Understanding frequency is key to a complete understanding of audio and how to manipulate it. We’ll cover this important aspect as a basis for many audio editing techniques.
Everything tangible has a frequency. From the light that lights your scene to the waves that are cast from a skipping rock. Understanding frequency is key to a complete understanding of audio and how to manipulate it. Frequency is measured in hertz and is defined as one cycle per second. A cycle refers to how often a sound completes one rise and fall of its wave. Think of a lake. You throw a rock into the lake. The ripples coming from the rock falling into the water can be compared to a sound wave. The distance between each wave and the next defines its frequency. In the world of audio, a low frequency would mean more space between the waves whereas a high frequency would mean the waves are closer together. To get a good idea of how frequency ranges work, let’s examine a piano. Each key on a piano makes up a given frequency. Pianos are tuned according to these frequencies. The very middle key of a piano is called middle c. The a note just above middle c is 440 hz. If you were to use your voice to hum at a constant note, matching the a above middle c, your voice would be at 440hz. Every musical instrument is tuned based on this standard. Instruments such as bass guitar have a lower frequency range, and instruments like a violin or flute are in the high range. The larger the hz number, the higher the pitch. For example a sound at 1,000 hertz is higher than one at 50hz. In line with that, the lower the frequency the lower the pitch. If you're looking to control frequencies, you need to look to an equalizer or filter. The difference between the two are more in their complexity than anything else. A filter is a simple frequency controller and an equalizer or eq, combines several frequency-specific filters into one device or plug-in. You'll want to use an eq when you want to add or subtract a frequency range. On one project, you may need to remove some rumble from dialog while another project needs a boost in a specific range. Eq is perfect for that. Let’s start out with the simple tools and expand to the more complex. Starting with the tools that are called first order filters. A high pass filter is an equalizer that eliminates frequencies below a specified cutoff point. It is also known as a shelfing filter. Since it is low frequencies that are removed, only high frequencies may pass. Use a high pass filter to remove unwanted low frequencies that are not needed and can be removed without changing the quality of the sound. Let’s take a look at what a high pass filter does: A low-pass filter is the opposite of a high pass, a low pass allows frequencies lower than a chosen cutoff point to pass through while cutting off higher frequencies. This filter is great for removing high frequency hum. It also is considered a shelfing filter because of its function is the same as a high pass, just on the other side of the frequency spectrum. Let me show you how a low pass filter works No matter high or low, remember when using these shelf filters, they are removing all frequencies past or before a point. Thus if the sound you are manipulating has qualities within those cut off frequencies, the sound will be negatively affected. A notch filter passes most frequencies while reducing those within a specific range to very low levels and is oftentimes used with noises of a known frequency, such as electrical hum. An equalizer will boost or remove certain frequencies or frequency ranges. The width of the affected frequency range is known as the q or q factor. The lower the q factor, the wider the frequency range being effected. The higher q, the more narrow the range. The graphic equalizer, aka graphic eq, is a powerful tool for audio production. You might find yourself dealing with a distracting hum from a refrigerator that, if you can’t remedy, will ruin a perfect take. Knowing how to use the graphic eq to solve this problem will save you from having to re-shoot. You're probably most familiar with the graphic eq used in everything-from a multi-channel mixer to your car stereo. Many people have encountered a graphic eq at one point in their life, but very few use them correctly. When you’re adjusting the bass or treble in your car stereo, for instance, it adjusts each corresponding band of eq. The main difference between your car stereo and a graphic eq used for audio production is the precision with which each frequency can be adjusted. Graphic eqs allow you to easily shape the tonal quality of your audio. They typically divide the frequency spectrum into ranges or bands and each band has its own controls. An average graphic eq might have anywhere from five to 31 bands or more, offering either very simple or very specific control of your sound. Their operation is quite simple, you just raise or lower one or more frequency bands to affect the sound. But go easy. If you're making drastic changes to achieve a listenable result, something else is probably wrong. Another type of equalizer is the parametric eq. Rather than divide the sound into specific frequency ranges, a parametric equalizer allows you to choose the frequency or frequencies and adjust accordingly. A typical parametric equalizer might have between four and six bands, but each of those bands is completely adjustable. Not only can you raise and lower the audio content in that band, but you can pick the frequency and the width of the band. A wideband impacts a larger portion of the sound while narrow bands have a more surgical effect. Operation of a parametric eq isn't quite as intuitive as a graphic eq, but with a little practice, you'll figure it out. It helps to find a plug-in with a graphical user interface, not just knobs. That way, you can see the impact of your adjustments on the screen as well as hear the results. Every sound has a frequency range — a male voice, a female voice, the note of the exhaust of a motorcycle and every musical instrument. Knowing what frequencies to remove to extract the offending sound is key. The constant hum of a water cooler, an annoying cricket and even white noise exist within specific ranges of frequencies. Identify the offending frequency by removing one band at a time until it’s lessened or removed. Problems like hum might exist in a very small frequency range, but things like the human voice make up a much wider range. For example, typical male speech lives between 85 hz and 180 hz, and female speech range is 165 hz to 255 hz. Knowing this is very helpful. Say you’re interviewing a man and you want to get rid of all unwanted frequencies. You can roll off frequencies above 155hz and below 85hz. Doing this removes the frequencies outside of those that make up the man’s speech. However, this can affect the quality of the voice, so in many cases, it’s better to be subtle with rolling off frequencies rather than just cutting them completely. Lowering them gradually along the frequency range to create a hill ramping up or ramping down will give more natural frequency removal. As we know, some people have high voices, and some low. Because of this, the range might slip around. This is a starting point, not the law of the land. It’s best practice to subtract frequencies rather than add to them. By adding to the presence of a frequency, you’re adding to the dynamics of that overall sound. This can create muddy audio. Also, many graphic eqs don’t do this transparently, meaning that they color the sound source, and this is typically unwanted. That being said, if you want to create more presence you might add a bit to certain frequencies so that the audio cuts through your mix. Adding and removing needs to be done in small increments as it’s very easy to muddy the sound by boosting frequencies or to change the quality by removing them. Understanding what things live in each frequency range is a huge advantage when combining or mixing many different sound sources together. Mixing your audio well creates a soundscape and allows you to bring focus to one thing over another. Having too many sounds within the same frequency range creates a chaotic soundscape. Think of a large room with many people all talking at the same time. With so many sounds happening simultaneously and within the same frequency range, hearing nuance is very difficult or impossible. By working the frequencies of your audio sources, you can create space for each to live, allowing for all sound sources to be heard as a cohesive whole. Every editor cringes at the phrase "we'll fix it in the mix," but the reality is that you can fix a lot of things with some simple frequency manipulation. Through tools like a high pass filter or a parametric eq you can actually shape the sound as you want it. Having control over the frequency of your sound allows for a more clear and clutter free mix.