How does the Equalizer work?

What Does the Equalizer Actually Do?

The equalizer is playfully referred to as the "louderlizer" (pronounced as "louder-leiser"). And indeed, that's exactly what the equalizer (EQ) does: it amplifies parts of the audio signal or attenuates them. The word "equal" means "the same," so it's essentially about equalizing specific parts of the signal to create a balanced sound image. It's also referred to as de-essing the sound. The parts processed by the EQ are the frequencies.

To keep the use of devices like the EQ from becoming too theoretical, the audible frequency spectrum is divided into a few areas for simplicity. Analogous to the relative speed of vibrations (20 Hz to 120 Hz is relatively slow, 120 Hz to about 6000 Hz (6 Kilohertz or KHz) is medium-fast, and everything above that is fast), we call these areas lows (basses), mids, and highs. More precise subdivisions are made with terms like sub-basses, low-mids, and high-mids. However, these are still within the audible spectrum.

BONUS KNOWLEDGE! A very brief - simplified - refresher of physics: the air surrounding us is elastic. It can be compressed and expanded. If, for example, you pluck a guitar string, it starts to vibrate. Sound is produced. The air can be stimulated by the sound and transmits it. This means there is an alternating compression and expansion of the air, propagating in all directions in what are called sound waves.

Our bodies respond to such sound vibrations depending on the frequency. Frequency is simply the number of these vibrations within one second. The human ear is responsible for receiving anything that vibrates roughly 20 times up to about 20,000 times per second. The eardrum in our ear receives the pressure fluctuations in the air and starts to vibrate. It converts these vibrations into information that our brain "interprets" as sounds, tones, and noises.

The unit for frequency is Hertz (Hz). 20 Hz = 20 vibrations per second; 1000 Hz = 1 KHz (Kilohertz) = 1000 vibrations per second.

As we age, our audible frequency range slightly decreases, especially in high frequencies, which are only audible up to about 16,000 vibrations per second.

By the way, everything below 20 Hz is called infrasound. Infrasound is used, for example, by giraffes for communication; humans cannot perceive this communication. On the other hand, everything above 20 KHz is ultrasonic. In this range, animals like bats communicate, utilizing frequencies up to 200 KHz. Children with healthy ears can sometimes perceive frequencies from 16 Hz up to slightly over 20,000 Hz.

The strength of the EQ lies in its ability to manipulate individual parts of the frequency spectrum. Each frequency range essentially gets its own volume control. This is much more effective than changing the overall volume of an instrument (relative to other instruments) because every instrument has at least one frequency range in which it expresses its character. It ensures a clean sound image when you selectively process this frequency range.

BONUS KNOWLEDGE! Apart from the fact that we all listen very subjectively and our ears can be easily deceived with simple tricks, a "technical" balance of frequencies by no means implies that we perceive the signal uniformly. This is because our ear doesn't hear linearly, but logarithmically. Low tones and sounds need to be delivered to the ear at much higher volumes (energy) to be perceived well. High tones require only little energy.

The ear is a part of evolution - it is adapted to nature. The lowest sounds of nature that are audible to humans are also the loudest (most energy-rich): volcanic eruptions and thunder. The rustling of the forest, which we find pleasant and safe, requires significantly less energy. Yet, our ears are very sensitive in the upper mids, allowing even the quietest snap of a twig to be heard, which in nature generally signals potential danger.

The human voice is also in the upper mids - we are able to identify a friend's voice even when we hear it through the telephone receiver, which only transmits upper mids. We perceive the speaking voice at an average volume.

What types of Equalizers (EQs) are there?

As mentioned, the basic principle of the EQ is to alter volumes in specific frequency ranges (frequency bands). Generally, there are three approaches for this.

The fixed EQ

The simplest design can be found on stereo systems or in car radios. Here, you typically find a knob or slider for each frequency band labeled "Bass" (Lows), "Treble" (Highs), and sometimes "Mid" (Mid-range, usually referring to the upper mids). So, you can roughly intervene in the sound profile. However, you're neither told in which band each slider works nor can you fine-tune this range. For stereo purposes, this isn't necessary since you'd generally distort the overall sound, for instance, trying to give small shelf speakers more bass punch. Fine details don't matter here.

The graphic EQ

The graphic EQ is essentially a more refined fixed EQ. It's "fixed" because the bands are preset and unchangeable. It's refined because there aren't just two or three, but rather 12, 20, 32 or even more bands to modify.

These devices are designed for professionals. The goal is to specifically filter out one or more narrow frequency range(s). The graphic equalizer is preferably used at the end of the signal chain. If, for instance, the acoustics in a concert hall cause an annoying amplification of a specific frequency, the graphic EQ can quickly locate and adjust this frequency. The bands are precisely labeled, indicating how neighboring frequency ranges are affected by each slider. By design, it's never exclusively the labeled frequency that's affected, but also a certain range of adjacent higher and lower frequencies.

The parametric EQ

For modifying the sound of individual instruments, it's crucial to be able to precisely target the relevant frequencies. It also matters how the adjacent frequencies are affected when adjusting.

The parametric equalizer allows for such fine adjustments. Simple (but not necessarily inferior) models offer three bands, complex devices four to five (software solutions sometimes even more).

Each band has several sliders. Firstly, you can set the frequency range you want to modify, so it's not fixed like in a graphic EQ. Hardware EQs provide bands in a relatively broad yet limited range, like 60-120 Hz for the lows, 100-5000 Hz for the mids, and 4-16 KHz for the highs. From these ranges, you choose the exact desired frequency. Software EQs might offer the entire frequency spectrum per band, allowing you to select the desired frequency with a slider.

Each band also has its own volume slider to boost or reduce the set frequency. After all, that's the purpose of an equalizer.

In the parametric EQ, there's a third slider. With it, you can adjust the width of the adjustment. This means you can decide if only a tiny portion of the adjacent frequencies should be affected, or if the altered frequency range should encompass a larger part of the adjacent frequencies.

In English, this is referred to as Quality, which is why the slider is simply labeled "Q", resulting in the term "Quality Factor" or "Q-Factor". It refers to the precision of the frequency intervention. The higher the Q-value, the narrower the selected frequency band.

What is a filter in an Equalizer?

The filter is the central element of the equalizer. Various designs ultimately determine the type of intervention (colloquially: the type of EQing). Whether there are additional sliders for intervention depends on the filter.

What does the pass filter (through-filter/cut) do?

As the name suggests, this type of filter lets certain frequencies pass while cutting off others. The pass filter can be found, for example, at the beginning and end of the frequency spectrum in a parametric EQ. Filters are named according to their function. Either the filter blocks frequencies below a set value, like 80 Hz. Depending on the perspective, the filter allows frequencies above 80 Hz to pass (Hi(gh)-Pass Filter) or cuts off frequencies below 80 Hz (Lo(w)-Cut Filter). So, Low-Cut and High-Pass mean the same thing. Conversely, the High-Cut or Low-Pass Filter is used for the other end of the spectrum.
With pass filters, you can create a clear sound profile by cutting off noise with Lo-Cut and Hi-Cut that exists and makes the sound system work - but is outside the audible range and is thus redundant. Anything that helps the sound system work "more relaxed" improves the sound! But with extreme pass filter settings, you can quickly achieve effective effects. For this, the function of the bandpass filter, explained further below, is used.

BONUS KNOWLEDGE! Most sound-modifying tools are not limited to the purpose of creating a clear, natural sound profile. On the contrary, very distinct sound-shaping effects can be achieved with extreme settings. The equalizer can also be viewed not so much as a technical device but more as an instrument in this context. There are virtually no limits to creativity. This is especially true when combining several sound-shaping tools.

It's even more crucial to understand how each tool works. Only then can you optimally utilize it for your purposes.

Just as with instruments, there are standard models, bestsellers, and celebrities among sound-shaping devices. Therefore, certain handling routines have proven effective for individual models, and certain models are used to achieve a particular sound. Famous producers and sound technicians can undoubtedly be understood as artists who can masterfully operate their instruments.

What is the slope steepness?

For pass filters, you can't adjust the quality using the Q-slider. However, with high-quality EQs, you can choose the slope steepness. This indicates how strongly the signal is attenuated from or up to the set frequency. By design, the signal isn't entirely removed.

The slope steepness is measured in dB/octave, where "dB" is the unit for sound pressure level. For example, if you want a Lo-Cut at 120 Hz, your high-pass filter at the EQ's input is set to 120 Hz. Setting a slope steepness of 6 dB/octave means that the frequency an octave lower (which is half the oscillation number, so 60 Hz) is 6 dB quieter. 6 dB technically means halving the sound pressure level, i.e., the volume. Another octave lower, at 30 Hz, the signal is another 6 dB quieter.

Common values are 6 dB/octave and multiples up to 48 dB/octave. You need high-quality filters for the extreme values, as significant sound distortions can occur in these areas.

Unfortunately, most drum modules don't offer a slope steepness setting for Lo-Cut and Hi-Cut. Therefore, you need to develop a feel for how the pass filters in your module work.

What is a roll-off?

The roll-off is related to the attenuation of highs. For Lo-Cut, a strong intervention is often desired to eliminate noise (24-48 dB/octave). For Hi-Cut, such a strong intervention can quickly sound unnatural. Thus, the intervention is gentler (6 dB/octave) but can start "earlier". A common practice is the gentle "rolling off" of high frequencies starting around 6 or 8 KHz to give the ear the impression that the sound source is farther away. With the right settings, you can also handle annoying noise quite well without sounding unnatural. So, a roll-off is simply a gentle slope steepness setting for Hi-Cut. Even here, most drum modules typically don't offer a slider. Experiment with the Hi-Cut in your drum module to get a feel for how it works.

What does the bell filter do?

The bell filter is the filter that is essentially "in the middle" of the parametric EQ. At the beginning, you find the Lo-Cut, and at the end, the Hi-Cut (both pass filters), and in between one or several bell filters. Each bell filter represents a frequency band that you can select. The bell filter boosts or reduces the chosen frequency. In doing so, it also affects adjacent frequencies, which, depending on the Q-factor setting, are boosted or reduced more or less strongly. The chosen frequency is always the most significantly changed. The influence on adjacent frequencies decreases the further they are from the chosen frequency.
When graphically depicted, this intervention curve looks like a bell, hence the name. With the bell filter, you specifically target a frequency to work on. For a more natural sound profile, it's proven effective to boost a frequency only slightly while choosing a low Q-factor to create a broad bell. When reducing, it's often more sensible to cut more significantly with a high Q-factor. The narrow high bell represents the targeted exclusion of a narrowly disruptive frequency.

What does the band filter (bandpass) do?

The bandpass filter is simply a combination of the Hi-Pass and Lo-Pass, or Hi-Cut and Lo-Cut. However, it is sometimes presented as its own filter type. Often, you can switch the bell filter to a bandpass. The filter works by narrowing the range of frequencies that are allowed through, which makes it easy to achieve creative effects quickly. For example, if the filter is set to cut frequencies below 1kHz and above 3kHz, you can easily simulate the sound of small announcement speakers or telephone receivers, which primarily transmit this range.

What does the Notch Filter (Notch Filter) do?

The Notch Filter is a very narrow bandpass filter designed to sharply attenuate or filter out a specific disturbing frequency. In a graphical representation, this appears as a deep, narrow notch across the entire frequency band, hence its name.

What does the Trim/Gain/Make Up knob do?

When you boost a frequency on an EQ, you naturally increase the overall volume of the signal. The same applies when you reduce frequencies. It's rare to boost and cut frequencies so evenly that there's no change in the overall volume at the end.

Some devices provide a knob for this purpose, allowing you to adjust the signal either before or after EQ processing. The knob can have different labels, but the idea is the same. Trim is often used before processing to initially reduce the signal a bit. This is useful if the signal is already quite loud, and you plan to further boost certain frequencies in the EQ.

If the signal is initially weak, you can amplify it using Gain before EQ processing. The better the input signal to the EQ, the more effectively your EQ adjustments will be.

Make Up refers to adjusting the signal after EQ processing. If you've boosted several frequencies significantly, then the resulting signal might be too strong and needs to be reduced. Conversely, a signal that has become too weak due to extensive cutting in the EQ can be boosted again with Make Up.

What should I consider when setting the EQ on my drum module?

Before you begin sound shaping with the equalizer, you should be clear about where you're sending the final signal. The four most common scenarios are:

  • You play alone and listen through your headphones
  • You use the Mix Out (Stereo) to play solo over a sound system
  • You use the Mix Out to play with a band over a sound system
  • You use the Direct Outs to send signals from each instrument separately to a live or studio mixing desk

Each of these scenarios affects how you optimally use the equalizer in your drum module.

How do I optimally use the EQ when playing alone through headphones?

This situation provides you with the most freedom to shape your sound to your personal preferences. The fact that the equalizer in the drum module is usually straightforward and offers little special features, like an additional frequency band or choice of slope steepness, is least problematic here.

The headphone output is always in stereo. For good sound, you should definitely use high-quality headphones. Since stereo sound requires two speakers or headphone cups, using a single repurposed earbud, such as the hands-free earbud from your mobile phone, won't suffice.

In stereo sound, you're adjusting the dimension of height with the EQ. Ensure that the signals adjusted with the EQ aren't boosted in the same frequency bands. Give each instrument its "sweet spot". This means you boost in the frequency range that defines the sound of that particular instrument. Achieving this on drums is straightforward, as the sweet spots for bass drum differ from snare drum, toms, and cymbals.

You can adjust the sound to fit your headphones perfectly. Check whether the volume is neither too soft nor too loud. If you have a pleasant volume with the headphone knob turned up to about 3/4, you're generally on the right track. You can assume that the EQ has a sufficiently good signal for processing.

Some drum modules also offer a Master EQ. This is an equalizer that affects the entire drum kit. Here you can make minor corrections to the overall picture, such as a broad, slight boost of bass and treble. The Master EQ is also suitable for quickly adjusting to different headphones.

How do I optimally use the EQ when I play alone using the Mix Out and a PA system?

Firstly, you need to determine if your sound system consists of a single monitor. Whether it's a simple floor monitor that projects sound upwards at an angle, or a full-fledged side fill located next to or behind you comprising a robust bass speaker and a top part for mid and high frequencies.

The crux is: if you only have one monitor or a single speaker, you're working in mono. As is known, this limits your sound design options. However, if you're only sending your drum set through the monitor, adjusting the individual instrument levels can still yield a good sound in mono. To do this, you'll need to be very precise with the EQ and fully utilize the height dimension.

When using the MixOut, the EQ serves two purposes. Firstly, remember that if you're boosting extensively, each instrument should be amplified in different frequency bands, as you can't pan anything out of the central stereo field. For example, if you have two identical crash cymbals and boost both in the high-mid range by the same amount, they might blend into each other in mono. To solve this, treat each crash differently within the frequency band.

BONUS KNOWLEDGE! There's a simple trick when two instruments play in similar frequency bands simultaneously. Decide which instrument you want to boost and which to attenuate. Take the combination of bass drum and snare drum as an example. While the bass drum primarily sounds in the low frequencies and the snare in the high-mids and highs, you might want to boost the beater attack of the bass drum in the high-mids. This aspect makes the bass drum rhythm audible. If the combined sound of the bass drum and snare is undifferentiated, adjust the EQ of one based on the other. This trick can be especially effective when dealing with a few instruments (two to three). Experiment and gather experience!

The EQ will likely require different settings when connecting to speakers compared to using headphones. The sound characteristics might differ significantly. Ultimately, you're not tuning the EQ to individual instruments but to the output medium. Your second goal, then, is to ensure the PA system itself sounds balanced. It'll sound different in each room due to room interactions, unlike headphones.

In setups with a Mix Out and PA system, using a Master EQ that affects the entire drum kit can be beneficial. Adjust for the room's sound characteristics. Even if you're playing alone, if you're sending the drum set through a sound system, ideally opt for stereo monitoring. Stereo provides more and better options for producing a punchy, clear drum sound.

BONUS KNOWLEDGE! "P.A." stands for Public Address, originally referring to speaker systems in places like train stations. This term was later adopted in the entertainment industry.

How do I optimally use the EQ when playing with a band using the Mix Out and a PA system?

First and foremost, each musician should ensure their instrument sounds good individually. However, this can cause problems in the overall band sound. Overlapping frequencies can become an issue. The trick described above with frequency "beds" can help, especially in distinguishing bass from the rest of the band. Most instruments, including male vocals, won't have significant frequencies below 80 Hz, with the exception perhaps of the bass drum. To enhance clarity, instruments other than the bass should apply a low-cut at 80 Hz with a minimum of 12 db/Octave.

Any other sound enhancements via EQ will come through experimentation and experience since the possibilities are too diverse to cover fully. With increasing instrument numbers, it may get more complex. But if every musician communicates and acts with the same approach for the sake of the band's sound, a lot can be achieved. This applies to EQ settings in your drum module as well. By the way, it's evident that a mono system is not an option for a band.

How do I optimally use the EQ when sending signals through Direct Outs to a mixing console?

A brief answer would be: Not at all!

However, this answer depends on the quality of the mixing console. It doesn't particularly matter whether it's for a live production or a recording session. The fact that you're using the Direct Outs to separately process each signal of your drum kit implies a relatively high-quality environment.

If it's an inferior mixing console, perhaps it's better to make adjustments in the drum module and send the signal directly to the console. But in a high-end setup, the instruments should be sent directly and unprocessed. Here, the EQ in the drum module can assist by cutting off frequencies that are outside of the audible range, especially useful for electronic sounds or effect sounds.

However, a sound engineer is best equipped to handle a drum set that sounds like an unprocessed acoustic set. It might seem odd to create an "unprocessed" sound with an EQ, but it can be helpful in certain scenarios. Every session brings more experience and improves the sound.

It's crucial to communicate with the sound engineer, make suggestions, and listen. Good communication is often more important than the actual settings.

How can I best prepare for a live performance or recording session?

If you have sufficient storage space in your drum module or can store externally, it's wise to copy the same drum kit and have two or three versions ready. You can pre-edit the various copies differently and quickly access them on-site, depending on the situation.

For example, you might have different Master EQs programmed, or you have a variant with EQ and one without. Experience will serve you well here. The better you understand the pros and cons of your setup and how to deal with them, the quicker you'll achieve good results, and the better you can prepare.

Creating at least one copy of your drum sets has another advantage. You can "mess up" the copy without any risks. If you end up making things worse with the EQ, you can quickly delete the copy, create a new one, and start over. Nothing happens to your originally optimized set.

By the way, this approach is also great to embark on your discovery journey with the EQ. Put on your headphones, copy one of the presets in your drum module that you like, and try out how you can make it sound even better with the help of the Equalizer!

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