Tech's Files: The Time and Space Continuum

Jun 1, 2009 12:00 PM, By Eddie Ciletti

UNDERSTANDING AUDIO PHASE AND POLARITY IN STUDIO ACOUSTICS

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X-Y Plot

Here's an unusual way to reveal the direction of the offending waves and ultimately determine the treatment location in real time. Back when disk mastering engineers were obsessed with out-of-phase, low-frequency information, there was always an oscilloscope set to X-Y mode to show the phase relationship between the two channels. (For more info on this, see the “Audio Science” sidebar.)

Start by setting up a pair of cardioid mics in X-Y mode at the primary monitoring position (the sweet spot). Connect the mic preamp's outputs to the 'scope's inputs, also in X-Y mode. Most workstations have a multifunction audio generator capable of pink noise, square and sine waves. Start with pink noise to get levels and X-Y balance. Prove to yourself that everything is as it should be by reversing the polarity of one mic channel as shown in the graphic of the 'scope displays.

Switch the audio generator to sine wave and slowly sweep from 250 Hz down to 40 Hz. Along the way, your ears are likely to notice bass bumps and holes — it's useful to document these — and, hopefully, the scope's X-Y display will concur. Set the oscillator to a “hole” frequency, preferably above 100 Hz. Grab a portable bass trap, like Real Traps' MiniTrap or equivalent (2×4 feet by 4 inches thick), and move around with the trap until you find a position that has the most dramatic positive effect on the 'scope's X-Y display. The trap will vibrate when it intercepts the wave and may do so more when at a 45-degree angle. Note that the position of the room's door — open, closed or in-between — can also affect the path that the sound takes around the room.

The goal is to find a location and orientation for the bass trap that improves the phase at the listening position. Once you get that far, you can experiment with the density of the trap. Good luck!


Eddie's acoustics toolkit includes Smaart Version 5 (for its 24th-octave spectrum analysis) and Wavelab 5 for its ability to turn an impulse recording into a Waterfall Plot.

AUDIO SCIENCE

Meet Lisa

An oscilloscope is typically used to measure amplitude and frequency over time, much like a DAW's waveform screen. But in X-Y mode, each audio channel becomes a horizontal or vertical line that individually shows only amplitude, but together opens a window into the relationships between them, generating a Lissajous pattern. In-phase mono is a 45-degree diagonal line. Reverse polarity on one channel flips the diagonal direction. A phase difference of 90 degrees yields a circle.
Eddie Ciletti






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