karalie7@aol.com
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low/hi shelf vs hi/low pass eq
Hi everyone, I use the low shelf eq on all my vocal tracks which cleans the song up very nicely.  Question one: What is the difference between low shelf and hi pass eq?  Question two: When would use the hi shelf, hi pass eq? and how would you? Thanks for your help. Karalie
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kwgm
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RE: low/hi shelf vs hi/low pass eq
2008/09/11 23:56:43
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I'm not sure of your question -- are you asking the difference between a shelf and a filter?
If so, the only essential difference is the intensity of the filter being used, which is denoted in analog circuitry by the number of "poles", or by the number of db per octave the filter reduces the signal.
In an EQ, a "shelf" is commonly implemented with a single-pole (-6db/oct) filter. Know that -12db attenuation reduces sound power by 1/2, and a 2-pole filter (called a 2nd-order filter by EE-types), is also known as a -12db filter. (Sometimes you'll find EQ's with adjustable shelves, or that use a fixed 2-pole filter.)
Let me clarify your second issue -- a high-shelf is actually implemented with a low pass filter, and a low-shelf is implemented with a high-pass filter. When you think of an EQ as an attenuation circuit, this will make sense. You use a high-shelf band to cut high frequency sounds above the set level, by passing the frequencies below that level -- low-pass. Likewise, you use a low-shelf band to cut all frequencies below the set level, by passsing all frequencys above that value -- high-pass.
If this is confusing, open your Sonitus EQ and play with both ends of the EQ and I hope all will become clear. Remember, an EQ is best used as an attenuation device, to cut frequencies. You use an EQ to enhance certain frequencies by removing others. This requires some rethinking, and is probably why engineers are a bit strange. :#)
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altima_boy_2001
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 01:29:22
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1. Shelf filters can cut or boost signals, pass filters generally only cut signals (except with bumps [resonance] created by high Q levels). 2. Shelf filters apply a constant gain change to all frequencies above or below the specified frequency. Pass filters have increasing gain change (cut) the farther you move away from the specified frequency. Try putting a high-pass filter at 1 kHz and a low-shelf filter at 1 kHz on the Sonitus EQ and look at the difference in the graphs. Set the graph range to +/- 20 dB and the difference becomes obvious.
post edited by altima_boy_2001 - 2008/09/12 01:38:41
You can use me as your eSoundz referral (altima_boy_2001).
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mixsit
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 02:53:14
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Maybe obvious, maybe not but a way of looking at it; With High Pass you're trimming or reducing unwanted bandwidth. Peak/dip/bell' you're correcting imbalance of a frequency spot (although it could be used at the bottom of the range as a trim). With a shelf you're adjusting the overall level of the range it's covering while keeping the relative frequency balance (within it's range) in tact. ..If the second pair of pictures is comparing a high pass and a shelf (too small to see the lables) it maight be that you have the shelf corner frequency low enough so that it looks like a high pass because what would be the flat part is off the end of the scale. (?)
post edited by mixsit - 2008/09/12 03:00:45
Wayne Smith Part time long time.. CathouseSound Mother Ships - StudioCat DAWs Portals - RME
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Marah Mag
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 05:02:32
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The way I came to understand the diff between a pass filter and a shelf filter was that a pass filter rounds off either end of the spectrum, at a certain rate or depth, while a shelf filter divides the spectrum into two levels or bands, each of which remains flat/unchanged relative to itself. Could just as easily be called a plateau filter. Which makes me wonder, do they make multi-shelf filters with more than two shelf levels? Hmmm... a multiband compressor, where only the gain level was doing anything, with adjustable bands, could simulate something like that, couldn't it?
post edited by Marah Mag - 2008/09/12 05:06:14
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altima_boy_2001
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 06:02:11
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Which makes me wonder, do they make multi-shelf filters with more than two shelf levels? Sonitus EQ could do this since it allows every band to be a shelf.
You can use me as your eSoundz referral (altima_boy_2001).
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mixsit
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 11:31:41
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That's an interesting tilt- a mid band shelf as opposed to the 'bell option. It would need to have top and bottom corner point settings unlike Hi and Low shelves. The existing way to do it would be a Hi and Low defining either side of the mid band -but you'd only get one.. edit.. Hmmm... a multiband compressor, where only the gain level was doing anything, with adjustable bands, could simulate something like that, couldn't it?
..Oops. It didn't even sink in the first time, of course that would be the ready-made soluton. A multi-band' with the compression off is just a string of cross overs. 
post edited by mixsit - 2008/09/12 12:14:04
Wayne Smith Part time long time.. CathouseSound Mother Ships - StudioCat DAWs Portals - RME
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mlockett
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 16:28:31
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Examples: On a vocal track, you might want a high pass filter. If you put a HPF at 100hz, it will cut 50hz drastically and 25hz more drastically... which may be desirable, since most of the noise in those ranges are room noise and ventilation systems and not actual vocals. If you want to add some extra presence to the vocal, you might use a high shelf, and boost it. A low pass filter obviously would not be the same at all. I noticed a while back when a hired pro mixed one of my songs, he put a high shelf on my vox.
post edited by mlockett - 2008/09/12 16:29:57
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jacktheexcynic
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RE: low/hi shelf vs hi/low pass eq
2008/09/12 23:47:32
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you can use a shelf and a pass in combination to seriously cut things off at either end as well without running into the issue of boosting the cutoff frequency with the high/lowpass. this is typically done before a mix is finalized to get rid of any unwanted lows (<30-40hz) and extreme highs (>15-18k), particularly if you are going to go to mp3.
i typically use a shelf EQ when i want to make a broad change to a track - for example, i've got a thick vocal i want to thin out or i want to add presence as mlockett said. i use a high/lowpass to roll off frequencies i don't want, such as everything below 100hz on basically all instruments but kick and bass and everything above 1.5k on bass guitar and everything above 5.5k on electrics, etc. (these are approximate numbers of course).
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