Ever wanted to create that cool stereo sound, but don’t know how? Unsure of how to properly use microphones for stereo recordings? Don’t worry, you have come to the right place. As a sound engineer, you will be required to produce stereo recordings professionally. Hence, the question that you should ask is – What is “Stereo Microphone Techniques”?
Today, you will learn about how to properly use microphones for stereo audio productions. Find out about the various stereo miking techniques that are available. Understand the details of these techniques, in order to perfectly execute them. We will also go through the advantages each technique has over the other. So, you ready yet? Then let us begin!
Introduction – What is “Stereo”?
Stereo is actually a shorthand term for stereophonic. It is essentially a sound reproduction method that produces an illusionary audible perspective that is multi-directional. Multiple audio channels are utilized through a configuration of two or more loudspeakers (or stereo headphones), with a method that creates the perception of sounds coming from various directions, similar to natural hearing.
The term stereophonic sound (stereo) is also applied to other audio systems such as “quadraphonic” and “surround-sound”, including the widely known two-channel, two-speaker systems. It is generally understood to be the exact opposite of monophonic, or “mono” sound, where audio is perceived to only come from one direction, usually centered in the sound field (analogous to a visual field).
The human brain uses two features of sound, when placing objects within the stereo sound-field (between two loudspeakers). The first is the difference in loudness level between the two channels, and second, is the time delay difference in arrival times for the same sound in both channels. Currently, stereo sound is prevalent in almost every sound and entertainment system (Hi-Fi, TV etc.).
The two main aspects of stereo recording that we will cover today is:
- Stereo recording methods
- Mono sound compatibility
Stereo Recording Methods
There are a few different microphone techniques that you can use to produce your stereo recordings. These methods may require some practice, in order to be well executed. Don’t worry too much though, as in this section we will go through the principles of these methods:
- X-Y technique: intensity stereophony
- A-B technique: time-of-arrival stereophony
- M/S technique: Mid/Side stereophony
- Jecklin Disk technique
X-Y technique: intensity stereophony
Two directional microphones are used here, placed together at 90° (or more) to each other. The difference in sound pressure level between the two mics, will produce the stereo effect. The sonic quality of this method however, is not as “open” and “rich” sounding as compared to an A-B setup. This is largely due to possible phase ambiguities, and also the lack of differences in “time-of-arrival”.
When microphones with a bidirectional quality are used, and placed facing around 45° in relation to the sound source, then this X-Y setup is known as the Blumlein Pair. Many authorities in the audio industry consider this configuration capable of producing sonic images that are very realistic.
A-B technique: time-of-arrival stereophony
In this method, two microphones (typically omnidirectional) are positioned parallel to each other, with some distance between them. This technique captures “time-of-arrival” stereo information, together with some sound pressure level difference information, which increases in intensity if the microphone is positioned very close to the sound source.
As a reference, it is good to take note that if the distance between the microphones is about 50 cm (0.5 m), then the time delay for a sound signal reaching the first one from the side, before it reaches the other microphone, is approximately 1.5 ms (1 to 2 ms). Be careful when attempting to increase the distance between the microphones, as it effectively decreases the pickup angle.
M/S technique: Mid/Side stereophony
This technique requires two microphones with different polar patterns. One would be a bidirectional microphone (Figure of 8 pattern) facing towards the side, and a cardioid mic (a variety of cardioid can be used here, although Alan Blumlein himself, has stated the usage of an omnidirectional microphone in his original patent) positioned to face the sound source.
The capsules of the microphones are vertically stacked and placed as closely as possible to each other, in order to reduce comb filtering caused by differences in arrival time. The production of both left and right channels are based on this simple matrix: Left = Mid + Side, Right = Mid − Side (“minus” means that the side signal with reversed polarity is added).
This method will produce a signal that is completely mono-compatible. Furthermore, if the Mid and Side signals are recorded (instead of using the matrixed Left and Right), the stereo width (together with the perceived distance of the sound source) can be attenuated even after the recording session.
Jecklin Disk technique
Quite similar to the A-B recording method, the jecklin disk technique employs 2 omni-directional microphones, positioned at 36cm apart from each other. A Jecklin disk (sound absorbing disk) with a diameter of 35cm, is placed right in the middle of the two microphones. The disk produces an impression of a wider stereo separation between the mics, as compared to an equivalent A/B recording.
Mono Sound Compatibility
Depending on the production requirements, you might need to convert stereo signals into mono. This will result in the cancelling of the out-of-phase parts in the signal. This could potentially reduce or completely nullify some parts of the signal. You need to seriously consider this factor, when choosing a technique to use.
Here are some pointers to look out for:
- The A-B technique is the least compatible with mono, as they rely on phase differences in order to produce the stereo image needed.
- Jecklin disk requires only a short distance between the mics. Sounds coming from the sides can only reach one microphone and will not interfere. Hence, Jecklin disk is reasonably compatible with mono. It also produces phase shifts and amplitude differences that are accurate to what the human ears would hear at this position, thus making the signal suitable for headphone playback.
- For X-Y techniques, the microphones have to precisely be in the same position, which is practically impossible. If they are slightly shifted to the left or right, some high frequencies may be lost during mono playback. Thus, they are usually separated vertically. This will only create problems with sound coming from above or below the height of the microphones.
- The M/S technique is perfect for mono compatibility, because the Mid signal can be re-produced by just summing Left+Right.
You would also want to consider the equipment needed for various techniques. For example, A-B techniques typically have two separate microphone units that are mounted on a bar, which defines their separation. X-Y microphones can also be mounted on a single unit, or even positioned on the top of a handheld digital recorder.
M/S arrays are generally very compact and works well with a standard blimp windscreen, which allows for boom-operated stereo recordings. This also makes the M/S method a commonly used stereo technique for film location recording. Often times, small ‘pencil microphones’ are mounted onto video cameras, and in some cases, coupled to the zoom.
At last, we have finally come to the end of this article. Hope you people enjoyed reading and at the same time, learned a few things on stereo microphone techniques!
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