Let's cover what tone controls can do and how to best use them.  In their simplest form, tone controls are used to boost (increase) or cut (decrease) the response of a particular microphone or instrument at certain frequencies.  They are very similar to controls found on most home stereo systems.  The tone controls on sound mixers are often called equalization controls, or EQ for short.  A mixer with three-band EQ would have bass, midrange, and treble controls.  If you are fortunate enough to have four-band EQ, the midrange is usually split into high-mid and low-mid controls.

For each control, the amount of boost or cut will occur around a specific frequency.  In general, the bass control will affect frequencies around 100 Hz (cycles per second) and below.  The treble or high control will affect frequencies around 10,000 Hz and above.  A single midrange control will be centered on or near 3000 Hz.  If two midrange controls exist, the high-mid control will center somewhere between 4000 and 6000 Hz, and the low-mid frequency will fall somewhere between 500 and 2000 Hz. The specific frequencies should be listed in your mixer manual.  Also, some mixers have an additional control(s) which allows you to adjust what frequency you want the boost or cut to be at.

Guidelines for Setting and Using EQ Controls

1.  Use EQ sparingly!  If all the EQ controls on a channel are boosted more than 3 dB (approximately 2 or 3 o'clock), return all the EQ controls to zero flat and try increasing the volume control.  It will probably sound better and you will have less feedback problem.  The same principle applies for excessive frequency cutting.

2.  In general, listen to the sound and first try cutting the frequencies that are too loud.  If a voice has too much bass, first try cutting the bass rather than boosting the highs.  And if you have a mid control, you may achieve a better result by gently adjusting two adjacent controls rather than a drastic cut or boost on one control.

3.  Tone controls affect a broad band of frequencies.  Excessive boost or cut may degrade the overall response of a microphone or instrument to a point that just sounds bad.   If you are using the EQ controls to try to remedy feedback problems, be very cautious.  Use your ears.  A slightly lower volume may be better to listen to than a very unnatural sound caused by too much EQ adjustment.

4.  Remember that your ears have frequency response which is quite dependent on how loud the sound is.  Check your level before turning the EQ knobs.

5.  Tone controls should not be used to improve the speaker system/room response.  That is what the system equalizer is designed to do.  You need to make sure that your speaker system is properly designed, installed, and adjusted.  Also, the tone controls cannot improve the acoustics of a room.  Problems with acoustics can only be fixed with physical treatment.  If you are hearing a "funny sound" from a microphone, try repositioning the microphone.  You may be hearing the result of a reflection combining with the original sound and now producing a result which is quite different form the original!  This especially can be a problem with a pulpit or lavalier microphone.

These items all point to a general rule of thumb when using tone controls on any mixer:

THE CONTROLS ARE THE LAST RESORT IN THE ATTEMPT TO COMPENSATE FOR SONIC PROBLEMS.

EQ controls should properly be used for subtle shading to enhance the sound, to make a sound more natural, or occasionally change a sound to achieve a special effect.

In closing, here's a tip on creative use of tone controls.  During a musical performance, you may want to highlight a certain vocalist or instrument.  Rather than use the volume control, use a small amount of EQ boost in a strategic frequency range of a source.  For instance, boost the treble control of the piano microphone to enhance its upper harmonics.  This method will create a more subtle change in the apparent presence (the feeling of being closer or in front) of the piano in the mix without the perception of a "solo" being added.

Keep practicing--and don't forget to listen!