In recording circles, you may have heard of the 2:1 rule of ambience when discussing different microphone polar patterns and their sensitivity to room ambience. If you're wondering about this rule, you've come to the right place!
What is the 2:1 rule of ambience? The 2:1 rule of ambience states that a cardioid microphone must be positioned twice as far from a sound source as an omnidirectional microphone in order to capture equal amounts of room ambience. This is due to the microphone polar patterns and the reflections within acoustic spaces.
In this short article, we'll consider this rule, having a look at the cardioid and omnidirectional microphone polar patterns as well as the basics of acoustics.
For more information on microphone placement, check out my article Top 23 Tips For Better Microphone Placement.
A Primer On Acoustics
Acoustics is a deeply complex study of the propagation of sound waves in the natural environment, including gases, liquids and solids.
When it comes to recording audio, we're nearly always dealing with gas (air) and capturing the sound waves that cause vibrations in the air molecules (though we can also record in liquids and solids).
The interactions between sound waves and the “acoustic environment” include the direct sound from the source producing the sound waves to being with along with all the reflections from different boundaries and materials in the environment.
Note that sound energy will be absorbed by the boundaries and the primary medium (i.e., air), and the rate of absorption varies across the frequency spectrum.
When it comes to simple absorption within the medium itself, sound energy follows the inverse-square law, which states that the sound energy will drop by 6 dB for every doubling of distance.
If you're interested in learning everything you need to know about decibels (dB), check out my article What Are Decibels? The Ultimate dB Guide For Audio & Sound.
This is just touching the surface of the complexities of acoustics, but it's important to touch on.
Now if we change our focus to the typical recording environment, at least for music production, we'll have a room. These “live” rooms (as opposed to control rooms) can range from small closets and dedicated vocal booths to carefully designed studio spaces to large cathedrals. The common thread between them is that they are filled with air and have physical boundaries.
The physical boundaries, as we discussed, will reflect sound around the environment, and we will perceive these reflections along with any direct sources in a way that tells us about the acoustic environment we happen to be in.
All else being equal, the further we are from the direct sound source, the greater the reflections to direct sound ratio will be.
When we place a microphone in such an environment, the microphone will pick up the direct sound and the reflections or “ambient sound of the environment” as well. As the microphone is positioned closer to the sound source, it will pick up more direct sound compared to the reflections, and vice versa.
So to sum things up for the purposes of this particular article, we have the direct sound, which travels directly from the sound source to the listener or microphone, and we have the ambient sound (typically “room ambience”), which is made of all the reflections within the space.
This is oversimplified, I know, but it sets the stage for our discussion on the 2:1 rule of ambience.
What Are Omnidirectional Microphones?
Omnidirectional microphones have omnidirectional polar patterns and are equally sensitive to sound from every direction. Unlike their directional counterparts, omni microphone capsules have only one side of their diaphragms open to external sound pressure.
If we were to consider a polar graph with 360° (0° being where the microphone is pointing in 3-space and 180° being to the rear of the mic in 3-space) and concentric circles to represent differences in sensitivity (in dB), we'd have the following polar pattern to represent omnidirectional microphones:
Yes, the solid black line around the circumference of the graph is the polar pattern. That's because an omnidirectional microphone is equally sensitive to sound from every direction.
This trait makes omnidirectional mics except for capturing sounds around a room, but not ideal choices for close-miking sources where having directionality can help to “pinpoint” the capture of those sources.
Now that we understand the necessary info about omnidirectional patterns, let's consider unidirectional or “cardioid-style” patterns.
For more information on omnidirectional microphones, check out my article What Is An Omnidirectional Microphone? (Polar Pattern + Mic Examples).
What Are Cardioid Microphones?
Cardioid microphones have unidirectional cardioid polar/pickup patterns. They're most sensitive to on-axis sounds (where the mics “point” toward). These microphones are generally 6 dB less sensitive to the sides with a null point to their rear. Cardioid mics are revered for their directionality and rejection of rear sounds.
Note that the cardioid polar pattern is also sometimes referred to as the unidirectional, kidney or heart-shaped polar pattern.
We can visualize the cardioid polar pattern in the following diagram, where we can concentric circles for sensitivity, a 360° directional graph, and a solid black line to represent the cardioid polar pattern:
So we can see that a cardioid microphone is the most sensitive on-axis (0°), with a null point of no sensitivity to its rear (180°).
It makes sense, then, that cardioid or “unidirectional” microphones are often used to close-miking sources to minimize bleed from other instruments in the same space and the room ambience overall. Let's carry that thought into the next section.
For more information on cardioid microphones, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).
The 2:1 Rule Of Ambience With Cardioid And Omnidirectional Microphones
Armed with the information we now know about acoustics and cardioid and omnidirectional microphones, let's revisit the 2:1 rule of ambience, which states that a cardioid microphone must be positioned twice as far from a sound source than an omnidirectional microphone in order to capture equal amounts of room ambience.
First off, we know from the aforementioned inverse-square law that any direct sound source energy reaching a microphone at twice the distance from another will drop by 6 dB. We can use that as a starting point.
So assuming the output levels of the two mics in question are the same, in order to have the same relative level from the direct sound source, the mic positioned at twice the distance (the cardioid microphone) would need to have 6 dB more gain than the other (the omnidirectional microphone).
Amplifying the cardioid microphone by an additional 6 dB would effectively bring the sides information being captured (at 90° and 270° of the polar graph), which are generally about 6 dB less sensitive than the on-axis capture, up to the same level as the omnidirectional pattern. Note that the rear sensitivity (at 180°) will ideally still be a null point.
Of course, there are variations in mic design that will cause this rule to vary, but I'm laying out the theory here.
The reflections within the room will happen everywhere, with different resonances in different locations for different frequencies and varying rates of reflections and absorption across different surfaces.
It's practically impossible to state the 2:1 rule of ambience as a hard rule for these facts, but it's certainly the case that the 360° sensitivity of an omnidirectional microphone will pick up these reflections more than a cardioid microphone, with limited directional sensitivity. The reflections or ambience-to-direct signal ratio will be higher in the omni mic.
If we're pointing a cardioid microphone at the source, we would have to back it up to about twice the distance in order to capture the same relative source-to-ambience ratio as the omnidirectional mic (which, by the way, we don't have to point).
This is a loose rule with difficult proof, but it holds up, more or less, as accurate in practical recording applications.
It's critical to note here that the cardioid microphone is being pointed at the source. If the cardioid microphone, for example, were pointing away from the source (with the source in 180° null point), it would only really be capturing the reflections of the environment with no direct sound.
So that is the “2:1 rule of ambience”. I wish you well in utilizing this knowledge in your recording sessions in the future!
What is the 3:1 microphone rule? The 3:1 rule of thumb states when two mics are capturing a source (or multiple sources in one space), the mics should be separated by a distance 3 times that of the shortest mic-to-source distance. Another 3:1 rule says a distant mic should be at least 3 times farther from a source than a close mic.
How far should stereo microphones be placed? There are 3 styles of stereo miking that affect distance:
- Coincident pairs are to have their capsules placed as close together as possible.
- Near-coincident pairs have their capsules placed 6-12 inches apart and angled symmetrically on either side of centre.
- Spaced pairs are typically separated by several feet (usually one-third to one-half the width of a sound stage) and point directly at a sound source.
• What Is A Coincident Pair Of Microphones? (With 2 Techniques)
• What Is A Near-Coincident Pair Of Microphones? (+7 Examples)
• What Is A Spaced Pair Of Microphones? (With 3 Techniques)