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We now know that EQ's frequency and gain controls can be used to shape a signal's frequency content by boosting or cutting various ranges of frequencies, but what defines the range of frequencies that these boost or cuts apply to? When increasing or decreasing the gain control of an EQ, even though we select a target or center frequency to work on, rarely is that the only frequency that is affected. For example, if I choose to boost a signal by 6 dBs at 2k, the frequencies around 2k also get boosted.
The range or bandwidth of frequencies around the target frequency that the EQ's gain control will affect is known as Q, short for quality. In other words, Q is a way of expressing the frequency width of a filter in relation to the center frequency of a specific filter band. A boost or cut with a narrow or higher Q value will cause the EQ to affect fewer frequencies around the target frequency, while a wider or lower Q value will affect more frequencies around the target.
Listen to this example of a 6 dB boost at 2.5k as I adjust the Q from narrow to wide. Notice that the wider Q settings are more obvious because I'm boosting a wider range of frequencies. (music playing) Traditionally, Q is often represented by this mathematical relationship.
Take the center frequency and divide by the Q value. For example, a Q of 2.0 at 1000 Hz would be 1000 divided by 2, which gives us a bandwidth of 500 Hz. This filter would span a frequency range of 500 Hz, 250 Hz below the center frequency and 250 Hz above. Different EQs use different values to measure Q, so don't be surprised if your EQ doesn't follow the traditional mathematical definition.
Generally, it's safe to assume that higher Q settings result in a narrower bandwidth and lower Q settings result in a wider bandwidth around the target frequency. And realistically that's all we need to know to shape our sound to our liking. Don't let the term quality lead you to believe that a higher Q actually makes the EQ sound better or of higher quality. It just means the band of frequencies that will be boost or cut is more narrow or refined.
Narrow Q settings are most commonly used for fine-tuning a signal's frequency content and honing in on very specific areas, or residences, like the ringing of a snare or vocal sibilance. EQs with extremely narrow Qs are often referred to as notch filters, because they can be used to notch out a very narrow range of frequencies, like an unwanted 60-cycle hum. However, be careful when applying extreme gain changes with very narrow Q settings as unmusical distortion and undesired resonance or ringing can be added to the signal very quickly.
Because frequency and pitch are directly related, large boosts with very narrow Qs can cause normally un-pitched elements like a kick or snare to take on a pitch, or pitched elements to take on a wah-wah effect. In fact, a classic wah-wah pedal is just a sweepable EQ with a large boost and narrow Q. Wide Q settings can be used for broad- brush tonal shaping and are often used in mastering applications, where disrupting the frequency balance with aggressive narrow Q filters would introduce undesired artifacts.
Another way I like to think of it is that narrower or higher Q settings can be used more for solving sound problems while wider or lower Q settings can be used more for tonal shaping tasks. Some EQs have fixed Q settings that you won't be able to control, while others have Q settings that change depending on the frequency selected or the amount of gain used. For example, on the famous Neve 1073 EQ, which has seven fixed frequencies in its mid-band, the Q increases or becomes more narrow as you switch to higher frequencies.
On the Waves Renaissance EQ the Qs are asymmetrical. A boost results in a wider Q, whereas a cut results in a more narrow Q. When possible, for general-purpose EQ tasks, erring on the side of wider Q settings rather than narrower ones should be used when shaping a signal's tone, as they tend to sound more natural. However, you always want to let your ears be your guide. Narrow Q settings can be just the signature sound you're looking for, either to get a certain instrument to pop out of the mix or tame a harsh resonant buildup.
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