We all have strong opinions about which mics sound best on particular sources, but in many cases there is more happening around a mic other than the source you are trying to capture. When positioning a microphone, do you ever consider how the off axis response colours the sound of other sound sources?
I’ve been accused of audio heresy in the past because I’m not a fan of the SM57. It’s a useful mic on guitars but I avoid them on snare drums because of their off axis sound, which I find colours the hi hat in an unpleasant way. In this article we will look at a couple of examples, including the SM57 on snare and offer some thoughts not just about why this happens but why it matters.
Why Is Off Axis Sound Coloured?
Considering how ubiquitous cardioid mics are, it’s easy to forget how clever they are and what an engineering achievement it is that they work at all.
How Do Different Polar Patterns Work?
This gets quite technical:
Omnis respond to differences between the air pressure in front of the diaphragm compared to the air (almost) sealed in a chamber behind the diaphragm.
Ribbons naturally have a figure 8 response because of the way what is happening in front of and behind the ribbon changes depending on whether the sound arrives from the front/back or from the side.
A cardioid response is created by allowing air from outside the capsule to reach the back of the diaphragm via a very carefully designed labyrinth which lengthens the path this sound reaching the back of the diaphragm takes. Sound arriving from behind the mic gets delayed in a different way from sound arriving from the front. The cancellation introduced creates the familiar cardioid polar response.
As clever as it is, it’s pretty astonishing that cardioid mics work as well as they do but considering that the mechanism by which they work is destructive interference, it’s no surprise that off axis sounds suffer from comb filtering, that’s part of how it works.
So is it sensible to only think about the sound of the thing the mic is pointing at and not to consider the off-axis sound? Surely the off-axis coloration coming into a mic needs to be thought about every bit as much as the on-axis wanted sound
Are All Mics Coloured Off Axis?
By “colour” I mean that the frequency response changes depending on the direction from which the sound arrives. The best omnidirectional small diaphragm condensers have a nearly identical response from all directions. Cardioid mics usually show a near perfect cardioid response at 1KHz but you’ll nearly always find that the response at 100Hz is moving towards omnidirectional and at 16KHz is closer to hypercardioid. This difference across the frequency range is what colours the off axis sound. An example of a cardioid mic which is very consistent off axis is the Neumann KM184. It is typical that small diaphragm mics have a more consistent off axis sound than large diaphragm mics.
If you look at the example polar plots in thee images and diagrams, you’ll see how off axis response varies with frequency and from mic to mic
The Neumann KM184 has very even off-axis response, small diaphragm mics tend to have better off-axis response than large diaphragm mics
The Shure SM57 is very different and follows the pattern found in most cardioid mics of where at low frequencies the response is more towards omni and polar response narrows at the high end.
The classic Sennheiser MD421 doesn’t have a particularly tight cardioid pattern, some cardiods like this one range from omni-ish through to cardioid, others maintain the cardioid response at the low end but often have a hypercardioid rear lobe.
Good omnis are very flat off-axis, as are good figure 8s like the Sennheiser MKH30, which has a near perfect off axis response.
When you go beyond the first order mic patterns things can get messier. See the polar plot of the very directional Sennheiser MKH416 shotgun.
The Sennheiser MK4 is a good example of a mid priced large diaphragm cardioid, it displays a tight cardioid patterns which narrows and develops a hypercardioid rear lobe over 2KHz.
Example 1 - Small Diaphragm Condenser Mics As Used With Singer Songwriter
In this example, I have a pair of Aston Starlight cardioid small diaphragm condensers. One is 6 inches from my mouth and the other is 9 inches from the neck body join of my acoustic guitar. Rather than impose my singing on you I explain the setup and audition each mic both on its intended source and on the other. This is an interesting one as, as you can see from the polar plot below the Aston has quite a wide pattern around 4KHz.
Example 2 - How Does The Hi Hat Sound In A M80 Or SM57 As The Snare Mic
In this video, you can hear the difference between James’ preferred snare mic - the Telefunken M80 and the ubiquitous Shure SM57, take a listen…
Why Does Off Axis Response Matter?
Even if you’re recording single instruments you will get coloured room reflections and if you’re recording in stereo using coincident or near coincident arrays (XY, ORTF) then the centre of the image is being captured by two, off-axis mics. This is one of the advantages of MS, the centre is picked up on axis by the mid mic. If you use mics live this colouration can cause feedback and, counter intuitively, feedback can be less of a problem when using omnis as although the gain ceiling will be lower, the feedback behaves more predictably and usually comes in more slowly.