Basics: Five Questions about Filter Response

By Craig Anderton 

You can think of filters as combining amplification and attenuation—they make some frequencies louder, and some frequencies softer. Filters are the primary elements in equalizers, the most common signal processors used in recording. Equalization can make dull sounds bright, tighten up “muddy” sounds by reducing the bass frequencies, reduce vocal or instrument resonances, and more. 

Too many people adjust equalization with their eyes, not their ears. For example, once after doing a mix I noticed the client writing down all the EQ settings I’d done. When I asked why, he said it was because he liked the EQ and wanted to use the same settings on these instruments in future mixes. 

While certain EQ settings can certainly be a good point of departure, EQ is a part of the mixing process. Just as levels, panning, and reverb are different for each mix, EQ should be custom-tailored for each mix as well. Part of this involves knowing how to find the magic EQ frequencies for particular types of musical material, and that requires knowing the various types of filter responses used in equalizers. 

What’s a lowpass response? A filter with a lowpass response passes all frequencies below a certain frequency (called the cutoff or rolloff frequency), while rejecting frequencies above the cutoff frequency (Fig. 1). In real world filters, this rejection is not total. Instead, past the cutoff frequency, the high frequency response rolls off gently. The rate at which it rolls off is called the slope. The slope’s spec represents how much the response drops per octave; higher slopes mean a steeper drop past the cutoff. Sometimes a lowpass filter is called a high cut filter.

 Fig. 1: This lowpass filter response has a cutoff of 1100 Hz, and a moderate 24/dB per octave slope.

What’s a highpass response? This is the inverse of a lowpass response. It passes frequencies above the cutoff frequency, while rejecting frequencies below the cutoff (Fig. 2). It also has a slope. Sometimes a highpass filter is called a low cut filter.

 Fig. 2: This highpass filter response has a cutoff of 100 Hz, and a very steep 48/dB per octave slope.

What’s a shelving response? This isn’t designed for “surgical” sound shaping, but can correct broad deficiencies such as lack of high frequency “sparkle” or excessive “boominess” in the bass. The QuadCurve EQ offers both high and low frequency shelving stages (Fig. 3). The name “shelf” comes from the fact that the response doesn’t continue to drop off like a lowpass or highpass filter, but reaches a particular level of boost or cut and then flattens out.

Fig. 3: This graph shows a substantial low-frequency shelf cut, and a slight high-frequency shelf boost.

What’s a bandpass response? This boosts only those frequencies around its resonant or center frequency, while rejecting higher and lower frequencies. The QuadCurve’s bandpass filter not only has a variable frequency control but also has variable boost (makes those frequencies louder) or cut (makes the frequencies softer), as well as the option to vary the range of frequencies that are boosted or cut. This is called width, Q, or resonance. When cutting frequencies, a bandpass response is called a notch response. The QuadCurve has four parametric stages (Fig 4); the name refers to each filter having multiple adjustable parameters.

Fig. 4: Four QuadCurve stages. From left to right: a bandpass filter with a low frequency boost that covers a narrow range of frequencies (brown); a shallow notch filter that covers a moderately broad range of frequencies (yellow); a steeper notch filter that covers about the same range of frequencies (green); and a broad, wide high frequency bandpass filter (blue).

What is resonance? Resonance produces a sharp peak at a filter’s resonant frequency or cutoff frequency. Because a bandpass filter’s response centers on the resonant frequency, narrowing the width or increasing the amount of Q creates a sharper response. With a shelving response, the peak occurs at the frequency where the shelf begins to increase (Fig. 5).

Fig. 5: A low-frequency shelf boosts the signal but has added resonance that creates a peak above the shelf, along with a high-frequency shelf that cuts the signal. Note how in the case the resonance reduces the signal level further.

Learn more about the QuadCurve EQ in SONAR X3 Producer