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WPI Technical Theatre Handbook: Equalizers
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As discussed in the sound board section, equalizers are used to shape the tone of a sound. Outboard equalizers are rarely (if ever) used with aux sends on a sound board. Instead, they are usually inserted on an input channel to equalize an input signal, or on output channels to equalize the output of the system before sending the signal to amplifiers and speakers. In applications where microphones are used (especially body mics), equalizers can be an extremely helpful tool in eliminating feedback problems. In addition to eliminating feedback, overall sound system equalization is used to adjust the system such that it sounds pleasing. Equalizers are also often used to overcome deficiencies in the speakers used, or in the venue that they are being used in.

Two main types of outboard equalizers exist. The most common is the graphic equalizer. Graphic equalizers consist of a set of slider controls and circuitry that allow boosting or cutting of a range of frequencies. Each slider is set to control a small range of frequencies, and can usually boost or cut this range fairly substantially. Graphic equalizers with ten to thirty bands are quite common. Graphic EQs with more bands afford the audio engineer much more control over what frequencies to boost or cut. This is a boon when eliminating feedback, as it allows the offending frequencies to be removed without disturbing other frequencies.

While graphic EQs work well for removing feedback, paragraphic EQs are often used instead. Like the parametric EQs mentioned in the sound board section, paragraphics allow the setting of the frequency to be boosted or cut. The only difference is that the control to boost or cut the signal is a slider rather than a knob. As a point of interest, the graphic portion of graphic and paragraphic equalizers comes from the fact that an idea of the signal shaping can be had by simply looking at the sliders on the front of the equipment. That is to say, it is a graphical representation of the equalization curve.

Many people often wonder how one goes about setting all of the sliders on an equalizer. To some, it seems to be black magic, and in some ways it is. There are two main approaches to setting equalizers, and both will not always apply in a given situation.

In cases where microphones are being used, there exists the potential for the system to feed back. This is a case where ringing out the system is often used. An equalizer is set up on the outputs of the sound board. The level of the microphones are brought up one by one, until feedback is heard. The frequency of the feedback is determined (either by experience or guessing), and removed using the equalizer. This process is repeated until all microphones are working at an acceptible level. Often, especially when wireless body microphones are used, actors will move around. Sometimes they move into a position that makes feedback extremely likely. This should be taken into account when ringing out a system by having someone walk around to all locations that the actor will be in and performing the above procedure at these locations.

In cases where no microphones are used, ringing out the system makes no sense, as the potential for feedback does not exist. It is this case (as well as the case when microphones are used) that equalizing to make the system sound pleasurable is used. This is a highly subjective topic, and is practically impossible to explain to someone. Since everyone has different tastes, it is difficult to agree upon one set of criteria that define what makes a sound system sound good. However, here are some general guidelines that can be useful:

  • There should be no noticable peaks in the sound. For example, booming bass or ``honking'' speech are not desirable. In the first case, the bass frequencies (100Hz and below) should be cut. In the second case, the low-mid frequencies (200-500Hz) should be cut. In general, the sound sytem should sound natural and smooth. Many people have never really heard a smooth and natural sounding sound system. This makes it difficult to know one when you hear one. When starting out, it helps to find someone who is experienced to expose you to a good sounding system. When you hear one, remember what it sounds like, and try to achieve that sound.

  • It is a common temptation to boost the low bass frequencies. Generally this just wastes amplifier power and reduces the overall headroom (ability to get louder) of a system. A large portion of the speakers used for sound reinforcement do not respond below 40Hz, thus boosting these frequencies on the equalizer does no good.

  • Equalize to a sound source you know, and that is similar to what the final sound system will be reproducing. If the system is just going to be reproducing voice, equalize to voices. Make them sound as understandable and clear as possible. If the system is going to be playing loud techno music, by all means, EQ to the latest Prodigy CD.

  • If you've got the time, put some music in that you know well, and play with the equalizer. Get to know what the different frequency bands on the equalizer do to the sound. This will help immensely when trying to make a system sound good.

next up previous contents index
Next: Compressors and Limiters Up: Signal Processing Equipment Previous: Signal Processing Equipment   Contents   Index
Steve Richardson 2000-07-06

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