis spill is captured remains fundamentally uncoloured.

The microphone’s sensitivity is given as 1.85mV/Pa, which is comparable with most other stage mics — an SM58 has a sensitivity of 1.88mV/Pa, for example, making it less than 0.2dB louder. However, the output impedance is slightly higher than usual, at 300Ω, and although this will technically mean a slightly higher noise floor than a mic with, say, a 150Ω output impedance, given the KSM8’s intended application I think such concerns are completely irrelevant.

Singing Praises

I wasn’t able to try the KSM8 in a live stage situation, but I did compare it directly with both a Shure Beta 58 dynamic and SM87A capacitor stage mic in the studio. Just commenting again on the visuals for a moment, the KSM8 is a very elegant and classy-looking mic indeed, and if I didn’t know better I’d assume from its shape and styling that it was a capacitor mic rather than a moving-coil design.

In isolation, the KSM8’s sound character is very obviously far more natural and tonally balanced than either the Beta 58 or SM87A, and it is much, much easier to maintain a consistent sound quality and level when using the mic handheld. The high end of the voice comes over with a smooth, neutral aspect, free from the almost resonant peakiness that most stage vocal mics deliver. The on-axis pickup pattern is well focused, and although moving substantially off axis results in a dramatic reduction of level, the tonality remains remarkably consistent. Side spill doesn’t suffer excessive coloration, and the amount of rearward rejection is impressive and unusually clean. Taken all together, these attributes should combine to make the KSM8 a joy to work with on stage, minimising the need for aggressive EQ or dynamic manipulation.

Handling noise is extremely low and never gave any problems, even when I was deliberately trying to be clumsy! I also noticed how the mic body’s tapering shape and weight distribution automatically guides a natural hand position well away from the grille’s rear entry ports.

Overall, the KSM8 appears to be a very impressive stage mic, both in terms of its practical sonic benefits, and in its innovative technology and sophisticated engineering. Naturally, the latter influences the price greatly, and the KSM8 is consequently the most expensive moving-coil stage mic currently on the market. However, even so it is still significantly less expensive than many high-end capacitor stage mics, while it boasts some very useful and unique sonic and pragmatic advantages. Shure can be proud of their achievements in creating the Dualdyne capsule, and of advancing microphone technology in such a practically valuable way.

Alternatives

None. The KSM8 is unique in being a dual-diaphragm, moving-coil stage vocal mic.

A Game Of Cardioids

In a directional microphone, sound waves are generally directed towards the rear of the diaphragm through an acoustic labyrinth, which effectively introduces an audio delay. The idea is that sounds from the rear take the same amount of time to reach the rear of the diaphragm through the labyrinth as they do to pass around the outside of the microphone and impact on the front of the diaphragm. If the two passage times are accurately matched, the sound pressure will be identical on both sides of the diaphragm, and there will be no net movement — which means no electrical output. Consequently, the polar pattern will have a deep rearward null, and we call that a cardioid response. Varying the labyrinth delay time slightly leads to other directional patterns, such as hyper- and super-cardioid, where the null angles are offset either side of the rear axis at 110 degrees and 126 degrees, respectively.

The cardioid capacitor microphone was conceived in 1935 by Dr Hans Joachim von Braunmühl and Dr Walter Weber, both working for the Reichsrundfunk Gesellschaft (RRG Electroacoustic Laboratories) in Germany during the early 1930s. Their design used a complex arrangement of perforations drilled into separate brass plates which, when sandwiched together to form the microphone backplate, created an acoustic labyrinth behind the front diaphragm. To prevent dust and moisture ingress, they decided to cover the back of this labyrinth with a second (passive) diaphragm which would still allow sound waves from the rear to pass into the capsule’s internal labyrinth. Braunmühl-Weber’s innovative capsule design was patented in 1936, and introduced commercially as the famous M7 capsule employed in all of Neumann’s early microphones.

Where the magic happens! The active and passive diaphragms differ in that the active (top) one has a  coil attached.Where the magic happens! The active and passive diaphragms differ in that the active (top) one has a coil attached.Although the rear diaphragm isn’t used as part of the electrical circuit in a fixed-cardioid capsule, it was subsequently discovered that it does play an important role in modifying the capsule’s proximity effect, which I’ll come back to in a moment. In a switchable-pattern capacitor mic the rear diaphragm is made electrically active, of course, and by changing the electrical biasing of the two capsule halves and combining the electrical outputs from both diaphragms, it is possible to create any desired polar pattern from omni through cardioid and on to figure-of-8.

An interesting side-note here is that Ben Bauer, working at Shure in the mid-1930s, developed a broadly similar acoustic labyrinth concept based on Braunmühl-Weber’s work for a single-capsule moving-coil cardioid mic. Bauer’s clever ‘Uniphase’ design omitted the rear diaphragm, but led directly to the iconic Unidyne Model 55 microphone, released in 1939: the world’s first single-capsule cardioid dynamic mic.

The science behind the Braunmühl-Weber capsule is a little complex, but if you want the full story with all the maths, Shure have produced a detailed white paper on the science and practical effects of the dual-diaphragm capsule, which can be found here: http://cdn.shure.com/publication/upload/340/pdf_ea_dual_diaphragm_mics.pdf.

The ‘Dummies Guide’ explanation is that the passive rear diaphragm acts as the first element of the acoustic labyrinth network, and provides significant compliance at low frequencies. In other words, the acoustic impedance of the diaphragm increases as the frequency gets lower. This compliance is completely absent in a single-diaphragm cardioid capsule, of course.

In effect, the passive diaphragm’s compliance acts to partially block or impede low frequencies from reaching the rear of the front diaphragm, thus altering the net pressure acting on the active diaphragm in a frequency-dependent way. In fact, for close sources, as the signal frequency reduces, the dual-diaphragm capsule behaves less and less like a directional pressure-gradient device, and more and more like an omnidirectional pressure-operated device.

Pressure-operated capsules have no proximity effect at all, of course, and consequently the dual-diaphragm capsule has a greatly reduced proximity effect for very close low-frequency sound sources. Usefully, and for related reasons, the mic’s susceptibility to plosive popping is also dramatically reduced as an added bonus!

However, the proximity effect is not the only aspect of the capsule’s behaviour that is changed by the addition of a rear passive diaphragm. The polar response also behaves differently with both frequency and source distance. In general, a single-diaphragm capsule provides more accurate results for distant (far-field) sound sources, having a more accurate cardioid response which rejects rearward sources more effectively, and also enjoying a more accurate on-axis frequency response. In contrast, the dual-diaphragm capsule achieves a superior cardioid polar response and on-axis frequency response for close (nearfield) sources.

It is for precisely these reasons that dual-diaphragm capacitor microphones have become the standard choice for recording close-up vocals, users often citing the preferred low-frequency character, and why single-diaphragm cardioid mics are generally preferred for more distant-miking applications.

Controlling The Proximity Effect

Controlling proximity effect in moving-coil mics is not a new idea, and different manufacturers have developed a variety of strategies over the years. For example, AKG developed their D200 range of ‘two-way’ or dual-capsule microphones in the late 1960s. In these microphones the front capsule mounted directly behind the grille captures only the high frequency components of the source sound, while a second capsule located behind it receives the low frequency sound components, via a damped acoustical tube from entry ports at the bottom of the microphone’s handle. A crossover at 500Hz combines the two capsule outputs to produce the final signal, much like a two-way speaker working in reverse!

The benefits of this complicated and expensive approach include a very smooth frequency response with an unusually wide (for the day) bandwidth, a very uniform cardioid polar pattern with negligible off-axis colouration, and the complete elimination of proximity effect. There were four models in AKG’s dual-capsule range, starting with the D200. The D224 became the flagship model, but the D202 and slightly smaller D222 models were far more recognisable thanks to their very distinctive shape.

In America, Electrovoice took a very different approach in their EV664 microphone launched in 1954. This was the company’s first model to use their ‘Variable-D’ technology, although the concept wasn’t patented until 1963. In essence, this system guides sound of different frequencies to the rear of the capsule through three or four separate entry ports spaced along the length of the mic body, each incorporating frequency-selective damping. The lowest frequencies enter via the farthest entrance port, with mid and high frequencies entering through ports closer to the capsule. At the rear of the diaphragm the acoustic combination of these separate signals with altered phases and amplitudes results in an accurate cardioid polar pattern with a greatly reduced proximity effect.

By 1967 Electro-Voice had improved the concept and the RE15 model was the first with ‘Continuously Variable-D’ technology. This improved design had even less proximity effect, achieved through the use of considerably more entry slots designed to provide an almost continuous variation of phase shift with frequency, instead of a small number of fixed discrete shifts.

Electro-Voice’s product range now includes several Variable-D mics although the most iconic and recognisable is undoubtedly the RE20.

Published May 2016