EQs and Filters
SOUNDWAVES
Sound travels in cycles, which are called waves. The sound is actually causing the molecules or particles that make up the air to compress and pull apart over and over, like the way a spring behaves when you hang a weight from it and let it bounce. As you can see in the picture of the sine wave, the wave basically repeats itself. (Sine waves are a good way to understand the way sound waves work because they are simple pure tones and only contain one frequency, but keep in mind that the sounds we hear are much more complex and that sine waves do not exist in nature). If you were to draw a horizontal line through the wave that evenly dissects it, the line would be at the equilibrium point, where the air molecules are not being affected. Above and below the equilibrium point corresponds to the compressing and pulling apart of the molecules. The peaks are called the crest and the trough of the wave (just like waves in the ocean) and the length of the wave-the wavelength-is calculated by taking a point on the wave and going through a complete crest and trough to that same exact point on the next wave.
Wavelength is important because the number of wavelengths of a certain wave that occur in one second is the waves frequency, and frequency is important in sound waves because it is how we perceive pitch. Since sound travels at a specific speed, if the wavelength gets shorter, more waves will go by in a second, resulting in a higher frequency, which we hear as a higher pitch. So, lower-pitched sounds have longer wavelengths and lower frequencies, and higher-pitched sounds have shorter wavelengths and higher frequencies. Human ears can detect frequencies from around 20Hz on the low end to around 20 kHz on the high end. The vertical distance from the crest to the trough of a wave is the amplitude. We perceive amplitude as volume-greater amplitude results in a louder volume.
Unfortunately, as was mentioned above, sine waves are very simple, but the sound waves that we hear every day are much more complex. This is because while sine waves contain only one frequency, natural sounds contain many, many frequencies, all added together to create the sound we hear. The fundamental frequency, which is like the base or the root frequency of the sound, is the pitch we hear in the sound, but all the other frequencies layer to alter the tone of the sound. This principle is what allows EQs to work. EQs boost or cut the volume of specific frequencies in the sound, which changes the tonal quality of the sound, and it’s very easy to hear the changes. The difficult part is understanding what changes to make and when to make them.
EQS
There are several different kinds of EQs. The most important for you to understand are the ones used in the channel EQs on the board, so we’ll start with those. The high and low EQs on the channel are called shelving EQs because they turn everything up or down at a specific frequency to the beginning or end of the frequency spectrum. The high EQ turns everything at or above 12kHz up or down together. The low EQ turns everything at or below 80 Hz up or down together. Since most of the defining frequencies are in the mid ranges, it’s less important to have precise EQing control over high and low frequencies. The mid range EQ on the channel is what’s called a semi-parametric EQ. In this case, you choose the frequency you’re boosting or cutting with the second knob, from 100 Hz to 8kHz, and select how much you’re boosting or cutting with the second knob. It’s semi-parametric because in a parametric EQ, there is a third knob, called a Q knob or bandwidth, that allows you to adjust how wide the EQ is; in other words, how many frequencies to either side of the selected frequency are affected as well.
The other type of EQ is a graphic EQ. A graphic EQ has many small faders, sometimes as many as 30 or more, each controlling an individual frequency, which allows a much more precise control over the sound. Graphic EQs are what’s used for room tuning, so they often come in stereo units (two graphic EQs in the same box). They are called graphic because the fader positions allow you to see graphically how the EQ is affecting the sound.
EQs have very specific uses, and it’s important to only use them when necessary. The most important use of EQs is in controlling feedback. Hopefully the room has already been tuned properly, so the chances of feedback should be lower, but feedback can still happen. When you hear feedback, after you find the channel that’s feeding back, you can EQ out the frequency that is feeding back. You have to use your ears and listen to the feedback. If it’s really high, turn down the high end. If it’s really low, turn down the low end. If it’s somewhere in the middle, it’s more difficult, but you can turn down the mid-range frequencies, then sweep through the frequencies until the feedback goes away.
The other main use of EQs is adjusting the tonal quality of a sound, clarifying it, or balancing the frequencies. When the frequencies of a sound are out of balance, it can sound very bad, and being able to adjust the sound with an EQ is a wonderful skill to have. An important thing to understand when EQing is that EQs are more effective when cutting than when boosting. If the mid frequencies of a sound are a little loud, which would make it sound like it was in a box, it works better to cut some of the mid frequencies to try to balance out the sound instead of boosting the high end to clear it up. Another important thing to understand is that the best way to learn how EQs work is just to do it and listen to the effects. After you’ve heard sounds with different frequencies boosted or cut, you’ll begin to recognize the same effects in other sounds. Generally, boosted high-end sounds tinny, while cut high-end sounds muffled. Boosted mid-range can sound boxy or canned, while cut mid-range can sound empty or dead. Boosted low-end sounds boomy, while cut low-end sounds weak and unsupported.
FILTERS
The only place you will see a filter is at the bottom of the EQ section of each channel strip, but since those are already set appropriately, you don’t need to worry about them. Since you’ll never use filters regularly, we’ll just discuss them briefly. Filters are EQs that only cut, and generally a rather broad range of frequencies. The most common filters are high-pass and low-pass filters, which, as their names suggest, allow either the highs or the lows at a certain a frequency pass while cutting the rest of the frequencies. So a high-pass (or low-cut, as it’s less commonly called) filter at 75 Hz, which is what the channel strip filter is, cuts everything under 75 Hz. A common low-pass filter would be set somewhere above 16 kHz and would be used to eliminate some extreme high-end frequencies that could cause more problems than they’re worth. A band-pass filter, which allows a band of frequencies in the middle of the frequency spectrum to remain and cuts both highs and lows, is really just a high-pass and a low-pass filter used together.
