Brief Summary

The ability of some mics to offer multiple polar patterns is, in our opinion, under-used. Getting the take is important and most decisions can be deferred to the mix stage. Committing to a sound at the recording stage is usually a good idea but choice of polar pattern can potentially make much more difference than compression tweaks and the like. In this article Julian looks at the multi pattern mic and considers some of the modern designs which allow polar pattern choice to be a mix decision.

Going Deeper

It’s safe to assume that cardioid mics outnumber other types many times over. It’s the default choice. When other patterns are used it’s often because a particular mic is chosen and it happens to have a pattern other than cardioid. For example I know I use hypercardioid far more than any other pattern but that because my trusty Beyer m201 which I use for VO most days happens to be hypercardioid. I use it because of how it sounds, not specifically because of its polar pattern.

The same can be said of people who favour ribbon mics. Ribbons compel users who favour the tonality of ribbons into using figure 8 mics. There are lots of desirable things about this pattern but I suspect that if physics didn’t dictate the polar pattern of ribbons, and they were available in any pattern, people would use cardioid ribbons by default in the same way as they do for condensers.

This brings me to the fact that you can have a choice. The multi pattern microphone makes polar pattern selection unrestricted, and even though we still use multi pattern mics in cardioid most of the time, what are we missing? Especially as in recent years new opportunities have been presented by some of the current models.

Some History

Fundamentally microphones come in two varieties: Velocity and pressure. A pressure microphone, as the name suggests, responds to changes in pressure. A velocity microphone responds to the movement of air. The velocity of air has a vector, in other words it comes from a particular direction. The same can't be said of air pressure which is the same, regardless of which way the microphone happens to be pointing. The original velocity microphone was a ribbon, and if you hold up a piece of paper and blow onto it, you find the amount the piece of paper is affected by you blowing changes when you turn the paper through 90°. That's the operating principle of ribbon mic, and if you understand this, then the figure of eight response of a ribbon microphone follows naturally.

A pressure mic is an omnidirectional microphone, more like a barometer which is both very small and very fast to respond. Sound being created by the compressions and rarefactions of air which make up sound waves contain periodic variations of a pressure and it is these to which pressure microphones respond.

The cardioid microphone was developed in the 1930s, and initially was created by combining an omnidirectional mic with a figure 8 ribbon mic in the same housing. By electrically combining the two outputs, a cardioid response can be derived. You can try this using two microphones set as close as possible to each other and you'll find you get a cardioid response and by inverting the polarity of either microphone the cardioid pattern is reversed. However the distance between the two capsules means that as frequency rises and wavelengths get shorter the cardioid pattern becomes less effective. For this combining of different polar patterns to work effectively the two microphone capsules have to be as close together as physically possible and in multi pattern microphones this is what happens.

Omnidirectional microphones work by preventing air from reaching the back of the microphone, diaphragm, meaning that as pressure changes around this sealed capsule the diaphragm is displaced. Cardioid microphone capsules are created by, very carefully, controlling the air which can reach the back of the diaphragm by creating a labyrinth of channels of different lengths which shift the phase of sound waves reaching the back of the diaphragm depending on the direction from which they arrive.

Rather than creating a cardioid response using an omni and a fig 8. Multipattern mics use two cardioid capsules mounted back-to back and by combining the output of these two capsules in different ways, any response from fig 8, through cardioid, to omni can be created. Because the capsules are as close as is physically possible the response is consistent across the whole frequency spectrum. So by combining two cardioid mic capsules in a single unit and adding some simple switching you can have all the first order polar patterns available to you. So why do we still have single pattern mics?

The first reason, unsurprisingly, is cost. Adding a second diaphragm adds significantly to the manufacturing costs. A second reason is that many mic types don’t exist in switchable multipattern form, it is only large diaphragm condensers which lend themselves to this treatment. Thirdly I’d suggest that the change in recording habits from purpose built studios to smaller, home studios has made the, already popular, cardioid mic overwhelmingly popular as users seek to keep as much of the room as possible out of their recordings. Why buy a mic with two capsules when you’re only going to use one?

Different Take On Multipattern Mics?

Times and technology are changing and there are several ways to keep your options open which haven’t been always been available, but before I get to them I’ll return to the point that it’s only large diaphragm mics which have switchable patterns. That’s true but there are alternatives which are sometimes overlooked.

Small diaphragm condenser mics have some advantages over their large diaphragm brethren. They can handle extreme SPLs and their off axis response is better at the top end. At high frequencies the wavelength approaches the diameter of the diaphragm of an LDC, causing issues. They inherently have a higher self-noise but a good SDC is plenty quiet enough for all but the most demanding applications. I’ve never seen a switchable SDC but the Shure KSM141 is a mic which features a slidable collar behind the capsule which can block the vents which allow air to the back of the diaphragm, converting it from cardioid to omni. Small diaphragm condenser mics with swappable capsules have been around for many years, the Collette series from Schoeps being an example of very desirable mics based on this modular system. Even the humble AKG C1000 came with a plastic converter which fitted over the top of the capsule and changed its response from cardioid to hypercardioid.

Moving to switchable multi pattern mics, These have been around for a very long time. The U47 was the first switchable pattern mic, released in 1947. In 1951 the M49 introduced fully variable pattern choice rather than the two patterns of the U47. Being able to access Omni, Figure 8, Cardioid, and the ‘in between’ patterns sounds ideal but unless being used with a specific intent, such as mid-side recording or rejecting a particular instrument or kit piece by exploiting a fig 8 side null, I find the predictable cardioid tends to take over. During busy recording sessions experimenting on other people’s time should be done with care.

Dual Output Mics

This is where my enthusiasm for multi pattern mics which allow you to record both capsules independently has come from. I’ve used a number of these and if I have the spare mic preamps available I have found them to be worth running an extra XLR for. A standard multi pattern mic takes the discrete output of each capsule and combines them internally. By adding a second output connector the two capsules’ signals can be recorded via two mic preamps (digitally controlled is best to ensure matched levels) and when in the DAW they can be combined to manipulate the polar pattern at the mix stage. Arbitrarily deferring decisions until the mix stage can be counter-productive but this is a case where I’m glad to be able to experiment away from the recording session.

Examples of such dual output mics are the Lewitt LCT 640 TS and the Austrian Audio OC818. The UA Sphere DLX mic, based on the Townsend Labs Sphere L22, also has this dual output functionality as part of its modelling and has the ability to manipulate the polar pattern post-record. The thing you can’t do with these mics is steer them in the way you can with an Ambisonics mic, however we’ll return to this later in the case of the OC818. But the ability to audition different polar patterns means that you can manage spill and the contribution of the room effectively and balance the positives and the negatives of each without holding up the session.

I’ve never used the Lewitt mics but I have experience of both the Sphere and the OC818 and there are several tricks they have up their sleeves. For example both can be switched to reverse cardioid in their respective control plugins. This is a feature in the new Aim Audio Inspire, you can set the mic to reverse cardioid mode using the on-mic switches for easy access to the controls when used close to guitar cabs and the like but the Inspire doesn’t offer a second output for making such changes post-record. I’ve used this feature on a tracking session when a Sphere L22 was in use on a guitar amp and halfway through the session the mic was moved and replaced by the player but put back the wrong way round. A disaster using normal mics but fixed with a mouse click in this case.

A second useful feature in both the OC818 and the Sphere is the ability to control proximity effect. If you want to close mic in fig 8 but don’t want the associated bass lift due to the proximity effect, you can dial it out via a dedicated control. But things get much cleverer than that in Austrian Audio’s PolarDesigner plugin. There is full control of polar pattern but also the ability to split the mic’s output into up to five bands via linear phase crossover filters and to set a different polar pattern in each band. Even cleverer you can set the plugin to listen to either the wanted signal or to the unwanted spill and set an optimum multiband setting to maximise or minimise respectively.

Stereo

With two independent cardioid capsules this kind of dual output mic can be used to capture stereo. The Sphere has had a ‘180’ plugin variant for stereo applications since the Townsend Labs days and while back to back cardioids is a very wide setting, it is legitimate and in the right application can sound great. I’ve used it as a complement to a single mono ribbon on drums before and it is very useable on its own too. Austrian Audio have a StereoCreator plugin which covers much the same ground as the UA Sphere in 180 mode but if you have a second OC818 you gain the ability to steer the mic.  

Ambisonics

If this sounds like its getting towards the steering abilities of a Soundfield mic it’s because it is, and Austrian Audio also have an AmbiCreator plugin (unfortunately none of these are natively Apple Silicon yet) which takes the output from a pair of OC818s and can create a B Format output suitable for Ambisonics. I was confused by this as I couldn’t see where the height information was coming from in what is effectively a crossed pair of mics. It turns out it uses the difference in height between the upper and lower mics to create an ‘endfire array’, a technique which is common in automotive and mobile phone applications and can create the necessary response over around 2kHz. This isn’t perfect but it can create the necessary fig 8 response over that area of the frequency spectrum - Clever!

These mics are just examples. If there are other multi pattern mics with dual outputs please share them in the comments. The value of these options is that, beyond the flexibility offered by the post processing in a plugin, there exists something of a chicken and egg situation with polar patterns, particularly for new users. Many experienced engineers are very aware of the possibilities presented by other polar patterns, particularly omni and fig 8, in recording sessions, but cardioid is undeniably the one nearly everyone would choose if they could only have one pattern. Because of this the more attractive pricing of the cardioid-only versions of multi pattern mics, coupled with the fact that most of the time cardioid is perfectly acceptable, means that other patterns get overlooked. The ability to experiment, to answer the ‘what ifs’ on your own time, means that rather than spending most of its time being just a more expensive cardioid mic, a multi pattern mic can really fulfil its potential. Imagine how it would have affected your development as an engineer if you could only audition different EQ or compression settings by recording another take? That’s how it is with polar patterns for most of us!

There aren’t many jobs cardioid is the wrong choice for, but that doesn’t mean its the best choice. We can change so many things which are less important than polar pattern after the band have gone home. Why not polar pattern? Share your thoughts on polar pattern choice and application in the comments.

Image Credits

Nicoguaro, CC BY 4.0, via Wikimedia Commons

Galak76, CC BY-SA 3.0 , via Wikimedia Commons