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Foundations of Audio: EQ and Filters

High- and low-pass filters


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Foundations of Audio: EQ and Filters

with Brian Lee White

Video: High- and low-pass filters

We know that shelving filters boost or cut energy at the target frequency and all audio frequencies either above or below it. A pass filter also affects all energy above or below the target frequency, often referred to as the cutoff frequency in a pass filter. However, instead of boosting or cutting that content by a specific amount of gain, it instead removes that frequency content completely. Any frequency content outside the cutoff frequency is attenuated or cut gradually in a downward slope, heading towards negative infinity.
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  1. 3m 28s
    1. Welcome
      1m 36s
    2. What you should know before watching this course
      35s
    3. Using the exercise files
      58s
    4. Using the "Get In the Mix" Pro Tools and Logic Pro session files
      19s
  2. 15m 46s
    1. What are frequency and amplitude?
      2m 27s
    2. Measuring frequency
      1m 48s
    3. Measuring amplitude
      1m 58s
    4. The perception of frequency and amplitude
      4m 18s
    5. Frequency and pitch
      5m 15s
  3. 36m 10s
    1. What is an equalizer?
      4m 14s
    2. Hardware and software EQ
      1m 58s
    3. Understanding frequency and gain EQ controls
      3m 41s
    4. Using the bandwidth, or Q, EQ control
      5m 35s
    5. Parametric equalizers
      2m 36s
    6. Shelving filters
      5m 11s
    7. High- and low-pass filters
      5m 42s
    8. Putting it all together with multiband EQ
      3m 43s
    9. Using graphic EQ
      3m 30s
  4. 46m 13s
    1. Creating focus
      3m 47s
    2. Get in the Mix: Using EQ to fix problems and place elements in the mix UPDATED
      8m 30s
    3. Get in the Mix: Creating complementary EQ curves UPDATED
      9m 7s
    4. Get in the Mix: Creative EQ with the telephone effect UPDATED
      5m 30s
    5. Get in the Mix: Frequency bracketing with filters UPDATED
      5m 44s
    6. Get in the Mix: Automating EQ UPDATED
      6m 18s
    7. Learning to listen
      3m 10s
    8. Balancing expectations from the recording process
      4m 7s
  5. 41m 14s
    1. Get in the Mix: EQing FX returns UPDATED
      4m 29s
    2. Using common vintage-modeled EQs
      5m 2s
    3. Using frequency analyzers
      3m 44s
    4. Using harmonic generators to excite frequency content
      5m 44s
    5. EQ or compression first?
      3m 3s
    6. EQ and room acoustics: Is your room lying to you?
      6m 15s
    7. Boost or cut? The relative nature of EQ and headroom
      4m 0s
    8. Building healthy EQ strategies
      8m 57s
  6. 19s
    1. What's next and EQ summary
      19s
  7. 5m 51s
    1. A session with Brian Lee White
      5m 51s

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Foundations of Audio: EQ and Filters
2h 29m Appropriate for all Jan 11, 2012 Updated Jan 17, 2014

Viewers: in countries Watching now:

In this installment of the Foundations of Audio series, author Brian Lee White shows producers and audio engineers how to properly apply equalization (EQ) and improve the sound of their mixes. The course covers the use of parametric and graphic EQs—and filters such as the high/low pass filters and shelf filters—in a variety of musical settings. These principles can be applied to any digital audio workstation platform, including Logic and Pro Tools, as well as analog workflows.

Topics include:
  • Measuring frequency and amplitude
  • Understanding the relationship between frequency and pitch
  • Working with EQ controls such as bandwidth (Q) and gain
  • Using graphic EQ
  • Understanding the shelving and high-pass and low-pass filters
  • Creating focus with EQ
  • Creating complementary EQ curves
  • Performing frequency bracketing with filters
  • Automating EQ
  • Using frequency analyzers
  • Using harmonic generators to excite frequency content
Subjects:
Audio + Music Mixing Music Production Audio Foundations Audio Effects
Software:
Logic Pro Pro Tools
Author:
Brian Lee White

High- and low-pass filters

We know that shelving filters boost or cut energy at the target frequency and all audio frequencies either above or below it. A pass filter also affects all energy above or below the target frequency, often referred to as the cutoff frequency in a pass filter. However, instead of boosting or cutting that content by a specific amount of gain, it instead removes that frequency content completely. Any frequency content outside the cutoff frequency is attenuated or cut gradually in a downward slope, heading towards negative infinity.

Let's listen to some audio examples. Listen to the following drum loop with a high-pass filter engaged. (music playing) Next, let's listen to the loop with a low-pass filter engaged.

(music playing) Like the name suggests, a high-pass filter allows the highs to pass through the filter unaffected, while the low frequencies are attenuated, or removed, starting at the cutoff frequency and lower.

Likewise, a low-pass filter allows the lows to pass through the filter unaffected while the high frequencies are attenuated or removed, starting from the cutoff frequency and higher. Sometimes these filters will be referred to as high-cut and low-cut filters. High cut is simply another name for a low-pass filter and low cut another name for a high-pass filter. Don't let the names confuse you. Fortunately, the names are actually quite descriptive. Low-cut filters cut the lows; high-pass filters let the highs pass through, et cetera.

When both filters are used simultaneously, one filtering out the lows and another filtering out the highs, it is often referred to as a band-pass filter, since in that case only a specific band of frequencies are allowed to pass through. Pass filters generally only have two controls: frequency and Q. The frequency sets the cutoff point of the filter, while the Q control, sometimes called slope in a pass filter, determines the steepness of the cutoff slope.

Q in this case is usually measured in dB per octave and determines how aggressive the EQ will begin removing frequency energy above or below the cutoff point. Q settings on high- and low-pass filters generally start at 6 dB per octave, which is a rather gentle slope, and work their way up to more aggressive settings--sometimes as much as 48 dB per octave, which looks basically like a vertical line here on the graph.

Let's look at an example of a high-pass filter. A setting of 12 dB per octave with a cutoff at 100 Hz means that after passing through the filter, a signal of 50 Hz or one octave lower will be reduced by 12 dB, while a signal of 25 Hz, two octaves lower, is reduced by 24 dB. High- and low-pass filters are especially useful in restricting or bracketing frequency content of signals in a mix. Many times rumble, or frequencies lower than the fundamental of the instrument, can be completely removed using a high- pass filter set just below a signal's fundamental frequency.

For example, a male vocal isn't going to have much below 80 Hz, so cutting off everything below 80 with a high-pass filter can help ensure that no additional low-frequency content unrelated to the vocal itself makes its way into the mix. Likewise, low-pass filters can be used to restrict and reduce high-frequency content. In the case of DAWs, unlike analog mixing consoles and tape, high-frequency retention is 100%, meaning no high frequencies are soaked up by the DAW's mixer.

This can lead to mixes that have too much high-frequency extension in too many of the instruments. This can result in a subtle building up of undesirable high-end frequency content that is hard to describe. I like to use low-pass filters on instruments that aren't contributing significantly to the high frequencies of my mix. For example, a 12-inch guitar speaker cabinet doesn't produce much useful audio information above 8K, so why keep it in the mix? Reducing or eliminating those high-end frequencies helps to remove any noise or hiss that may be present in the recording, but not part of the actual instrument.

And reducing these frequencies on multiple tracks keeps them from adding up in a mix. Using high- and low-pass filters to help tighten up frequency content and improve focus has been a secret of the pros for years, and I would find myself hard-pressed to do a mix without them.

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