The Inquirer: EQ Part 1

November 10, 2017

The electronic source of our instrument’s sound is a boon for tone shaping, but it’s also a minefield. Consider a cellist, pianist, horn player, or vocalist: Their focus is set sharply on their instrument, manipulating its timbre through technique alone. We bass players have that ability, too, but it’s coupled with an entire post-instrument signal chain, each part of which has the potential to impact our sound dramatically. Moreover, electronic tone-shaping, for example by equalization (EQ), isn’t an abstract and intuitive result of our musicianship; it’s a technical pursuit, more like engineering than playing. No wonder, then, that many bass players eschew that side of the craft altogether or misuse their gear, doing more sonic harm than good. Understanding EQ will not only improve your tone, it’ll make you a savvier musician generally.

EQ is frequency-selective volume adjustment. To understand EQ, a brief overview of sound is in order. When you pluck a note, the vibrating string produces a complex spectrum of energy at various frequencies. The lowest frequency—and usually the loudest—is called the fundamental. But the signal sent to the amp consists of much more than the fundamental; a string’s complex vibration pattern includes a broad range of overtones, which are multiples of the fundamental frequency that can be described in terms of musical intervals. For example, the first overtone is an octave above the fundamental, and the next one is a 5th above that. An instrument’s overtone series produces its timbre or tone color, and the relative balance of an instrument’s overtones is largely responsible for the differences in sound between, say, a trombone, piano, harp, and cello all playing the same note.

If an instrument’s timbre comes from the strength of its overtones relative to the fundamental frequencies, it should be obvious why EQ is such a potent tool. By giving us control over our signal’s frequency content, EQ can fundamentally alter our instrument’s personality. The degree of precision depends largely on the type of EQ.

LOWPASS & HIGHPASS FILTERS

The most basic EQ consists of filters that allow low or high frequencies to pass, blocking frequencies above or below a given cutoff frequency. A passive bass’s tone-control circuit is a lowpass filter. The more it’s rolled down, the more high frequencies are removed from the output signal. While bass-preserving lowpass filters might seem to make the most sense for our instrument, highpass filters are useful, too, especially when recording. Since most music-playback systems struggle to accurately reproduce low frequencies, cutting some lows with a highpass filter can make speakers operate more efficiently, since they aren’t being tasked with producing low frequencies they can’t handle.

SHELVING EQ

Shelving EQ filters are typically found on the bass and treble bands of onboard bass preamps and bass amps. Shelving EQ boosts or cuts frequencies above or below a specified cutoff frequency by a particular amount. The amount of boost or cut follows an amplitude slope from the cutoff frequency until it reaches a maximum or minimum, past which the EQ boosts or cuts all frequencies equally. Shelving EQs are broad-stroke tools for quickly tilting a bass signal toward the bass or treble end.

NOTCH FILTERS

Not common on bass equipment, a notch filter is a narrow-bandwidth filter that targets a specific frequency. A graphic EQ is a series of notch filters typically controlled with faders that visually represent the EQ’s frequency response. In the studio or live, notch filters are an excellent way to tame a nasty resonant frequency or control feedback.

PARAMETRIC EQ

By far the most flexible EQ circuit is the parametric filter, so called because its effect relies on a variety of parameters (settings). Three separate settings govern a “fully parametric” EQ. A player can select the filter’s frequency, the amplitude of the boost or cut, and the bandwidth or “Q” that the filter controls. This bears explanation: The maximum amount of boost/cut occurs at the chosen frequency, but the surrounding frequencies are also affected. The Q parameter determines the breadth of frequencies that the filter boosts or cuts. Semi-parametric EQs are more common on bass gear. They feature a preset Q, but allow control over frequency and amplitude.

Next time, we’ll explore how to achieve different tones with the EQ on your bass and/or amp, compensate for a bad-sounding stage, or help put a shine on an inadequate bass.

Bass Player Senior Contributing Editor Jonathan Herrera is the magazine’s former Editor-in-Chief. Catch up with him at jonherrera.com and at airshiplaboratories.com.

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