Special mic techniques employed on stereo recording are explained this month.
In the recording studio, the stereo image is created with the pan pot during the final mix. When recording on location, however, the proper placement of two microphones in a suitable stereo configuration will ensure a good stereo spread as well as the right balance of instruments and proportion of direct sound to indirect sound (reverberation). A good knowledge of stereo miking techniques will go a long way towards good location recordings, both for the quick demo job in a band's rehearsal room or a full-blown live recording in a hall.
These techniques are also highly suited for recording backing vocals, strings or brass, indeed any acoustically well-balanced ensemble. Most useful for home recording, they may be utilised to 'process' polyphonic background layers, polysynths, string machines and the like.
In this configuration, two unidirectional mics are crossed, or angled apart. The stereo position of an instrument is reproduced by a level difference between channels (the microphone aiming more towards the instrument will deliver more signal). Naturally, the stereo spread will be widened by increasing the crossing angle between the mics. Try out different angles between 90° and 180° (extreme separation). Listen to the stereo spread as well as to the reverberation.
A safe suggestion is the ORTF configuration: a near-coincident arrangement. Crossing angle is 110°, and the mic grilles should be spaced 7 inches apart. X-Y technique is sometimes used for drum overhead mics in the studio.
A-B arrangements use two parallel mics pointed to the sound source and spaced apart. It is the time delay difference due to different path lengths from an instrument to the two mics that establishes the stereo position of that instrument. When mixed to mono this arrangement may cause phase cancellations. Spacing distance between the mics determines the stereo spread and separation between instruments.
A third microphone (panned in the middle of the stereo image) may be employed in the centre between the two stereo mics to cover central instruments that tend to get lost when the mics are spaced too far apart. Experiment with the spacing, starting with about 6 feet.
Personally, I achieve good results with two omnidirectional mics hanging from the ceiling to record my rock band when rehearsing - no extra mic stands in the way, and when the band itself is balanced soundwise, you get a wonderful stereo picture.
When recording a concert or the like, raise your mics at least 10 feet high on a stand. (If you're cutting a bootleg, though, you'll probably need to take a more secret approach...!) Aim the mics down at the band. Now check the distance from the musicians: short distances mean dry sound, giving a 'close' feel. Increase working distances and you will get more room ambience, indirect sound (reverb), and a 'distant' feel. Generally, any acoustically good seat in the hall will also be a good place for your microphone stand.
Stereo miking relies on a well-balanced musical ensemble. Of course, most musicians try to blend the sounds from their instruments but, being used to the benefits of multitrack recording as well as to skillful monitor mixing on stage, they have lost some of their ability to balance their own sound as compared to the old days of recording. Back then, a big dance band was recorded with three or four microphones. Musicians were not only able to perfectly blend their horns into their own section, but also moved closer to the mic for a solo or blew away from the mic when playing loud high notes.
To get a good balance, move musicians around. If an instrument is too quiet move it closer to the microphone.
When recording large ensembles like symphony orchestras, quiet instruments are usually reinforced with 'spot' mics. These are placed as close as two feet and often phase-reversed to avoid phase cancellations. Time lags caused by the longer sound travel path to the stereo mics are often compensated for with a digital delay line.
An inversion of this principle was used on the last Grateful Dead live album. In addition to the close-miking onstage, ambient mics placed in the hal I were fed to the mix to give the record a touch of the sound the concert-goer would usually hear. Again, the onstage mics were delayed to ensure correct time alignment.
Microphones used for stereo applications must be well matched ie. their frequency and polar responses must be equal. Also important is a flat frequency response from 40Hz to about 20kHz, depending on the quality of the other components in your recording chain. The best choice will be condenser mics. Stereo microphones are manufactured by the major companies in the studio microphone market, like AKG, Neumann, Schoeps. But they are pretty expensive! Figure 5 shows the AKG C34 that is used in the pop field, eg. for ABBA's vocals.
The impression of space that is created by stereo miking, especially with spaced omnis (A-B), can be used to add an interesting depth to home recorded keyboard sounds. I used it successfully on my old Solina string machine as well as with polyphonic synths.
The technique is to simply play back your directly recorded tracks through a pair of monitor speakers and re-record them with a stereo mic pair. Choose a rather 'live' room to get a good ambient feel. This may then be mixed with or used to replace the original tracks. This procedure makes for a more diffuse, distant sound that I like for taking the direct edge off the synths when combining them with softer, acoustical instruments. With the string machine it came closer to the desired sound of a large orchestra way back in the mix.