Monday, January 14, 2013

Low Shelf vs. Low Cut

A couple times recently the difference between types of EQ has come up. Specifically what the sonic difference was between the low EQ on a channel and the low cut button. In this instance both were labeled at 80 Hz so I can understand how the different uses could be confusing.

The difference is the shape of the filter. On less expensive consoles you'll often just get a low cut button near the trim pot. It turns on a filter with a very steep roll off below the labeled frequency. It's more properly called a high pass filter as it does a good job below the cutoff frequency and lets the highs pass relatively unaffected. On larger consoles it will often be labeled high pass and additionally have a sweep knob so you can set the cutoff frequency.

It's used not so much for shaping the sound as it is removing unwanted content. In a lot of cases the kick drum and bass guitar are the only channels without the low cut engaged. On a console with a sweep high pass you can do quite a nice job of not only removing stage rumble and popped consonants, but of preserving headroom in the system (or on the album). It can be used artistically as well. With a four band EQ at my disposal for vocals I'll sometimes roll the high pass up as far as 300 or 400 Hz on a female vocal. When things start to sound unnatural I back off a little.

Moving on to the channel EQ, the low EQ is most often a shelf EQ. That means that as you roll off gain the shape of the filter is such that all information below the cut off frequency is reduced the same amount. Again on larger consoles you'll often get a sweep knob to control where that cut off is. The frequency response graph looks like a shelf.

Really nice consoles and a lot of plugins will also let you switch the shape of the filter between shelf and band, which is the bell shaped curve seen on the mid EQs. A fully parametric EQ will even give you control over the width of the curve.

Knowing exactly what's behind those knobs at the top and bottom of the EQ section is what sets the Brethren of the Knob and Fader apart from the posers.  The next time you step up to an unfamiliar desk take a good look and make sure you know what you're dealing with before you start working your magic.

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