Omni-pattern microphones have a relatively even, spherical response, which makes them very useful for natural-sounding recordings. They're ideal for recording instruments such as strings or acoustic guitar - provided that the instruments are being played in a pleasant-sounding room!
Knowing your cardioid from your omni can help you to achieve better recordings. If you're confused about what it all means, our guide to mic polarity should be a step in the right direction.
Every music technology textbook includes a description of the different microphone pickup patterns, but what most users really want to know is what benefits the different patterns have to offer, and in what situation you might choose them. In this article I'll be concentrating on 'single microphone' applications rather than stereo, which we'll look at in a future article.
Basics
It is worth remembering that although the printed polar patterns are in two dimensions (as below), the actual pattern is three-dimensional. For example, an omnidirectional microphone pattern drawn on paper looks like a circle, but in reality it is a sphere.
Despite some mics on sale offering multiple switchable pickup patterns, there are only two fundamental patterns: the omni and the figure-of-eight. All the other patterns in use today, including the popular cardioid, are created by combining these two in differing proportions.
Omni mics are often referred to as 'pressure microphones', because they essentially measure sound pressure at a point in space. A diaphragm is fixed across the mouth of a sealed cavity, so in effect the mic behaves like a very small barometer capable of following audio frequency pressure changes — but it has no means of detecting the direction of the sound waves, hence it has an omnidirectional polar pattern. Because this arrangement only senses pressure, it doesn't matter what direction the sound approaches from. All that matters is the change in pressure at that point in space, so it is more or less equally sensitive to sounds from all directions — hold this thought, as I'll come back to it in a moment.
This diagram shows the three basic polar patterns found in microphones. All other patterns are variations on these themes. The blue circle is an omni pattern, the red circles show a figure of eight pattern, and the green line shows the cardioid.To prevent the microphone behaving too much like a meteorological barometer and responding to changes in the weather, the cavity is engineered with a very small air leak or vent built into it, so that very low-frequency pressure changes due to weather (or altitude) don't force the diaphragm in or out permanently. At audio frequencies, however, the cavity can be considered to be sealed.
The mechanical simplicity of this 'pressure operated' design means that off-axis sound is still picked up reasonably accurately (in terms of frequency response), but the physical size of any microphone's diaphragm will always result in some high-frequency loss as you move off-axis — and the larger the diaphragm, the more pronounced this high-frequency loss will be. If you imagine a sound approaching a diaphragm at, say, 45 degrees off-axis, the sound will reach one side of the diaphragm slightly sooner than it reaches the other. This results in some phase cancellation at high frequencies and hence a degree of high-end loss. That's why precision measurement microphones tend to have very small-diameter capsules. This creates another problem, as the smaller amount of sound energy captured by a small diaphragm requires more amplification, which in turn leads to a higher level of electrical noise. That's why this sort of microphone is unsuitable for most music applications.
Another advantage inherent in the design of pressure-operated microphones is a well-extended frequency response at the bottom end — typically an octave more than a similar-sized cardioid microphone. They are also less susceptible to picking up noise and rumbles from mechanical vibrations than cardioid mics.
A figure-of-eight microphone uses a diaphragm open to the air on both sides, so, rather than responding directly to pressure, it responds to the difference (or gradient) in pressure between the front and the rear of the diaphragm — hence the generic term 'pressure gradient' microphone (sometimes also referred to as a 'velocity' microphone, because it detects the velocity of sound waves). This arrangement of the diaphragm makes the microphone very sensitive to sounds approaching from either the front or rear axis, while sounds approaching from the side cause no diaphragm movement at all, as the pressure each side of the diaphragm always remains equal. The practical outcome is a microphone that is essentially 'deaf' 90 degrees off-axis but is equally sensitive to both front and rear. Sound picked up by the rear of the diaphragm also produces an inverted electrical signal compared with the same sound picked up by the front of the diaphragm (this is pretty logical if you think about it, as the two scenarios result in the diaphragm being pushed in opposite directions).
The frequency response of on-axis sounds is reasonably consistent within the limitations set by diaphragm size. In other words, the smaller the diaphragm, the greater the accuracy when picking up off-axis sounds. One critical aspect of the design of a pressure gradient microphone is that the output level falls with decreasing frequency. This is because the pressure difference across the diaphragm gets smaller as the wavelength of the sound wave increases. To overcome this problem, the diaphragm's suspension is usually arranged to respond more easily to low-frequency sounds than to high-frequency sounds, which results in a more even frequency response. A side effect of this is that the microphone becomes much more sensitive to mechanical vibrations.
Proximity Effect
Another important factor to be aware of is that all pressure-gradient microphones exhibit, to different degrees, a 'proximity effect' — a low-frequency boost that occurs when the microphone is used very close to the sound source (hence the other common term, 'bass tip-up'). The effect is due to the physics of the way the mic works, and is quite a complicated topic, but in practical terms, for recording, it can be both a strength and a weakness — it depends on what you're trying to achieve.
Here, both mics have a figure-of-eight pattern. The side rejection is being put to good effect so that the guitar mic rejects most of the vocal and vice-versa, giving better separation than you would get with cardioid or omni mics. However, given that they also pick up from the rear, you need to be particularly careful to avoid sounds hitting the rear of the mic.
If you were to combine pressure-operated and pressure-gradient capsules in a single mic, or arrange a single capsule to have the properties of both, the result would be a cardioid shape. On-axis, at the front, the omni and figure-of-eight polar patterns produced by the two fundamental mic elements add together to make the combination very sensitive. At the sides, the figure-of-eight element has nothing to add, leaving just the omni's pickup. Consequently, the sides of the cardioid are less sensitive than the front. At the rear, the figure-of-eight response is the same sensitivity as the omni, but the electrical output is in the opposite polarity, so the two cancel out, making the mic extremely insensitive to sounds coming directly from the rear. A plot of the microphone's sensitivity at different angles is roughly heart-shaped, hence the cardioid tag, though in reality it looks more like a vertical cross section through an apple, with the stalk being the rear of the pattern.
The cardioid or unidirectional pattern is widely used because of its ability to discriminate against sounds arriving from the sides or rear of the microphone. However, when you look more closely at how a cardioid microphone behaves, it soon becomes evident that it isn't the one-size-fits-all solution that it might at first appear to be.
Although the very first cardioid mics used two separate capsules, the majority of cardioid microphones these days are built using a single capsule, where a sonic labyrinth behind the diaphragm is used to manipulate the phase of sounds reaching the rear of the capsule in such a way as to produce the desired cardioid pattern. In general, this system works extremely well and is at the heart of most hand-held stage dynamic microphones, as well as many studio capacitor models, but its weakness is that the cardioid pickup pattern isn't the same at all frequencies, so while the mic might produce very accurate results in situations where the incident sound is directly on-axis, off-axis sounds will in effect be filtered by the directional characteristics of the microphone, most often characterised by a drop-off in high-frequency sensitivity. Try talking into the side of a cardioid microphone and you'll soon hear just how coloured the off-axis pick up can be.
In the real world, sound rarely arrives only on-axis, as most environments produce a significant amount of reflected sound, and this can arrive at the microphone from pretty much any angle. The practical outcome of this is that the (otherwise accurate) on-axis sound is mixed with significantly coloured reflected sound and, in untreated rooms, this can lead to a noticeably nasal or boxy characteristic. It is also worth noting that cardioid microphones are classified as pressure-gradient microphones (because the rear of the diaphragm is not sealed off) so, like the figure-of-eight, they also exhibit the proximity effect — which can result in a significant rise in bass when used very close to instruments or vocalists.
If the mic diaphragm is open to the air on one side but closed at the other, it is considered to be pressure-operated: although it reacts to air pressure, it is not sensitive to direction, resulting in an omnidirectional mic pattern. Where the diaphragm is open on both sides, as in this diagram, it responds to the pressure-gradient (the difference between the pressure at the front and the back of the diaphragm). In this case, sound from the side results in even pressure on both sides of the diaphragm, which is why figure-of-eight mics reject sound from the side, but are responsive to both the front and rear .By varying the mix of pressure-operated (omni) and pressure-gradient (figure-of-eight) elements, we can produce wider or narrower cardioid patterns and, once again, these variants can be replicated using a single-diaphragm capsule by modifying the acoustic labyrinth behind the diaphragm. Narrow-pattern mics, such as supercardioid and hypercardioid, show a small lobe of sensitivity at the rear, where the larger pressure gradient component starts to leave its mark in the form of a small opposite-polarity rear tail. As a result, the least sensitive axes on these types of mic tend to be between 35 and 45 degrees off the rear axis, rather than directly behind. This becomes critical when placing foldback monitors for a stage vocalist, for example, and also affects how you must position the mic in a studio to reject unwanted spill. In general, where you aim the 'dead axis' of the microphone is at least as important as what you point it at, and often more so!
Because these mics have a narrower pickup pattern than a regular cardioid, they are more sensitive to positional changes in the sound source, so it is important to minimise movement when working close to them. Physical absorbers placed behind the microphone can help reduce the level of sound reaching that sensitive rear lobe, so in situations where good separation is paramount, the careful use of narrow cardioid mics is a valid option.
Theory Into Practice
Although separate mics may be preferable, you can use a cardioid pattern to capture both toms, as the pattern is reasonably wide to the front of the mic, while it provides significant rejection to the rear, helping to avoid spill from cymbals.How does this very basic overview of microphone patterns and their characteristics help us when it comes to actually making our recordings? Returning to our old friend the cardioid, we now know that we have to make allowances for an inaccurate off-axis frequency response and, if used really close to a sound source, for proximity bass boost. The directional qualities help keep instruments separate in the recording and also help minimise the amount of reflected sound reaching the microphone, but you can be sure that any spill or reflected sound that does reach the rear and sides of the microphone will be significantly coloured in comparison with an omni mic used in the same situation. An omni will, of course, pick up more of the room sound, but it will pick it up with much less coloration than a cardioid.
Armed with this knowledge, you can try to arrange your recording setup to minimise the amount of off-axis sound reaching the microphone. One way to do this is with sound absorbers, such as heavy, folded blankets, duvets, or panels of acoustic foam. For example, when recording vocals you need something behind the singer's head that will intercept and absorb sound that might otherwise bounce off the wall directly behind them and get into the front and sides of the cardioid pattern. It also helps to have absorbers around the rear and sides of the mic, not forgetting the ceiling above the mic and singer. While improvised screening can be very effective, commercial solutions such as the SE Reflexion Filter are somewhat neater and less obtrusive when it comes to screening the microphone itself, but the reflective surfaces behind the singer must also be treated to achieve optimal results.
Using effective absorbers will significantly improve the quality of recordings made using a cardioid-pattern microphone in a highly reflective space, but few cardioid microphones sound as natural as a good omni model, simply because of the way the acoustic labyrinth behind the diaphragm affects the purity of the sound.
The cardioid pattern is a combination of the basic omni and figure-of-eight patterns, resulting in a directional mic that responds to sound from the side and front, but virtually rejects it from the rear. Rather than using two capsules to achieve this, most cardioid mics now incorporate a vented 'labyrinth' in a single-capsule design that manipulates the phase of sounds hitting the rear, to produce the desired cardioid pattern. The supercardioid and hypercardioid designs use the same principle to create a more focused pattern to the front, at the expense of reducing the rear rejection. If you notice vents at the side of the mic head, the mic probably has a cardioid pattern (or a variation on it).Excessive proximity effect is most problematic when recording vocals, but it is easily dealt with by positioning a pop shield so that the singer can't get closer than a couple of inches from the microphone. The decision-making process here involves weighing up the consequences of using an omni microphone and suffering more spill, or choosing a cardioid mic, where the amount of spill is reduced but the spill that is left will sound more coloured — in fact, the basic sound may be less natural too. In many instances, you'll actually get noticeably better-sounding results by using an omni-pattern mic and then arranging acoustic screens to reduce the amount of spill getting into the rear and sides of the microphone.
In my own studio, I used to always reach for a cardioid mic when recording acoustic guitar, but I now often choose to use an omni microphone in conjunction with a Reflexion filter. Not only is the result more natural-sounding, but the exact positioning of the microphone seems less critical than when using a cardioid model. Of course, we're not always going for a natural sound — especially in pop music, where a musically pleasing result is of more importance than absolute fidelity. This is why large-diaphragm cardioid mics (arguably the least accurate in terms of fidelity) are so popular for vocal recording.
While it doesn't take too much imagination to explore the pros and cons of cardioid or omni microphones in typical recording situations, it is less obvious where the figure-of-eight fits in. After all, why would you want a microphone that is as sensitive at the rear as it is at the front, other than for specialist stereo applications such as M&S (Mid & Side)? Well, sometimes it's because the physical makeup of the microphone gives you no choice. Ribbon microphones, for example, have a natural figure-of-eight polar pattern and, back in the '50s and '60s they used to be popular with live bands, as they allowed two backing singers to sing into opposite sides of the same microphone. In the studio today, we choose ribbon mics for their tonality.
However, one of the main reasons to choose a figure-of-eight microphone is not so much where it picks up from as where it doesn't. Remember that a figure-of-eight mic is totally deaf to sounds arriving from 90 degrees off-axis. This means that where you have two sound sources in close proximity, you can often significantly improve the separation between them by arranging a couple of figure-of-eight mics so that the 'deaf axis' of each mic points towards the sound source you're trying to reject. Often the rear sensitivity of the mic can be countered by using acoustic screening. This technique can be very successful when recording an acoustic guitarist who also sings, as it helps to separate the guitar and voice, though the separation will never be perfect for two reasons: the sounds don't originate from a single point and room reflections can reach the mics from many different angles. For this last reason, using acoustic absorbers to isolate the recording area from excessive room reflections is highly recommended.
Polar patterns aren't always as simple as they first appear: the 'squashed spider' polar response pattern of the supercardioid Sennheiser MKH416 'shotgun mic' for high frequencies (above 8kHz) is shown in red. The mid-frequency (1-8kHz) polar pattern of the same mic is shown in blue, as compared to the typical cardioid response, shown in green.Multi-pattern microphones work by combining the outputs from two capsules, the most popular arrangement being two back-to-back cardioid capsules. By controlling the level and polarity of the two capsule's outputs, any of the basic polar patterns can be created. However, where purity of sound is important — such as the critical recording of classical or ethnic instruments — choosing a dedicated omni or figure-of-eight single-diaphragm microphone (ideally with a small diaphragm) is likely to produce more accurate results, with less coloration of the off-axis sound.
When it comes to cardioid-pattern mics, it makes little difference whether you use a single-capsule model or a dual-capsule, multi-pattern model. Both will perform similarly, as both are based on cardioid capsules. In theory, the smaller diaphragm variants should produce more accurate results for off-axis sounds, but the benefits may be overshadowed by the coloration caused by the acoustic labyrinth used to create the polar pattern in the first place.
A Final Word...
I've deliberately kept this overview as simple as possible, in order to get across a few important concepts. Perhaps the main thing to appreciate is that, while the large-diaphragm, cardioid-pattern microphone might be the mainstay of project studio recording, there are situations in which it may not be the best choice. I hope that I've also adequately stressed the importance of the recording environment, as controlling what reaches the microphone — especially by way of reflection — can make a huge difference to the end result.