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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The phase relationship between two loudspeakers is also critical. Tweaking a digital crossover network on a KRK control room monitoring system was a revelation for me, as it allowed me to manipulate the phase relationship between the midrange cone driver and the dome tweeter. When the timing is wrong, the sound is like every bad monitor you've heard, and when it's right, your ears just know it and no other piece of test equipment is necessary. Considering that the crossover frequency is typically in the middle of the ear's most sensitive region, optimizing crossover phase can also significantly reduce ear fatigue.

A Slice of the Sonic Pie

Just as an equalizer can manipulate bass, mids and treble bands, a spectrum analyzer divides the audio band into many smaller slices to reveal the energy in each band. This can be useful when the user is sorting out a mix, but when combined with a pink-noise generator and an omnidirectional mic, it can also help unravel the mysteries of a control room's frequency response. As pink noise is random, the display can be a bit erratic. A good spectrum analyzer allows the information to be averaged — like watching in slow motion — which is especially useful for observing LF response.

Initially, it's better to look at one monitor at a time, yet it's also important to find the room's acoustic center. Start with both monitors on, place the mic on a boom in the approximate center and then slowly pass through the “center” (between both monitors) to get the best, smoothest high-frequency response. If the response looks like a comb filter, then the mic is not in the center. Any untreated reflections will complicate the process (think mirrors). Once the mic is centered, look at one monitor at a time. The response should be the same, assuming the room design and monitor placement are symmetrical and the monitors are correctly wired and properly functioning.

Back when large studio monitors were the norm, a third-octave EQ was typically inserted into the monitor chain. While this was called “room tuning,” the more correct term might be “voicing.” IMHO, the cure was often worse than the disease, especially when radically different EQ settings were applied to each channel. The treatment should be identical on both channels; otherwise, left/right phase anomalies can seriously degrade the stereo image.

When a spectrum analyzer shows positive “bumps” in the curve, it doesn't indicate the bigger problem of resonance. Sure, a little subtractive EQ helps and in some cases may be enough. Similarly, no amount of EQ can fill in the holes. Understanding why requires more than the two-dimensional analysis of amplitude and frequency, but suffice to say, trying to force a solution with EQ is not the way to go.

Waterfall Plot

You might expect relatively flat response from a close-miked monitor, yet the room will interact with the monitor in a way that the spectrum analyzer can't fully reveal. Spectrum analysis is great at assisting the ear in finding the trouble spots and for documenting the before/after curves. The third dimension is reverberation time for large spaces and decay/resonance for small spaces.

With eyes closed, the ears know when a space is absorptive, reflective or diffuse by how sound bounces around (or doesn't). Enter the Waterfall Plot, a 3-D representation of frequency, amplitude and decay time. Instead of random pink noise, an impulse is used to stimulate the space. This can be electronic or mechanical, such as a simple handclap, a balloon popping or two drumsticks clicked together.

Resonance implies that some frequencies take longer to decay than others, whether due to room dimensions and their ratios or to construction-related sympathetic vibration (windows, walls, cavities, etc.). Mid- to high frequencies behave similarly to light, but low frequencies are more squirrelly. The Master Handbook of Acoustics, by F. Alton Everest, states, “All room modes terminate in the corners of a room.” Surely, you've noticed how bass frequencies are more intense in the corners, and other boundaries often louder than at the monitors.






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