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Using a Stereo Microphone – a Primer

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Using a Stereo Microphone – a Primer Using A Stereo Microphone – A Primer Stereo Recording stereo microphone can prove useful in the multitrack mixing environment. Practically all of our recordings today involve stereo. While surround Hearing In Stereo releases are becoming more common, even these are largely The two primary cues that our brain based on a primary stereo image uses to help us locate a sound enhanced by effects and ambient source in space are differences in sound to the rear and sides of the arrival time (phase) and amplitude listener. between our two ears. The process works pretty well in real acoustic The standard studio technique for spaces, but it’s confounded popular music production is to create somewhat by loudspeaker a stereo sound field from reproduction, where we hear sound multitracked recordings of coming from two discrete sources monophonic sources panned left- (the loudspeakers) as well as right, with delays and reverberation reflections from the surfaces of the added to create the sense of space. room. True stereo recording - capturing the full acoustic environment from a We can move the perceived location single vantage point using a process of a sound between the speakers similar to that of the human ear simply by changing the amplitude of provides a distinctly different kind of the signal fed to each speaker - this sound. is what a pan pot or stereo balance control does. Another way of Single-point stereo miking has long changing the apparent location of the been used for classical music sound between speakers is by recording. Here, the goal of the sending the identical signal to both recordist is to capture and convey to speakers at the same amplitude but the listener the experience of delaying the sound to one speaker actually being in a concert hall with relative to the other. The sound the orchestra. This is probably still arrives first in one ear, then the the most common application of the other. Our brain tells us that the single-point stereo microphone due source of the sound is coming from to its convenience and its ability to the direction of the ear which hears it accurately capture the sound of the first. acoustic space. Stereo microphone techniques need not be limited to In general we need an amplitude classical recording, however, and difference of about 15 dB in order to are often appropriate for recording in make a sound to appear to be an intimate jazz club or a church. In coming from one speaker or the addition, recording such physically other, but this is dependent on the large instruments as pianos, drum program material. At low kits, or large vocal groups with a frequencies, a 15 dB amplitude difference between speakers makes demonstrated mathematically that for practically no difference in apparent lower frequencies, level differences location. The bass would have to be between speakers are translated into 25 to 30 dB louder on one channel phase differences, with the ear on than the other in order for it to be the side of the louder speaker perceived as being off-center, and leading the other ear in phase. many playback systems do not have sufficient dynamic range to allow This phase shift varies approximately this. linearly with frequency, so it represents a difference in time-of- Mixing engineers almost always pan arrival fairly accurately. At high bass to the center for this reason. In frequencies, the phase differences addition, engineers who were raised get swamped out by amplitude on vinyl learned that moving a large differences, which is what our brain amount of low frequency energy off uses as a cue to their position. center causes the needle to jump out of the groove or damage the disk The real world throws in some cutter. Back when “mastering” was wrinkles like the shape of the the first step in the record listener’s head, but the bottom line is manufacturing process, it was fairly that in spite of a lot of theoretical and common for the mastering engineer practical obstacles, we can actually to separate the lows from the highs, hear stereo when listening to mix the two low frequency channels loudspeaker playback. As recording to mono, and put the tracks back engineers, it’s our job to capture the together, assuring that the bass was sound field so that it can be centered. realistically reproduced through loudspeakers, and there are several With CDs, those mechanical ways to do so. problems went away, but the problem of dealing with a large low Stereo Microphone Applications frequency amplitude difference between channels in order to create During the 1930’s, both Blumlein in the illusion off-center bass still the UK and Harvey Fletcher (of the remains. Phase difference due to the Fletcher-Munson loudness curves) at different path lengths from a source Bell Laboratories in the US to each ear still works at low developed techniques for using frequencies however. This is the microphones in pairs for recording in mechanism by which we are able to stereo. The results of their locate low frequency sounds in the experiments and demonstrations are stereo field. all still relevant and applicable today. Alan Blumlein, an engineer and Typically a stereo mic setup is acoustician at EMI in England is appropriate in situations where generally considered to be the father acoustics are good (or at least not of stereo recording and reproduction detrimental to the sound) and when as we know it today. He the group being recorded is playing all together and is well balanced. As foot, and sometimes with a baffle you might surmise, this best placed between them. describes orchestras, jazz bands, and acoustic folk artists. Since An ideal coincident pair has the classical music is traditionally diaphragms of the two microphones performed in good sounding halls, a occupying the same physical space. well-placed stereo microphone setup Since that’s physically impossible, is frequently all that’s required to we make do with “pretty close.” This capture the performance accurately. can be accomplished by positioning And there’s nothing like a stereo two individual microphones with their recording from a good seat in the heads just about touching, and with house to hear what your band an imaginary line passing through actually sounds like. both diaphragms. Typically classical stereo recordings The important concept here is that are plug-and-play with no overdubs the two microphones work as a pair or punch-ins, no mixing or ducking, to give us both the amplitude and and perhaps only some editing phase differences we need in order between alternate takes. Stereo to locate a sound in space. miking isn’t limited to recording of a fully integrated performance, Coincident Arrays however. In the modern music studio, recording single instruments Coincident arrays require that the with a wide frequency range and microphone diaphragms be placed large radiating area such as acoustic as close together as possible. The guitars, pianos, and percussion LSD-2 incorporates two multi- instruments can add a useful pattern microphone elements in a dimension to a multitrack mix. In single case with the diaphragm of addition, a stereo recording of a well one positioned directly above the balanced instrumental or vocal other, making it very near to an ideal section as part of a multitrack project coincident stereo mic. can add realism as well as save studio and mixdown time. The mics in a stereo array can have a cardioid, bi-directional, or in certain Stereo Microphone Arrays cases omnidirectional directivity pattern. Since the mice are either Stereo microphone arrays fall into pointed in different directions or are two general categories: coincident some distance apart, each mic gets and spaced (sometimes called “near a different perspective on the sound coincident”), which describe how the field. Since a directional mic’s two microphones are positioned sensitivity varies with angular relative to each other. position, any sound originating off center will appear at a different level A spaced stereo pair has the two in each mic of the pair, giving us the microphones separated by a fairly amplitude cue to locate the sound in small distance, usually less than a space. Because the microphones are weaknesses and it’s up to the located in essentially the same engineer which arrangement to use. space, a coincident pair has the advantage of mono compatibility – As a broad generalization, the that there’s little or no loss of content coincident approach produces a when a coincident stereo recording stereo image that’s more focused, are summed to mono. This is more allowing the listener to more important than you might realize. precisely pinpoint a sound source. Although you may not consciously By contrast, the spaced array yields listen in mono very often, consider all a less precise stereo image, tends to the people who wake up to clock produce recordings that are more radios or still hear television audio open-sounding with a wider stereo through a single loudspeaker. If your spread. stereo recording contains too much out-of-phase information between Either technique can work well in a the channels, certain frequency good recording environment, but in a ranges or overall ambience can drop room with a lot of nasty reflections out when mixed to mono, leaving a that you can’t do anything about, thin sound. spaced omnidirectional mics or coincident bi-directional mics are Spaced Arrays more likely to save the day than the more common coincident cardioid Although the LSD-2 by nature of its arrangement. construction doesn’t lend itself to use in a spaced array (unless you buy Some engineers have used a two of them), we would be amiss in a combination of spaced and general discussion not to mention coincident setups, combining them to such an arrangement.
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