Before you decide to use an equalizer to modify an audio signal, take a moment to think carefully. While it might seem like a powerful tool, overusing it can lead to both benefits and drawbacks in the overall listening experience. The equalizer is not a magic solution—it has limitations and can significantly affect sound quality in ways that aren't always obvious.
The equalizer was first developed in the 1830s to address sound deficiencies, primarily used in Hollywood studios. With the help of what's now known as a fidelity filter, it proved effective for long-range amplification, which contributed to its widespread adoption—and eventually, misuse. Many generations of sound engineers have either misunderstood or been unaware of how the equalizer truly affects sound, leading to poor results when it’s applied improperly.
Equalizers are commonly used in mixing consoles, on input channels, or in wiring devices, such as on the output line to a speaker. Although using an equalizer on a microphone input channel allows for shaping the sound of a voice or instrument, one often overlooked issue is whether the need for an equalizer could actually indicate a problem elsewhere in the audio chain.
Equalization may be used to fix issues with the audio chain, speaker characteristics, microphone selection, or studio acoustics—even in well-equipped studios with top-tier equipment, equalizers are still frequently employed. Using an equalizer on the speaker output is particularly interesting because it's easy to assume that all acoustic problems in a room or speaker reverb issues can be fixed with it.
Today, two main types of equalizers are in use: parametric and graphic. A parametric equalizer allows precise control over frequency, bandwidth, and gain or attenuation. It enables continuous or step-by-step adjustments of frequency and amplitude, with a wide range—up to 20:1—while also adjusting the sharpness of the peak, known as the Q factor (ranging from 0.29 to 5.0). Typically, the highest and lowest frequency ranges can be switched between peak and flat modes.
When correcting for acoustic or speaker issues, operators may set the equalizer either subjectively through listening or objectively using an audio analyzer. The analyzer's microphone captures the sound at a specific location, and the equalizer's calibration curve is adjusted to counteract the measured response, aiming for a more linear output. However, this approach only works for one point in space.
Since the operator's ear or the analyzer's microphone records sound from a single spot, moving to another location can cause the equalizer settings to become ineffective. What sounds good in one place may not work anywhere else, especially considering complex acoustic phenomena like standing waves, resonance modes, and the mix of direct and reflected sound. This makes it difficult to achieve consistent sound across different positions.
The human ear, separated by about 19 cm, is highly sensitive to sound wave diffraction around the head, allowing us to perceive direction, intensity, and time differences between direct and reflected sound—this is called binaural hearing. In most cases, unless a simulation head is used (which mimics the exact placement of microphones in the ears), the data collected by a mono or stereo microphone will differ from what a listener actually hears.
Figure 2 shows the calibration curves for 1/3 octave equalizers set at +2, +4, +6, +8, +10, and +12 dB. Even with a simulation head and subjective corrections, it's important to remember that any frequency adjustment can have unintended consequences across a broader range, depending on the type of equalizer. As shown in Figures 1 and 2, the multiplier equalizer (Fig. 1) has a much wider correction range compared to the 1/3 octave equalizer (Fig. 2), due to its design characteristics.
Even when using a 1/3 octave equalizer at its highest setting, the correction range can still be relatively broad. At lower gain levels, such as +4 dB, the effect becomes even more pronounced. Therefore, it's crucial to approach equalization with a calm, rational mindset, making logical and balanced decisions to avoid unnecessary distortion or degradation of the original sound.
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