EQ or Filters
EQ stands for equalization, but really it refers to filtering. Filters can lessen or remove certain frequencies vs. others (which can be used to equalize the levels of certain frequency ranges in the signal, hence equalization). They are used to a very large extent throughout most guitar signal chains, with tone knobs, stomp pedals, amps, mixing boards, and DAW's all performing some filtering.
Contents
Cutting vs. Boosting
Good advice is to try to use EQ to cut frequencies instead of boosting. Filters cannot add frequencies - what they do to boost is to cut all other frequencies, then amplify the resulting signal. All filters reduce the signal level relative to the noise floor, reducing SNR. Boosting a small range of frequencies is actually cutting most frequencies and thus dramatically raising the noise level.
Also, the way we perceive volume we often mistake volume for SNR. In other words, we tend to prefer a louder signal over an identical but softer one. By boosting with EQ, you increase the volume while decreasing SNR. You may initially perceive this as an improvement, but it may sound better to simply turn up the volume without filtering. By restricting yourself to cutting, you do not run this risk.
By boosting with EQ you also run into the risk of clipping the signal, even subtly.
Digital EQ
Digital EQ's use phase-shifting to perform the task of analog filters. While this gives more control over exactly how you EQ the sound, it can also lead to artifacts such as transient-smearing, depending on exactly how the EQ operates.
Center/Cutoff Frequency
EQ's have a center or cutoff frequency that determines where on the frequency domain the change will be focused. For filters that use a center frequency, this is the frequency that will be most impacted by the filter, with neighboring frequencies also affected but to lesser degrees. For filters that use a cutoff frequency, it basically describes the frequency above or below which other frequencies are impacted. For low/high pass filters, it refers to the frequency that has -3 db of gain reduction.
Resonance
Some filters have resonance around the cutoff frequency, where a (usually) small band of frequencies actually receive a gain boost. This essentially emphasizes the cutoff frequency.
Q Factor
Q factor refers to the size of the range of affected frequencies. Another way to look at it is how noticeable or subtle the EQ change is in relation to the center/cutoff frequency compared to neighboring frequencies. Q is measured as 1/n octaves; a Q factor of 1 means the range of impacted frequencies is 1 octave wide. As Q increases, the size of impacted frequencies shrinks.
| Center Frequency | Q Factor | Low Frequency | High Frequency |
|---|---|---|---|
| 1,000 Hz | 0.5 | 500 Hz | 2,000 Hz |
| 1,000 Hz | 1 | 750 Hz | 1,500 Hz |
| 1,000 Hz | 2 | 875 Hz | 1,250 Hz |
Gain
Gain simply refers to the desired impact to the selected frequencies. In a band-stop/pass filter, this refers to the amount of gain boost/reduction applied to the center frequency. In a shelf filter, it refers to all frequencies past the cutoff beyond any transition band specified by Q factor. High/Low-Pass filters typically do not have a gain setting - the amount of gain reduction is determined by the distance from the cutoff frequency and the Q Factor setting.
Common EQ's
- Band-Pass/Band-Stop - a band-pass cuts all frequencies other than the specified frequency band, essentially boosting that band in relation to the rest of the signal. A band-stop does the opposite, cutting the specified band.
- Low/High-Pass - a low-pass attenuates frequencies higher than the cutoff frequency, while a high-pass attenuates those lower. The further the distance from the cutoff frequency, the greater the attenuation, usually adjustible to gradually roll off frequencies past the cutoff or blatantly remove them. Useful to remove droning lows or fizzy highs that sound noisy or inconsistent compared to the core part of the instrument's tone.
- Low/High-Shelf - A shelf filter is able to boost or cut all frequencies below (low shelf) or above (high shelf) a cutoff frequency. The amount of boost or cut is consistent for all frequencies above the cutoff frequency, unlike a low/high-pass which attenuates more strongly the futher from the cutoff.
- Parametric EQ - A band-pass/stop filter that allows adjustment of center frequency, Q, and gain to dial in the desired EQ change.
- Graphic EQ - Several band-pass/stop filters arranged in an array of frequency bands with vertical sliders to control gain. Visually, the EQ looks like the frequency domain, so the user can clearly see the impact the EQ settings will have on the signal.
- Notch EQ - Provides one or more band-stop filters with variable center frequency and gain, but fixed very narrow Q. Useful to remove small "problem" frequencies from the signal.
Common Guitar EQ
Most amps have Bass, Mids, Treble, and Presence controls, or BMTP. The exact ranges for each control depend on the amp, and often they actually interact with each other, shifting the shape and center frequency of the others. They roughly correspond to the following frequency ranges:
- Bass - 60 - 350 Hz
- Mids - 350 - 1,800 Hz
- Treb - 1,800 - 5,000 Hz
- Pres - 3,000 - 10,000 Hz
Some amplifiers or pedals simply use a tone control. Often this is just a band-pass filter with a wide Q, and the control shifts the center frequency. Thus, low settings allow low frequencies to pass while attenuating highs and vice versa.
Some amplifiers or pedals feature a graphic EQ with 5 or more bands, allowing greater control over the frequency response. The popular Mesa/Boogie Mark EQ labels its frequencies: 80, 240, 750, 2200, 5500 (although some analysis may reveal somewhat different ones in actual operation).
Sometimes, amps use switches, labeled things like "deep", "modern", "mid-shift", or "bright". These typically use EQ to alter the tone of the amp; however, there is often more to it than that, involving feedback and other non-filter based circuitry or algorithms that affect frequency response.
Pre vs. Post EQ
The way filters impact the audible tone of the guitar signal depend on their location in the signal chain, particularly in relation to distortion stages. Because most signal chains only feature one (or at least one main) distortion stage, EQ is often refered to as pre-EQ if it processes the signal before that distortion stage or post-EQ if after.
Distortion typically works by boosting a signal level past a threshold that the circuitry or algorithm can reliably process. The part of the signal that exceeds the threshold is clipped off. This reduces overall dynamics, serving to compress the signal. Frequencies that were barely audible before distortion may be boosted near the threshold, becoming just as audible as those that were attempted to be boosted past the threshold. By clipping the signal, distortion makes the waveform more similar to a square wave, adding lots of high-order harmonics above the clipped frequencies.
Pre-EQ
Pre-EQ can alter which frequencies get pushed into distortion vs. which ones are simply boosted up towards the threshold and thus become more audible. I classify distortion into one of three types:
| Distortion Type | Characteristic Sound | Created by Pre-EQ |
|---|---|---|
| Fuzz | Loose, flubby low-end and raucous, buzzy tone; intervals and chords sound like each note is fighting for prominence | Prominent bass and either scooped or flat mids |
| Crunch | Crisp rhythms and creamy leads; neither loose nor tight | Flat or prominent mids, low to medium bass |
| Metal/Lead | Very tight, thick bass response and searing high-end with lots of high-frequency harmonics | Prominent upper midrange and treble with low bass |
These roughly correspond to dark, neutral, and bright tones. Of course, the distortion stage used also has a large impact on what kind of tone you will get. Don't expect to get a metal tone from a fuzz stompbox simply by pre-EQ'ing.
Here's a guide what to expect tonally based on whether the tone is pre-EQ'ed to make certain frequencies prominent or deficient:
| Freq. Range | Prominent | Deficient |
|---|---|---|
| 0 - 150 | Muddy | Thin |
| 150 - 250 | Fuzzy | Thin |
| 250 - 500 | Creamy | Cold |
| 500 - 800 | Flat | Tinny |
| 800 - 1,500 | Djenty | Buzzy |
| 1,500 - 3,000 | Crispy | Sterile |
| 3,000 - 7,000 | Searing | Honky |
| 7,000+ | Gritty | Smooth |
Post-EQ
Post-EQ gives you pretty much what you expect - attenuation or boosting of certain frequencies vs. others. It does not change the distortion tone, but it can bring different aspects of the tone into focus.
| Frequency | Description |
|---|---|
| 0 - 100 | Thump |
| 100 - 200 | Boom |
| 200 - 350 | Punch |
| 350 - 600 | Warmth |
| 600 - 850 | Honk |
| 850 - 1,400 | Cold Djent |
| 1,400 - 2,600 | Hot Djent/Presence |
| 2,600 - 5,000 | Fizz |
| 5,000 - 10,000 | Sizzle |
| 10,000+ | Broken Glass |