How to Use a Parametric EQ

EQs are fairly intuitive to operate. We have all used them before; they are found, in simple form, in the tone controls of home stereo systems. The EQs that you use in mixing are not radically conceptually different from those tone controls: you have a frequency band, and you have a gain amount. But there are some important differences.

For the purposes of mixing, you want to be using a parametric EQ. A parametric EQ is a particular type of EQ which is well-suited to precise and nuanced adjustments of frequency balance. It consists of several “filters;” each of these filters creates a boost or a cut in a frequency range, and its behavior is controlled with three adjustable parameters: frequency, gain, and Q.

The “frequency” parameter sets the center frequency of the filter’s action. The filter will not act on only this frequency; it will act on the center frequency and all of the frequencies surrounding it, with the intensity of the action steadily decreasing with distance from the center frequency.

The width of the affected frequency range is controlled by the “Q” parameter. Lower Q values result in wider ranges; higher Q values result in narrower ranges. A sufficiently high Q will result in essentially only the center frequency being affected.

The “gain” parameter is the simplest of the three parameters of a filter. It simply sets the amount of volume adjustment; specifically the amount of volume adjustment at the center frequency. A negative value will result in a cut, and a positive value will result in a boost.

So how do you decide on values for the frequency, gain, and Q of a given filter? As with leveling, there is a procedure that you can follow. In this procedure, first you find the frequency, and then you find the gain and Q more or less together.

Setting the Frequency

In finding the center frequency, you first need to decide what general frequency range you want to affect, and then what exact frequency you want to center on. Sometimes, particularly as you begin to develop your ear, you will know just from listening what frequency range you want to affect. If you don’t know, then you need to spend some time analyzing the frequency content of your sound.

A spectrum analyzer can tell you where the critical ranges are (they will be the loudest portions of the frequency spectrum), and it can also tell you about the presence of any nonessential frequencies that you might want to cut. To get a more nuanced perspective on the frequency content of your sound, to really figure out what’s what, you can also employ a method known as the “sweep technique.”

To perform the sweep technique, set your filter to a medium Q and a high gain, and simply sweep it across the frequency spectrum, listening as you go. The sweep technique will tell you what the “ingredients” of your sound are, by letting you hear each frequency range individually. Once you have done a sweep, you will have a better idea of what each frequency range is contributing to your sound, and you will be better equipped to decide which ranges you want to boost and cut.

The sweep technique should be avoided whenever possible, for two reasons. First, it is very tiring to the ears. Second, after sweeping, your perception of the sound will be distorted, and you will no longer be in a good position to make judgments about EQ. Don’t go to great lengths to avoid sweeping, but don’t do it when it’s not really necessary. You’ll find that it becomes necessary less often as you begin to develop an ear for what the different frequency ranges sound like.

Presumably at this point you’ve decided on a frequency range that you want to boost or cut. Now you have to decide on a precise frequency to set as your center frequency. Sometimes it doesn’t really matter; just put the center frequency in more or less the center of the range you want to affect. But if you have a tonal sound, then you can sometimes achieve a better effect by setting your center frequency to a prominent tonal frequency.

To do this, you will want to employ the sweep technique again, except over a narrower range, and with a very high Q rather than a low Q. The high Q will allow you to “tune” your center frequency to a strong tonal frequency in the sound. You will know that you have done this when you hear a loud ringing sound.

Setting the Q and Gain 

Once you have found your center frequency, you should fiddle with the gain and Q values until you arrive at a satisfactory result. When boosting, I find myself generally using low to moderate Q values (0.2-10) and less extreme gain values (0.2-4dB), while when cutting I find myself using higher Q values (7+) and more extreme (-2dB or lower) gain values. This is the case for a variety of reasons, as follows.

When boosting, typically I’m boosting a critical range, and often it sounds best to also give the frequencies around the critical range a slight boost, just to make the sound more natural. This accounts for the low Q value. The mild gain value is simply because it seldom sounds natural to give a single region of a sound an extreme boost, and it can actually sometimes result in noticeable phase “smearing,” particularly with high Q values. This smearing can manifest, in its most blatant form, as sustained ringing near the center frequency.

You can, of course, cut critical ranges, in which case similar principles apply in terms of Q and gain settings. But, simply due to the nature of critical ranges, I don’t usually want to cut them. More often I’m dipping in between critical ranges to try and remove undesired frequencies, and I don’t want to cut the desired frequencies, so a high Q value gives me the precise action necessary to do this. I often use a fairly extreme gain value, simply because of the nature of what I’m trying to achieve; I’m trying to remove or substantially reduce undesired frequencies, not subtly reduce undesired frequencies.

None of these things should be taken as rules. These are merely common patterns. Don’t be afraid to do a boost with a high Q and a high gain if the situation calls for it. As always, your ear is the final judge.

Evaluating Your Results 

It can sometimes be hard to judge the results of your EQing. One technique that is helpful is to toggle the “bypass” button on your EQ on and off, to see what your EQing has done to the sound. Is it making the sound better, or worse? With extreme EQing the effects will be very obvious. With subtle EQing, particularly boosts and cuts less than 2dB or so in magnitude, they may be less so. In these cases, just sit back listen to the music for a while, and it should soon become apparent whether the EQ adjustments are helping or hurting the sound.

One final reminder. Always bear in mind that you’re not EQing the sound to sound good by itself; you’re EQing it to sound good in the context of the mix. So while listening to the sound by itself can be helpful, ultimately your judgments have to be based on how it sounds in the mix.

High Shelf/Low Shelf Filters 

Thus far I have made an important omission. Parametric EQs usually supply you with a few different types of filters. In the preceding discussion we have examined only one type of filter: the bandpass filter. The bandpass filter is the most common and important type of filter, but a few other common types of filters also require discussion.

The next types of filter we will look at are the high shelf and low shelf filter. High and low shelf filters have the same parameters as bandpass filters: frequency, gain, and Q. A high shelf filter boosts or cuts all of the frequencies that are higher than its center frequency. A low shelf filter boosts or cuts all of the frequencies that are lower than its center frequency.

That is a simplification. A high shelf filter does not simply adjust the volume of all frequencies above its center frequency, and none of the frequencies below its center frequency. As with bandpass filters, there is a curve involved, with the Q value controlling the steepness of the curve. The center frequency is the frequency at which the volume adjustment is half as much as is promised by the gain value. The same applies to low shelf filters.

High/low shelf filters are most useful when adjusting the balance of critical ranges when those critical ranges happen to be all frequencies above or below a certain frequency. They are also useful for reducing, but not removing, undesirable frequencies of the same description. To entirely remove frequencies above or below a certain frequency, you should use a highpass or lowpass filter.

Highpass/Lowpass Filters

A highpass filter cuts all frequencies below a certain frequency. However, rather than cutting all of them by the same amount, as would a low shelf filter, the gain reduction becomes progressively more extreme with decreasing frequency, until the gain reduction is so extreme that it amounts to complete removal. A highpass filter has just one parameter: the cutoff frequency. The cutoff frequency is the center of the action of the filter; the filter has already begun to act somewhat at the cutoff frequency, but not very much.

A lowpass filter is just the opposite of a highpass filter. Rather than cutting all frequencies below the cutoff frequency, it cuts all frequencies above the cutoff frequency. Other than that it behaves the same.

Some lowpass/highpass filters will also have a “resonance” parameter, which may also be called Q. This resonance/Q parameter is rather unlike the Q parameter for bandpass filters. What it does is it causes the frequencies in a narrow band around the cutoff frequency to be boosted. The higher the resonance value, the more the frequencies are boosted.