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Equalization Filters

An audio equalizer allows you to increase or decrease the strength of an audio signal within selected frequency ranges, or bands. For example, a three-band equalizer may have a gain control for the lows, midrange, and highs, so you can change the sonic “shape” of a sound by turning up some frequencies or reducing others. In general, it is much better to subtract frequencies than to amplify them, as this eliminates the possibility of distortion.

If you find that a sound is lacking “brilliance,” or high-end frequencies, try filtering out some of the bass or midrange frequencies. The overall effect is that the high-end frequencies are stronger than the lower-range frequencies. It’s easy to go too far when amplifying some frequencies, so get in the habit of reducing frequencies first. Particular kinds of sound—men’s voices, women’s voices, tape hiss, and traffic noise—all appear at different frequencies of the audio spectrum. Equalization (EQ) filters can be used for many things, from minimizing background noise in a recording to accentuating a narrator’s voice over background music. EQ filters can also create effects like making a voice sound as though it were coming through a telephone or loudspeaker (this is because telephones and loudspeakers generally don’t reproduce the high and low frequencies, only the midrange).

Frequency Ranges and Equalization

The entire range of human hearing, from 20 Hz to 20,000 Hz, can be broken into a spectrum of frequency bands: low, midrange, and high.

Note: Different devices define these ranges differently; the following ranges are approximate.

Low Frequencies

Low frequencies range from approximately 20 Hz to 250 Hz. Audible bass frequencies start around 20 Hz, though many speakers cannot reproduce frequencies this low. This is an example of how audio meters can be deceiving, because the meters may show very high signals but the speakers are not capable of making sounds that low. The lowest frequencies are felt as well as heard and require the most power to amplify. Often, subwoofer speakers are used just to handle the low frequencies in the mix (the 0.1 channel in a 5.1-channel surround sound mix is a dedicated low-frequency effects channel).

If you are trying to increase the impact of sounds like kick drums or explosions, add gain around 30 Hz or so. Filtering out 60-80 Hz removes a lot of low-end noise and rumble from wind or microphone handling. Between 150 and 250 Hz, you can add “warmth” to the audio signal (or subtract it).

Midrange Frequencies

Midrange frequencies range from approximately 250 Hz to 4000 Hz. Humans are most sensitive to this part of the audio spectrum. Most of the frequencies that make speech intelligible are in this range. You can make audio tracks stand out more in the mix by subtly increasing the frequencies in this range. At the top of this range, around 4 kHz, is where vocal sibilance occurs. Too much sibilance can be grating, but a little bit can make the voice sound crisp and detailed. If your track has too much sibilance, try reducing the 4 kHz range.

High Frequencies

High frequencies range from approximately 4000 Hz to 20,000 Hz. The high end of the frequency spectrum adds “brightness” or “brilliance” to a mix but does not affect factors such as impact (bass) or speech intelligibility. High-end frequencies can be grating, so don’t boost these frequencies too much.

Using Equalization Filters in Final Cut Pro

The following equalization filters are included with Final Cut Pro:

3 Band Equalizer
Band Pass Filter
DC Notch
High Pass Filter
High Shelf Filter
Low Pass Filter
Low Shelf Filter
Notch Filter
Parametric Equalizer

All of the Final Cut Pro EQ filters use a combination of three controls. For example, the Parametric Equalizer filter contains the following controls.

Figure. Filters tab showing controls for the Parametric Equalizer filter.

Frequency: This slider lets you select the audio frequency you want to boost or attenuate. The lowest available frequency varies from 10 Hz for the High Pass filter to 80 Hz for the 3 Band Equalizer. The highest available frequency for all EQ filters is 20,000 Hz.
Q: This slider adjusts the bandwidth of the filter resonance, whose frequency value has been set with the Frequency slider.

Gain: This setting controls how much you’ll be boosting or attenuating the specified frequency range.

Basic Guide to Understanding EQ

By Kendall Scott

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Using equalization may be second nature for the experienced, but it can be a veritable minefield for those new to the details of the concept. Perhaps you have a rudimentary idea of basic EQ from your home sound system, or from making simple adjustments on the audio mixer that you may have in your recording/editing setup. Learning to use EQ means experimenting to discover which adjustments work and do not work for a given situation. By using your ears and making intuitive adjustments you may eventually achieve desired results, but if you have a good understanding of the nuts and bolts you will be able to focus and zero in on the sound you are seeking, with speed and purpose. Maybe you are getting your ears acclimated by making your own home recordings, or perhaps you are trying to come to grips with your first software recording/editing package. No matter what the situation, knowing the basics will help you hear in equalization an indispensable tool for adjusting, mending, or sweetening your audio tracks.

Enjoying a live performance or presentation is completely different from a recorded experience. In a live setting, the sound of instruments and voices interacts with the listening space and creates a psychoacoustic event that can never be exactly recreated because it is a human experience. A recording is a reproduction of an event, and in order to provide the listener with an appropriate experience that approximates a three-dimensional listening environment with only two speakers, good recording techniques are paramount. Getting all of your audio sources to sit well together is the mixer's art, and EQ is one of the most important tools of the trade for music, film, and live sound.

Basic Components

In its most basic form, think of EQ as a set of filters that can alter the incoming signal. A filter is a device that allows you to selectively alter what passes through it, so think of a filter as a shaping tool. An audio filter lets you alter some frequencies while leaving others untouched. There are many types of filters that can be used for audio, but three basic types are:

Low-pass	Band-pass	High-pass

A low-pass filter lets low frequencies pass through it while attenuating higher frequencies above a desired cutoff point.

A band-pass filter may be thought of as a low-pass filter and high-pass filter working together to allow only a selected center region of frequencies to pass between two selected points. Telephone speech, for example, is limited to a narrow range from between 300Hz to 3200Hz reasons of intelligibility. By dialing these numbers into your band-pass EQ you can approximate the sound of a voice on a phone line.

A high-pass filter lets high frequencies pass through it while attenuating lower frequencies below a desired cutoff point. A microphone roll-off switch acts as a high-pass filter to remove unwanted low-frequency noises, such as handling and plosives.

These filters work by attenuating a selected portion of the original source. Some of the more interesting types of filters that will allow you to both boost and attenuate your signal are:

High Shelving	Low Shelving

Shelving filters cut or boost all frequencies equally above or below a desired cutoff point. You can see in the high shelving illustration that all frequencies above the cutoff may be cut or boosted without altering any frequencies below this point. The exact opposite holds true for the low shelving filter.

Shelving filters are excellent tools for getting your audio tracks to sit well together while mixing. If the piano and bass guitar tracks seem to be clashing because of overlapping frequencies in your mix, you may want to separate them sonically by applying a high-shelving filter to the bass and a low-shelving filter to the piano to keep them out of each other's way. By doing a little research to find out the frequency ranges of your instruments, you will be able to carve a comfortable niche for every element in your mix.

Overlap	Minimized Overlap

A peaking filter may be used to apply boost or cut to a specific frequency area. This is great for dialing-in very focused adjustments, such as adding more crack to a snare drum, or adding some presence to a vocal track.


Peaking Filter

Getting Familiar with Peaking Filters

If you own an equalizer with continuously adjustable frequency selection, apply some positive or negative gain from the EQ and try slowly sweeping through its entire range. Listen closely to your audio source to see if you can define what changes are occurring at different points along your sweep. Where does a hi-hat cymbal become overly bright and where does it become muffled or dull? On the other hand, what is the right frequency to make it cut through to be heard or add more "stick" to the sound. Can you locate that 60Hz hum on your miked guitar or vocal track and effectively reduce it?

In software it may be as easy as dragging your frequency marker across the display.

On a hardware unit, such as a mixer with a sweepable mid-section, you will have to first apply gain with one control and then adjust your frequency with another.

Putting It All Together

These are the basic building blocks of both hardware and software equalizers, and they can be combined in a variety of ways to make extremely powerful sound-shaping tools.

At its heart, a graphic EQ is a collection of peaking filters that allow you to reach for the frequency that you want to adjust, easily. Each filter is tuned to affect a specific frequency, and its amount of positive or negative gain is measured in decibels. Even though you are choosing a specific frequency to adjust, the limitations of most filter circuits produce results that will have an effect on neighboring frequencies as well.

The acuteness of the attenuation is measured in dB per octave. An octave is simply a doubling of a frequency, and as the amount of attenuation per octave is increased, so does the ability to pinpoint the selected frequency.

Fully-parametric EQ is a very powerful tool that allows for the adjustment of three signal parameters:

1) The amount of negative/positive gain

2) The frequency at which the negative/positive gain will occur

3) The width of the frequency range being effected

Like a graphic EQ, a parametric lets you choose the frequency you want to adjust and the amount of boost or attenuation you want to apply. But, the parametric goes one step further in allowing you to adjust how wide of an effective spread there will be to either side of the selected frequency.

This value, labeled Q, Resonance, or Bandwidth on different units allows for some very strong tone-shaping that can drastically change the overall character of the original source.

Gain - A little bit goes a long way

Remember that by adding gain to any of your selected frequency areas, you are also increasing the output of the EQ, so boost your signal judiciously.

Hardware, Software, and In-between

Equalizers may be as simple as a single tone control or they may be as complex as you can imagine. Good qualityhardware units have been designed to behave in a very "musical" manner, making it easier for you to dial in the sound you want. Beware of units that may do more harm than good by adding noise to your signal path, ultimately degrading your results.

Almost all audio recording/editing software will come with some form of EQ. To add to this, there are hundreds of specialized software EQ's that offer many combinations of features that are either unavailable or very expensive to incorporate on a hardware unit. Software also provides the option of dynamic EQ where changes can be automated over time, and settings can be stored with your session. There are even digital rackmount units that offer the benefits of both hardware and software.

Whether you encounter an EQ section on a large console or in your digital audio workstation, these symbols and concepts are a universal key to getting to the heart of any equalizer you may use in the future.

Thanks for checking out our Basic Guide to Understanding EQ. Should you have any further questions about EQs, we encourage you to contact us on the phone, online, or in person at our SuperStore in New York City. 1-800-947-9923.

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Tuesday, October 30, 2012

Audio Equalization Filter design