Although most equalization is done by ear, it is helpful to have an idea of which frequencies will affect an instrument in order to achieve a desired effect. On the whole, the audio spectrum may be divided up into four frequency bands: low (20-200Hz), low-middle (200-1000Hz), high middle (1000-5000Hz) and high (5000-20,000Hz).

When frequencies within the 20-200Hz range are modified, the fundamental and lower harmonic range of most bass information is affected. These sounds are often felt as well as heard, and boosting in this range will add a greater sense of power or punch to music. Reductions in this range will weaken or muddy the lower frequency response.

The fundamental notes of most instruments are within the 200 to 1000 Hz range. Changes in this range often result in a dramatic variation in overall signal energy, with an increase adding to the overall impact of a program. Due to the ear’s sensitivity in this range, a minor change in levels often results in a major audible effect. The frequencies around 200Hz can give the bass a feeling of warmth, without a loss of definition, while those in the 500 to 1000Hz range may make an instrument sound hornlike. Too much boost within this range often causes listening fatigue. 

Higher pitched instruments are most often affected in the 1000 to 5000Hz region. Boosting these frequencies often results in an added sense of clarity, definition, and brightness. Too much boost within the range of 1000 to 2000Hz may have a "tinny" effect upon the overall sound, while the upper mid frequency range (2000 to 4000Hz) will affect the intelligibility of speech. The most common crossover ranges for a loudspeaker often rest within this range, resulting in a possible peak or dip when such crossover characteristics become exaggerated. Boosting this range may also make music seem "closer" to the listener, while too much of a boost will tend to cause listening fatigue.

The high frequency area (5000 to 20,000Hz) is composed almost entirely of the harmonic structure for most instruments. For example, boosting frequencies within this range will often add sparkle and brilliance to a string and woodwind instrument. Boosting too much may produce sibilance on vocals and awkward-sounding percussion instruments. Boosting at around 5000Hz. Has the effect of making music sound "louder" - a boost of 6dB at 5000Hz can make the overall program level sound as though it has been increased by 3 dB; conversely, attenuation make music seem more distant and transparent.

One way of using an equalizer is to set the amount of boost to near maximum and change the boost frequency until the desired range of the instrument to be EQ’d is found. The amount of boost can then be decreased until the desired effect is obtained. Attenuation of a frequency range can be achieved in a similar manner.

If boosting one range of an instrument creates the need to boost the other ranges, the effect achieved is simply that of raising the overall level. This is more easily done with the input fader. If the increased fader level does not make the sound satisfactory, it may be that one range of frequencies is too dominant and requires attenuation.

As far as recording with EQ goes, there are varying opinions. If an engineer other than one who records the multi-track session is to mix it, he or she may have a very different idea of how the instruments should sound and may have o work very hard to counteract the EQ used by the original engineer. If everything is recorded flat, however, the producer and artists will have to strain, while they are trying to pass judgment on a performance, to imagine how the instruments will sound later. It is also important to know how the instruments will sound with EQ during overdubbing so that the producer and artists can decide when a song has been "sweetened" enough. When several mics are to be combined on one channel, they can be Eq’d individually only before recording, so that recording flat, as a rule, will prevent optimization later during the mixdown of sounds picked up by each microphone. In addition, while recording with EQ does not change the perceived noise level, playing back with EQ does. EQ used on playback is also added to the residual background noise of the track. So boosting highs during playback can make the background noise on that channel more audible than if the highs were boosted before recording. (this was more critical for tape noise in analog recordings, than background noise on digital sessions) If the same engineer is to record and mixdown the session, then recording with EQ is usually not a problem. In any event, unless a special effect is desired, EQ should be used moderately, and microphone selection and placement should be used to obtain a good instrument sound. If an instrument is poorly recorded in an initial recording session, it can rarely be corrected later during mixing.

An equalizer is a powerful tool and its proper use can greatly enhance or restore the musical and sonic balance of a signal. Experimentation is the key to equalizer use.

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