Behind The Mixer

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You could potentially improve your mix quality by taking one little step;  learning how the channel EQ controls affect sound frequencies.  They may not work like you expect.

Turning knobs

I recall seeing the EQ knobs labeled “High” and “Low” as I looked upon the sound board during my first time mixing.  “I know what those mean,” I thought.  “I can increase or decrease those high and low frequencies.”  I was only partially right.

The high and low knobs do allow me to boost/cut those appropriate frequencies, but it begs the questions;

  • What are the actual frequencies being affected?
  • How are they being affected?
  • Are they all being treated equally?
  • What about the mid-range is so important that it gets its own “Sweep” knob?
  • And if your channels have a Q option….what in the world is Q?

Before digging into these questions, let’s first cover the two types of channel EQ controls…

Parametric vs. semi-parametric EQ

Parametric equalizers allow the control of three primary parameters:

  • The primary frequency point in which changes will occur. This is called the frequency or center frequency.  A sweep-able control allows you to change this point.
  • The type and amount of change.  For example, a boost or a cut and the degree to which you do that, such as a 3 dB cut or a “-3 dB” change.  This is known as the amplitude or gain.
  • The range of frequencies which will be affected by the change.  This is known as the Q value or bandwidth.  Changing the Q will increase / decrease the range of frequencies around the center frequency.

Using a parametric equalizer, you would alter a sound by;

  1. Picking the center frequency with which you want to focus your tonal changes.
  2. Selecting the bandwidth (Q) which represents the equal number of frequencies to the left and the right of the center frequency which you want affected.  For example, you can select a wide bandwidth to affect a large number of frequencies or a small bandwidth to affect a lesser number of frequencies.  Ever heard the phrase, “cut narrow, boost wide?”
  3. Altering the amplitude by applying a cut or boost to that specified frequency range.

A semi-parametric EQ is a parametric EQ with one or more missing features, typically the bandwidth (Q).  You might see this in a mid-range EQ control that doesn’t give an option for controlling the bandwidth (Q).

When it comes to digital EQ controls wherein you are using a computer screen to visualize the EQ curves and make changes, you are using a parametric EQ that also can allow for using multiple frequency center points so you aren’t limited in the number of points you can control.

Two Forms of Control

So far, the terms frequency center point, amplitude/gain, and bandwidth (Q) have been discussed.  What hasn’t been discussed is the forms of EQ in which these can be controlled.

There are two forms of EQ control;

Peaking EQ. Photo provided by Iainf

Peaking EQ: A peaking EQ is exactly as it sounds.  You select the frequency center point which is set up on the top of a bell curve.  If you have a Q control, you can adjust the width of that bell curve.  Your mid-range controls are peaking EQ’s.  You might even have an EQ control labeled “High-Mid” which would qualify as a mid-range peaking EQ.  In this image, the blue line indicates a wide boost while the red line indicates a narrow cut.

Shelving EQ. Photo provided by Iainf

Shelving EQ: A shelving EQ is one in which only frequencies on one side of the frequency point are affected.  It will still have a natural curve but that amplitude control is only on one side.  This will be your High and Low EQ knobs.  In this case, all frequencies are affected equally.  For example, on the low EQ, all frequencies below 100 Hz are cut or boosted equally.  In this image, the red line indicates a low frequency shelving EQ with a cut to the frequencies.  The blue line indicates a high frequency shelving EQ with a boost to the high’s.

Know your frequency points

There is one critical bit of information we haven’t discussed and it’s the one I overlooked when I first started mixing; the baseline values.

The high and low shelving EQ’s have a set frequency point.  Your mid-range peaking EQ’s have a baseline frequency.  Do you know what they are?  I hate to tell you, but you need to crack open your manual.  For example, in my mixer, the frequency point of the High EQ knob is 10 kHz and the Low EQ knob is 100 Hz.

Knowing the baseline frequencies, especially with the shelving EQ’s, you can identify the exact frequency point in which you will base your cuts and boosts.

Going to the next level

Let’s look at what you can discover given all this information.  I’ve got a mixer with a fixed channel high pass filter of 80 Hz.  Engaging this, all frequencies below 80 Hz are significantly cut with a shelving ratio of around 18 dB per octave.  Considering my Low EQ frequency point is 100 Hz, then when I boost or cut my lows with the HPF engaged, I’m affecting a very small range of frequencies.  If I really wanted to get some thump out of my kick drum, I wouldn’t want that HPF engaged.  But what if I could use that HPF to get a sound from my kick drum that fit the song?

Take-away

The channel EQ buttons give you a lot of control over how you modify a sound.  Knowing exactly how they control them and knowing their frequency characteristics, you might see them in a new light with new possibilities.  And knowing those baseline frequencies…just one more way you can master your mixer and improve your mixes.

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  1. We are using a Behringer SX3282 Eurodesk 32-Channel Mixer With 24 mic channels with XENYX preamps, 8 more stereo channels, 8 subgroups and 8 aux sends at our church (new church). At former we used a Smaller board Eurodesk.
    I read the article but haven’t fully understand it as yet.
    I have not been taught the meaning of the numbers and the lines in between the numbers around each knobs on the EQ furthermore i was not even taught what any number is meant on the board.
    They taught us by feeling out/by hear so to speak. All this was at the previous church i attended.
    I am now trying to learn and to pass on what i will learn to others at this new church. What does the lines/strokes/ dashes in between the numbers around knobs represent? Do i count in 10 tens or by 5’s if you now what i mean?

    Reply

      • Thank you so much. Is information about this in your books? I desire to be a professional sound engineer for church Ministry (I may do it otherwise in the future) and I want other persons at church to be as well. I want to know to if I move to certain tick on knobs say about 15db, should i adjust another knob to 10db & so on then the fader at about 5 db (just examples). Is there a formula?

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  2. EQ is situational, what to do, what do the numbers mean? who knows, you just try stuff and trust your ears. Low volume, high volume, takes different EQ, head set, FOH, takes different EQ, small crowd, no crowd, big crowd, all take different EQ. As for the mid range parametric sweep, with the unity knob with add and cut, I have no clue, but then you don’t need a clue, just be willing to experiment with the knobs and hear what sounds like what you want for the situation. That been said, i look for that point in the sweep that to me seems to be a sweet spot, something that really defines the sound, then I shade that with the high and low EQ knobs. Sometimes I add, sometimes I cut.
    Watch that perfect studio sound turn thin out in space, adjust, then take that back into the studio and it sounds like mud. It can vary with your mood even, my suggestion, never consider an EQ set in stone.

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  3. Correct me if i’m wrong but isn’t it so that shelving EQ e.g HF and LF on mixing console start to affect frequencies way below(HF) or way above(LF) frequency point and first at -18db almost everything below frequency point is gone?

    Reply

  4. Thanks for the first article I’ve read which explained a parametric equalizer in a way that I as a layman could understand!

    I’m hoping you won’t mind giving me a little advice. I have a lot of recordings to transcribe that consist of vocals plus a lot of background noise, and I want to reduce the background noise so that the vocals are easier to understand. I need to do this in such a way that the volume of the vocals stays pretty much constant and only the background noise drops (i.e. if the background noise drops by 90% and the vocals drop by 20%, that’s no good – I need the vocals to drop by no more than maybe 5%).

    I have a piece of parametric equalizer software into which I can enter gain, centre frequency and width, and which allows me to set up as many bands as I like.

    Of course all recordings are different, as are all voices, but can you advise what numbers I should be using to make the male voice easier to understand, and, separately, the female voice? I need across-the-board settings that will work fairly well with a wide variety of voices and recordings.

    At the moment I have this:

    Band 1: cf 60 Hz, width 60, gain -70
    Band 2: cf 250 Hz, width 250, gain -20
    Band 3: cf 1000 Hz, width 1000, gain 21
    Band 4: cf 3500 Hz, width 3500, gain 10
    Band 5: cf 11000 Hz, width 11000, gain -10

    Any thoughts? Of course I can have more or fewer than five bands. Thanks in advance for any help you can offer!

    Reply

    • Patrick, you can likely simplify your process by using the free Audacity audio file program. You open up the audio file, select a portion with the background noise, and then use the built-in noise filter to remove it from the whole track.

      Reply

      • Thanks for the quick reply, Chris. Unfortunately, I can’t use Audacity or a similar program, because I have to come up with something that works in real time without requiring the user to convert. Also, it has to work across the entire PC’s sound. Software exists that does this – I’m using this one: http://sourceforge.net/p/equalizerapo/wiki/Documentation/ – it’s just a question of finding numbers that work across the board.

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        • Well, that’s a tough one! You’ve got the right idea. It’s a matter of defining “background noise.” If it’s the sound of other people / sounds in a room, then that’s tough because your best bet (if you have control) is better microphone usages – closer to source, mic with tight cardioid pattern. If you can’t control than, then I’d focus on dropping anything below 80-100 Hz and knocking down those highs a little. If it’s line noise then it depends on the frequency range in which the noise is coming in. The higher, the easier to cut. The closer to the mid-range, the more that’s going to affect volume level.

          If you can, get a recording of the typical stuff coming through along with the spoken word. Then, play around with that recording and see what you can do.

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  8. I don’t think you sated this clearly regarding a High Pass Filter: “Engaging this, all frequencies below 80 Hz are completely cut out. ” They typically are a shelving EQ at 12-24 dB/Octave (18 dB/Octave is common)! So the word “completely” is not really correct!

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  9. I’m not sure why I need to know the center frequencies on the fixed EQ controls. I do, but if I didn’t I doubt I’d do anything differently. The only time when I think about frequency data on the desk is on the sweepable mid band if I want to, say, dip the 400-500hz area on a vocal mic. I’ll point the sweep control at 450 and bring the gain back a bit. This saves bringing the gain down first then sweeping the cut around which wouldn’t sound good.

    Other than that I don’t think it matters what frequency the fixed high and low bands are centered on. It obviously affects the sound, but I don’t actually need to know what frequency that is, only how it sounds.

    Knowing what frequencies should be adjusted for a better sound is of course a useful skill, but in my opinion it only helps when you can actually change them. I got called to a local church at short notice recently to cover something recently where their normal person was busy. When I slightly boosted the high frequency band on their pulpit mic channel, I heard that boosted band was higher than I would have liked. I couldn’t change it so I backed it back down again. This told me how all the rest of the HF EQ’s would sound on all the other channels. It turned out to be 12k, but I don’t think that knowledge would have benefited or hindered me in any way. What would probably have helped though is if I could have selected a peak rather than a shelf.

    We’re hoping to do a new installation soon, I’ve been reading your article on the Studiolive 24 and am thinking of getting one of those. Centre frequency AND bandwidth control on that :)

    Reply

    • It’s good to know because it gives you a frame of reference. The mixing board is also akin to our musical instrument and in that way, I think it best we know as much as we can surrounding the stuff that directly affects us. For example, I don’t think you need to know how the circuitry works in the mixer. When it comes to articles on EQ’ing, frequencies are tossed around a lot, even though ultimate the ear is the best judge. However, when you know the Low EQ center point on your mixer is 100 Hz and you’re mixing a bass….well, I believe the more we know, the more we can do.

      I also brought in peak and shelving into it because those are also properties that are oft overlooked. I’d rather be complete and let the reader decide on what to focus than to be incomplete. I mean that in a good way.

      Reply

      • I didn’t mean to come across at all as saying that peak/shelving shouldn’t have been explained. I was actually trying to give an example of how knowing that would have helped (in a situation where peak/shelf could be selected). The internet isn’t always great for properly conveying meaning :) I meant that in a good way too.

        Reply

  10. Good article for sure. Our board didn’t actually have all of the specs, as I recently found out. I had to search and eventually found the engineer’s spec (that’s the sheet where they use the word “shall” a lot). The console shall have 32 channels and shall have 8 aux sends, etc.

    In my case, missing information from the manual was the HPF slope (18db per octave in my case), the Q setting for the hi/low mids and even the preamp gain when “off”.

    If you can’t find the information in your manual, the engineer’s spec might be a good option.

    Reply

  11. that’s great stuff you’ve written. It really helps us become better sound techs. thank you. keep going.

    Reply