Jan 182013
 

Welcome back to the Auria for iPad Blog’s three-part series examining the ChannelStrip. The ChannelStrip is available on the FX panel of each channel in Auria and has three components: the Expander, EQualizer, and Compressor. In this series, we’ve been treating the kick drum track from the Auria demo song, “The Approach.” In the first post, we used the Expander to clean up and isolate the kick drum. In the second post we applied the Compressor to refine it and give it a bit more punch and consistency. In this post, we’ll be adding the EQualizer to add a few final touches.

Just like the previous posts, we’ll go over some basic principles of equalization before jumping into the tutorial.

EQ

An equalizer is essentially a collection of one or more filters that can be used to augment or diminish certain frequencies across the audio spectrum. Each filter has a set of parameters that determine what range of frequencies are affected and how drastic the effect is. The two most common types of filters are shelving (first order) filters and band-pass (second order) filters.

Shelving filters look like this:

 

and band-pass filters look like this:

 

Shelving filters will often be referred to with terms like “high-pass” or “low-cut,” because of the effect that they have. A high pass filter will substantially diminish the frequencies below it, letting the higher frequencies pass through unaffected. A high-pass filter might be used on vocals to eliminate unwanted “B” or “P” sounds, while a low-pass filter might be used to diminish hiss on a noisy guitar track.

Band-pass filters are a more targeted type of filter—they are generally used to affect a particular frequency or range of frequencies, while leaving the rest of the spectrum untouched. Band-pass filters might be used to augment the strumming sounds on a guitar track or remove overly resonant overtones in a snare drum.

With any EQ, its filters will have a set of parameters that determine their behavior. The three most common are Frequency, Gain, and Q-value. Frequency and gain are pretty self-explanatory: frequency is the value at which the filter is centered, and gain is the number of decibels by which the filter is either boosting or reducing the affected area. Q-value can be thought of as the “slope” of the filter. It dictates whether the range of frequencies the filter is acting on is narrow or more widespread. A band-pass filter with a high Q-value would affect a small range of frequencies, useful for making precise, detailed adjustments. A low Q-value would make a broader adjustment to the area surrounding the frequency value.

Let’s look at a situation in which a high Q-value would be used. In a live performance, there are often specific frequencies that trigger feedback (due to a number of factors like the size of the room, the position of the stage, and the type of loudspeakers). If you’ve ever heard feedback like this, you know that it is a screeching, single frequency. In this situation, the live engineer would locate such a frequency and reduce it using a high Q-value. Since such a small range of frequencies is targeted (often less than 10Hz), the overall sound will not be affected much, but sounds with activity in or near this area of the spectrum will be much less likely to cause feedback than before.

Pro Tip: When trying to eliminate specifically problematic frequencies like in the example above, it is often much easier to locate them by first boosting the frequency instead. Using a high Q-value, turn the gain up substantially. Then, sweep the filter in the area where the problem frequency should be. You should hear it jump out when the filter sweeps over it, due to the combination of high-Q value and excessive gain. Once the center frequency is set, turn the gain down so that these frequencies are being attenuated.

A low Q-value should be used when a less targeted adjustment is desired. For example, vocals are often mixed with a augmentation of the “presence” range. This span of frequencies, often stated as being between 4 kHz and 6kHz, makes vocals or instruments sound closer and more distinct (thus the term “presence”). Since a broader emphasis of this range is desired, a lower Q-value would be used to boost the general area.

On some equalizers there are more parameters than Frequency, Gain, and Q-value, but an understanding of these three should get you started. Let’s take a look at the controls on the EQualizer:

The EQualizer

High Pass Filter – Sets the cut off frequency of the high pass filter. Use the IN button to engage this filter.

Low Pass Filter - Sets the cut off frequency of the low pass filter. Use the IN button to engage this filter.

Low Middle Filter – Use the frequency knob to adjust the middle frequency of the bell type low mid filter. Use the gain knob to set the amount of gain for this filter. The gain knob is not scaled in precise dB as the actual gain value varies depending on the Q setting. Use the switch to control the Q factor of the filter. Use the IN button to engage this filter.

High Middle Filter – Use the frequency knob to adjust the middle frequency of the bell type high mid filter. Use the gain knob to set the amount of gain for this filter. The gain knob is not scaled in precise dB as the actual gain value varies depending on the Q setting. Use the switch to control the Q factor of the filter. Use the IN button to engage this filter.

Low Shelf Filter- Use the knob to set the corner or middle frequency of the low filter. Use the gain knob to set the amount of gain for this filter. The gain knob is not scaled in precise dB as the actual gain value varies depending on the steepness/type setting. Use the switch to control the steepness of the shelf filter or to switch to a bell mode. Use the IN button to engage this filter.

High Shelf Filter – Use the knob to set the corner or middle frequency of the high filter. Use the gain knob to set the amount of gain for this filter. The gain knob is not scaled in precise dB as the actual gain value varies depending on the steepness/type setting. Use the switch to control the steepness of the shelf filter or to switch to a bell mode. Use the IN button to engage this filter.

EQ -> CMP/CMP -> EQ switch – This switch determines which module will be first in the signal chain, the equalizer or compressor module.

EQ – This button toggles the EQ module in or out of the ChannelStrip.

Output – Adjusts the output gain of the EQ module.

Now that we’ve covered the basics of equalization and we’re familiar with the EQualizer’s controls, let’s move on!

Tutorial

We’ll be using the EQualizer to put the final touches on the kick drum track from the demo song, “The Approach.” It should be stated up-front that EQ is probably the most subjective of the three treatments, compared to applying an expander or compressor. As is always the case, your ear is the ultimate reference, but these guidelines will point you in the right direction.

EQing a kick drum requires the treatment of a few key areas.

The first is in the 60-100Hz range. This is often called the “boom” or the “thump.” Big, bass-heavy kick drum sounds come from emphasizing the lower limit of that range, while a tight, punchy sound can be achieved by boosting the frequencies near the upper end of it.

The next is the “smack” or the “snap.” This is the sound of the beater actually hitting the drumhead, and is usually somewhere between 3 and 5kHz. Augmenting this characteristic of the kick drum will help it cut through the mix and give clarity to its attack.

Some may also choose to highlight a higher aspect of the interaction between the beater and the drumhead, often called the “click.” It can be found between 6-10kHz. Boosting it will noticeably bring the attack of the kick drum to the forefront of the mix. In metal or heavier rock, the “click” is often brought out substantially. However, it gives the kick-drum a slightly more aggressive sound, which isn’t to be desired in all situations.

For these three areas, you will most likely wish to reinforce them by adding small amounts of gain using narrow filters. Hone-in on the characteristic you’re looking for and add a few dB with a high Q-Value. However, what’s known as the “mud” can be adjusted more substantially.

The “mud” is in the middle, and it makes the kick drum sound overly resonant and flimsy. Cutting out a wide portion of this mid-range, known as “scooping,” will help. This filter is generally centered near 500-600Hz. In rock or hip-hop, this scoop should be pretty drastic. In jazz or folk music, it can be less so, as a more natural sounding kick drum is more appropriate.

Now, let’s apply this knowledge to our kick track.

We’ll start by using the Low Shelf Filter for the “boom.” Let’s pick the narrowest Q-setting and add an (excessive) 10db of gain so we can clearly hear the effect. The three frequency centers we would consider dialing in are the 68Hz, 87Hz, and 112Hz settings. Listening to all three, 68Hz adds a bit too much boom, while 112Hz is encroaching on that flimsy, box-like territory. We’ll go with 87Hz, and dial the gain back to +3db for a slight boost.

Next, we’ll use the High Middle Filter to boost the “smack.” Again, we’ll use the narrowest Q-setting and add 10db of gain. The three settings in this range we should consider are 3138Hz, 4104Hz, and 5367Hz. 5367Hz sounds a bit too high and unnatural, but the other two are both acceptable. Picking one is really up to your ear and what’s happening in the rest of the mix. For these purposes, we’ll go with the 3138Hz, and we’ll scale the gain back to +4db for a noticeable accentuation.

Augmenting the “click” doesn’t seem necessary on this tune, but if we wanted to, we could use the High Shelf Filter. The 7368Hz setting is really the only one that fits, and it could be brought up a few dB on the narrowest Q-setting if you so desire.

To lessen the “mud” we’ll use the Low Middle Filter. To get a sense of what we’ll be eliminating, start by adding 10dB again using the middle Q-setting. Check the 382Hz, 500Hz, 654Hz, and 1118Hz settings. Notice how they all sound pretty terrible, like kicking a cardboard box. The settings to the immediate outside of this range start to sound less bad, so one of these four is probably the right place. Since we’ll be using either the middle or lowest Q-setting, and therefore affecting a wider area, we probably want to use one of the middle two settings as to center the scoop appropriately. We’ll go with 654Hz and set the gain at around -5dB. We could choose to reduce this even further, as well as potentially use the widest Q-setting. Again, use your ears and determine what sounds best.

The last thing you may want to do is roll-off the extreme low-end using the High Pass Filter. Below 60Hz, things can get pretty muddy, and attenuating them on the kick-track wont affect it much. Set the High Pass Filter to 60Hz and we’re done!

We should now have a great sounding kick drum. We got it isolated and clean by using the Expander, consistent and punchy by adding the Compressor, and now it’s full and snappy after applying the EQualizer. Let’s listen to the original track:

 

And now the track with all of our adjustments:

 

Sounds great! This concludes the three-part series on the ChannelStrip. Happy mixing!

 

  2 Responses to “Closer Look: ChannelStrip (Part Three): EQualizer”

  1. Good article… But it would be better if you could label the diagrams with the knob functions! E.g High Middle Filter, Low Shelf Filter etc. Which knobs are they?
    Andy

  2. [...] While we were all thrilled to get the news that a massively powerful portable recording studio like Auria had arrived on the iPad, many of us were a bit overwhelmed with the complexity of the app.  It’s been a while since Auria hit the App Store, but many of us are still dealing with the app’s massive learning curve.  Fortunately, the folks at WaveMachine Labs feel our pain, so they have been putting together a great series of blog posts on the numerous features of Auria.  The latest post continues their look at the Channel Strip, with a major focus on the Equalizer.  It’s a good look at a piece of Auria can make a huge difference on the overall sound of your music – you can find it HERE. [...]

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