The variety of different microphone polar pickup patterns may be confusing at times, but when it comes down to the facts, we can make sense of it all. To put it in perspective — If you take some time away from a conversation at a busy restaurant to take in the whole scene, you will realize that you ears are capable of absorbing a variety of sounds at the same time. For instance, you will be able to listen to the music while everyone else is busy talking. In fact, you can overhear what members of the next table are talking about while the music is playing and you say hello to a nearby friend. Frankly, it is amazing the variety of audio signals that human ears can take in and especially, the level of detail with which each of these signals is processed to ensure that you understand each and every bit. However, this is made possible by the powerful and intricate processor located between the ears — the brain!
In a more technical setting, a microphone works in the same manner as a human ear but with polar pickup patterns in place of the brain. So, what is a pickup pattern? In layman’s terms, a pickup pattern is generally the description of a mic’s intrinsic directionality. In more specific terms, polar pickup patterns refer to a microphone’s sensitivity to incoming sound signals from various angles in relation to its central axis.
Understanding Polar Pattern Charts
Given the different microphone pickup patterns, different microphones tend to detect and record sound in different ways. As such, different types of microphones are designed to function differently. While some have the ability to detect and record sound signals coming from all directions, others can only take in sounds emanating from in front of them. Therefore, manufacturers specify a microphone’s directionality as a way of explaining to users where sound signals should come from in order for the microphone to record them. In other words, directionality refers to the specific position where the subject should be for the mic in question to function.
As a description of the microphone’s directionality, manufacturers will include a polar pickup pattern chart alongside their microphone specifications on the packaging or documentation. Unfortunately, most of these lines and axes are usually left unexplained which makes it difficult for the majority of users to understand these illustrations. Ideally, a typical chart will show the front of the mic facing the top of the circle and the back facing the circle’s bottom end. The diaphragm, ion the other hand, is positioned on the horizontal line. There are multiple lines extending towards the circle’s circumference from the microphone, each representing a 30 degrees deviation from the ‘on-axis’. To this end, the circle(s) that radiate from the center or from the microphone represent the degrees from which the mic is able to pick up sound signals.
What’s more, the plotted graphs usually include inner circles that represent the mic’s level of sensitivity. There is a difference of 5 dB (decibel) for each circle-the nearer to the center the circle is, the lower the sensitivity.
Types of Mic Polar Patterns
Just as the name suggests, a mic with an omnidirectional polar pattern is one that accepts sound signals from all direction. It is equally sensitive to all signals regardless of their direction, thus, operates as per the pressure principle. They are resistant to vocal plosives when in excess and show zero proximity effect. Also, with little to no gain-before-feedback, it can be challenging to place such mics near loudspeakers since they lack null points. Unlike other types of mic pickup patterns, omnidirectional mic diaphragms have only their front ends exposed to sound pressure. The back sides are usually encapsulated in minute chambers and are exposed to steady pressure.
They are receptive to signals from all direction, but although this is generally applicable across the board, small lapel mics are considered to be the most effective omnidirectional mics in the market. This is because a good number of omnidirectional mics tend to become unidirectional when operating in areas with high frequencies. This is attributed to the nature of short-wavelength and high-frequency sound signals, in relation to the surface area of the mic. As such, smaller surface areas are more ideal. Lastly, they do not conduct off-axis coloration, hence, are able to collect sound in its most natural form from all angles.
- Bidirectional Polar Pattern
Also common known as Figure-8, bidirectional polar patterns are unreceptive to signals coming from their sides but are equally sensitive to those coming from the back and the front. As such, their off-axis coloration is symmetrical. They contain the highest proximity effect and are regarded as the most ideal pressure-gradient microphones in the market. Being that there are rings of silence present at each side, sound pressure coming from external sources is equally felt on either side of the mic’s diaphragm. These null points, or rather the rings of silence, are positioned at 270 degrees and 90 degrees. Ultimately, this creates more of a cone-shaped scope of functionality such that all audio signals coming from the side are rejected fully.
With regards to vocal plosives, bidirectional mics are at risk of overloading. The pressure-gradient principle shapes this polar pattern, hence, allows them to react to plosives. Lastly, given that each side is subject to external pressures, acoustic labyrinths do not hinder sound waves from reaching the far-end of the mic. In turn, this implies that the distance between the back and the front of such mics is shorter than in omnidirectional mics. It is therefore expected that bidirectional mics have the most proximity effect.
- Cardioid Polar Pattern
This is by far the most popular out of the many different polar patterns out there. Such microphones are most sensitive in their on-axis direction while the opposite direction has a null point. In-between the two is a slow attenuation that hits -6 dB at two points; 270 and 90 degrees. Looking at the image, this pattern forms the shape of a kidney or an inverted heart.
Similar to the bidirectional mic, cardioid patterns also operate by means of the pressure-gradient principle. This implies that each side of the diaphragm is subject to variations in external pressure. Nonetheless, as opposed to bidirectional patterns, unidirectional patterns have an acoustic labyrinth positioned at the rear end. In some cases, lowered amplitude and time delay are usually brought about by dampening of acoustics at this point. With a null point at 180 degrees, this mic setup is the most sensitive to sound signals from a single direction. Also, its gain-before-feedback is relatively solid and it exhibits a proximity effect.
Aside from these three, there are other less common polar patterns. These include:
- Super-cardioid Polar Pattern
Made up of a 3:5 ratio of omnidirectional and bidirectional patterns, super-cardioid patterns are highly directional in nature. Compared to cardioids, they are more directional but they have a cone of silence in-between 233 degrees and 127 degrees, the null points. Such patterns also operate using a pressure gradient principle whereby both sides are exposed to external pressure. Notably, they are commonly used in movies largely due to their focused directionality and are most sensitive at 0 degrees on their null axis. The sides are less sensitive by 10 dB and since these patterns rely on the pressure gradient principle, they are also sensitive to vocal plosives.
- Hyper-cardioid Polar Pattern
As per the title, this is a hyper directional microphone polar pattern whereby both omni and bidirectional patterns are mixed at a ratio of 1:3. Compared to super cardioids, they have a bigger lobe of sensitivity with its null points at 250 and 110 degrees. They are unidirectional which implies that they are responsive to sound coming from one direction and also operate using the pressure-gradient principle with pools of silence on its sides. Its lobe of sensitivity is 6 dB less sensitive than its on-axis and 12 dB less on its sides i.e. at 270 degrees and 90 degrees. Also, they portray a proximity effect and show high levels of sensitivity to vocal plosives.
- Wide Cardioid (Sub-cardioid) Polar Pattern
Despite its unidirectional functionality, this polar pattern encompasses a considerably wide scope as they are somewhat superposition of cardioid and omnidirectional patterns. They operate using a pressure-gradient but lack null points. The difference between its side and on-axis response is 3 dB which makes it to a great extent omnidirectional. At the back, i.e. 180 degrees, the response is less sensitive by 10 dB, but the pattern portrays a proximity effect and is sensitivity to plosives.
- Lobar (Shotgun) Polar Pattern
It is this pattern that allows shotgun microphones to be as directional as they are renowned to be. Such a pattern is based on super-cardioid or in other cases, hyper-cardioid, pattern. To acquire directionality, interference tubes are used and their lobes of sensitivity are positioned at the back and the sides.
Unlike with other different polar patterns, an interference tube is a mandatory requisite for a shotgun patterned mic to work. While other mics have capsules positioned at either end, shotgun microphones have their capsules at the middle of their skinny tubes, thus, the need for an acoustic labyrinth to extend their reach. By default, such mics rely on the pressure-gradient principle since they are based on super or hyper cardioid polar patterns. On the other hand, its irregular shape implies that it has 4 null pints i.e. at 300, 240, 120, and 60 degrees, while its sensitivity is less 10 dB at 180 degrees (back) and less by 18 dB at 270 and 90 degrees (sides). Lastly, it portrays a proximity effect and is sensitive to vocal plosives.
- Boundary (PZM) Polar Pattern
This pattern usually takes a hemispherical form which is made possible by a boundary that is in close proximity to the diaphragm. This boundary is meant to be extremely close to the diaphragm in a bid to get rid of rea reflections that have the ability to bring about phase problems whenever the mic is positioned too close to noise producing surface or subject. On the other hand, such mics can work either unidirectional or omnidirectional.
Basic Microphone Pickup Terminologies
- On-Axis vs. Off-Axis
As aforementioned, the lines extending from the mic to the graph’s circumference represent deviations from the ‘on-axis’. On-axis implies that the sound reaches the mic from the front while the Off-axis implies that sound reaches the mic from either the rear or the side. In essence, the 0° point is used in reference to the on-axis direction of a microphone. All the other sound sources, that is, sound emanating from other directions apart from the on-axis is referred to as the Off-axis. To this end, the on-axis runs outward in a perpendicular position to the mic’s front center. It is the direction that the microphone points towards. Inversely, Off-axis represents all other directions from which the mic is unable to collect or record sound signals. For instance, if a mic has a 180° off-axis, this implies that the off-axis is situated behind the microphone.
- Side-Address vs. Top-Address
Noteworthy, while most off and on-axis parameters are represented in 2D on most polar pattern charts, physically, they occupy space in 3D. With that in mind, when trying to identify the on-axis line of different mic pickup patterns, the answer mainly lies in whether the mic in question is a side-address or a top-address. In a side-address mic, the on-axis line is situated on its side, and its points outwards. This implies that they are more sensitive to sound signals emanating from their sides rather than from the front. In the current market, most side-address mics are large diaphragm condensers or ribbon microphones.
On the flip side, in a top-address mic, the on-axis line is situated at its apex and points outwards. Usually, such mics have capsules positioned at the bottom or end of their bodies since they derive sound from the direction in which they point. Majority of hand-held and pencil microphones are top-address.
- Pressure Principle
A pressure mic is one with only one of its sides receptive to external sound waves while the opposite side is unreceptive to external variables due to its fixed pressure system. With sound pressure being a scalar quantity, such microphones are categorized as omnidirectional. More importantly, given that only one side is subject to external pressures, pressure microphones are mostly immune to vocal plosives and show zero proximity effect.
- Pressure Gradient Principle
Contrary to the former, pressure-gradient microphones are those with both sides receptive to external sound waves. Pressure-gradient capsules are utilized in making majority of omnidirectional patterns present in mics with multiple patterns. Also, they are used in all condenser and directional dynamic mics. In its most natural form, such a mic produces a figure 8 (bidirectional) kind of response. This is a pattern whereby a figure similar to the number 8 is exhibited, due to the sunken sides which represents their openness to external pressure.
- Acoustic Labyrinth
These are pathways and ports that have been designed specifically to delay sound from reaching either end of the microphone’s diaphragm. Capsules or cartridges with such a system are able to attain unidirectionality with only one diaphragm. In essence, there has to be some phase or delay in-between the back and front of microphone diaphragms in a bid to attain unidirectionality. In reality, acoustic labyrinths are used to help mics realize this delay. Noteworthy, bidirectional and omnidirectional mics lack acoustic labyrinths.
- Proximity effect
Usually, when mics are brought closer to the subject or source of sound, pressure-gradient microphones experience a rise in their bass responsiveness. This increase is what experts describe as the proximity effect. Bass goes up as a result of increased need of differences in phase between the back and front diaphragms with reference to amplitude variations. Notably, this effect is only applicable in pressure-gradient microphones since each side of the diaphragm needs to be subjected to identical sound waves in order to realize optimal functionality of the mic.
- Vocal Plosives
When speaking, it is impossible to prevent wind/gases from interfering with your sound. As such, plosives are a term used to describe the robust gusts of wind energy that interfere with sound, especially when produced via the mouth. Normally, plosives are created when specific parts of the speaker’s mouth close during the pronunciation of consonant sounds. These parts include the tongue, teeth, rear side of the mouth, and the lips. In the English dialect, plosives occur during the pronunciation of K, G, D, B, T, and P. While some polar patterns are suited for such pronunciations, others merely make them sound as pops which can be very unpleasant to the ear.
- Off-Axis Coloration
This is the difference between frequency response on the off-axis and its response specifications as detailed by the on-axis. Ideally, off-axis coloration is the reason why sound captured in off-axis sound different.
The Importance of Polar Patterns
Developing a sober understanding of the different mic pickup patterns is of utmost importance as it enables users to more easily and quickly visualizing the directionality of any kind of recording mic. As such, users are in a better position top pick the most effective mic for different sessions. More impotently, one is able to identify the least and most sensitive areas of the microphone as per its polar pattern. As such, users are in a better position to make informed decisions determining the mic’s placement. This way, users are able to maximize on sound recording and avoid unwanted frequencies.