Equalization (EQ)
Next on the channel strip is what are known as “filters” and “equalization” (EQ). Every sound that you hear falls within a specific frequency range and is measured in a unit of frequency known as Hertz (Hz), and as you get higher on the frequency spectrum, Kilohertz (kHz). The approximate range of human hearing is 20Hz to 20kHz. While the Hz is the unit of measurement, the number that precedes it is the specific frequency. The frequency 20Hz refers to a sound wave that cycles (going from 0° to 360°) 20 times a second, while 20kHz (or 20,000Hz) refers to one that cycles 20,000 times a second. The lower a frequency and the lower the amount of cycles per second it has, the lower the pitch of the sound. The higher the frequency and cycles, the higher the pitch of the sound. Kick drums, which of course have a lot of low frequency information, will usually fall within the 50-60Hz range, while a human vocal will fall in the 2kHz to 5kHz range. There are many different ways for you, as an engineer, to tonally manipulate the sound that arrives at the mixing console. One such way is through the use of filters and EQ which allow you to either boost or turn down specific frequencies. This can be done on almost every channel strip on a console, if not every channel strip, thus allowing you to remove or boost frequencies in your instruments to make it sound better and help get rid of feedback.
First, located after the analog gain, many consoles have a filter known as a High-Pass Filter (HPF) (Figure 8). In the case of the Allen and Heath channel strip, this HPF is set to 100Hz. However, some other consoles have the frequency set differently. For example, some Mackie consoles have the HPF set to 75Hz. Whatever the specified frequency is, the HPF will remove most of the frequencies below that specified frequency. In the case of the Allen and Heath, most frequencies below 100Hz will be removed from whatever input source it is applied to, allowing all frequencies that are higher than 100Hz to pass unaltered to the EQ. This can be very beneficial when your mix has too much low frequency information in it. A lot of times, when you are running a full band, the only instruments that really need frequencies underneath 100Hz are your kick drum, floor tom, and your bass guitar. If this information isn’t filtered out of the rest of your instruments, it will typically remove clarity from your kick drum, bass guitar, and other instruments which will make the mix “muddy.” Muddy is a term thrown around by engineers which means there’s too much low and low-mid frequencies in the mix, and there is an overall lack of clarity.
While not the case on most analog consoles, there is also sometimes a Low-Pass Filter (LPF) that is located at this point in the channel strip. It operates similar to a HPF, but instead of allowing high frequencies past a certain point to proceed to the EQ, it will attenuate high frequencies and allow anything lower than a certain point to pass through to the EQ unaltered.
Figure 8
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After filters we get into our EQ section (Figure 9). Most consoles will have at least a low frequency, mid, and high frequency attenuation control. Some will also include a low mid and a high mid frequency attenuation, which allows for a little bit more control of what frequencies you want to boost or cut. Each instrument has its own place that it falls within the frequency spectrum, which oftentimes will determine what frequencies you are going to boost or attenuate depending on your sound system and room. But before boosting any frequencies, I will more often than not see what frequencies I can first remove from an input source. It will take you a while to train your ear to be able to hear what frequencies you want to remove or add. However, in the meantime, the best technique you can use to find trouble frequencies is known as “sweeping.”
Figure 9
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With sweeping, what you do is grab the knob of a specific range (be that low, mid, or high), and boost it. If your console also has a frequency selection knob, you can move that knob around and hear the boosted portion move. In live sound, you are trying to remove as many problem frequencies which cause feedback as possible, as well as removing problem frequencies that make your instruments and vocals sound unpleasant. As you are sweeping the frequency spectrum, you will oftentimes hear a specific frequency that pops out and is undesirable. This is where you’d stop sweeping and then attenuate it to at least -3, if not more. Again, for each instrument there are different frequencies that you want to target because of the different tonal makeup of each instrument. A simple mistake that can be made on the Allen and Heath console is to not push the “EQ in” button. In fact, almost every engineer has spent time trying to fix a sound only to realize that their EQ has not been engaged for the last 5-10 minutes while they’ve been working on it. Many other consoles come with the ability to engage/disengage the EQ, all the way up to the highest digital consoles, so make sure your EQ is turned on before making all your changes. Now, for a more in-depth look at EQ, take a look at the following video. I will show you a live recording that I made from Cherry Hills, show you techniques like sweeping, HPF/LPF, EQ, go through each instrument of the band, and give you some other tips that should get you on your way to EQ your band in your room.