Wednesday, September 08, 2010

Comparison of Two Low Frequency Equalization Techniques

Download QuickTime 4 Channel Surround Comparison movie [ 6mb  .zip ]
Download QuickTime Stereo Comparison movie [ 4mb .zip ]

It is fairly common post production practice to use High Pass (HP) or "Roll-Off" equalization in addressing exaggerated low frequencies in field recordings. This movie compares this technique to one that does not use any HP filtering but rather a number of parametric "notches" at offending lower mid-range frequencies under 500 Hz. An EQ "plug-in" is inserted between two M-S matrix plugs in the mixing application so that separate EQ can be applied to the center and sides of the stereo field. (Here's a sample Reaper template using different, native plugs .zip]. Lower-mid range attenuation is most effective in the center of the stereo field as sound waves under 500 Hz pass freely to both sides of stereo mic arrays where they are summed, in phase. Boosting the level of the out of phase sound energy on the sides under 100 Hz can also improve the sense of spaciousness in some recordings. 

In the movie, two samples are repeated three times: (1) Flat, as recorded without EQ (2) Equalized only with Lower-Mid-Range Parametric Attenuation and  (3) Equalized only with 6 dB/Octave High Pass Filtering.  The samples come from an outdoor surround array comprised of two different stereo mic arrays: [Front S3]   [Rear PMBM1]. The arrays are separated 400 feet in a wooded valley. I chose material to demonstrate that the EQ technique works with different types of stereo arrays and that equalization is dependent upon the differing acoustics at the two positions. Overhead thunder provides ample, low frequency content that I found I could adjust more realistically than man-made low Hz "pollution."   I find lower-mid-range parametric filtering is effective at improving the clarity of most ambient field recordings. I've often wondered if sound wavelengths on the order of 4-24  feet (100-500Hz)  could be more prone to acoustic confusion in large spaces somehow. Mic technology is probably playing a role as well, but the wavelengths in this range seem to reinforce each other and form tight bundles of discordant frequencies that "mask" sounds with similar color very efficiently. There is more description of the steps I use when equalizing here

If you are already set-up for 4 or more channels of surround monitoring with QuickTime, download and play the 4 channel version on your computer.  If you play this version on a computer that is not set-up for surround playback, the rear stereo channels will automatically "fold" or mix into the front stereo pair. For those with stereo only, it will probably be easier to evaluate the results by downloading and playing the stereo version that has samples only from the rear array:  [ 4mb .zip ]

Both comparison movies have no equalization adjustments over 500 Hz and the attenuation of the lower mid-range content can make the overall tonal balance seem too "bright."  Here is a longer mix of the 4 channel surround recording made with with Lower-Mid-Range Parametric Attenuation and above 500 Hz equalization:

Download QuickTime 4 Channel Surround Comparison movie [ 25mb  .zip ]


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