Equalizers

             The reason that we need an equalizer in a sound system is that every environment you place your sound system in has some effect on how flat your system reproduces your sound.  A perfectly flat system is the goal and that can only be achieved by learning how to properly use a graphic equalizer.  The graphic equalizer seems to be the most complicated part of the audio path, but in reality is a very straight forward piece of equipment that takes a little time to master but only minutes to understand.  An equalizer is comprised of a master volume, an in/out button and 5-30 slider controllers which are band pass filtered volume controls and are labeled between 20Hz and 20KHz.  A band pass filter is a type of filter that is centered on a particular frequency and will cut or add volume to only those frequencies near its center.  The entire audible sound spectrum is comprised of approximately ten octaves.  If an equalizer has five controls, then each slide control raises and lowers the volume of two octaves around its center frequency.  If an equalizer has 9-10 controls then it is a one octave equalizer.  If there are 30 or more controls the equalizer is a 1/3 octave equalizer.  That means each control will effect the volume of a 1/3 octave slice of the entire sound which boils down to four keys on a piano.  If you move all the controls equally up or down you will effectively bring the entire volume of the sound up or down.  If you turn up the volume in the 400-500 Hz region on the equalizer and play the piano through it, when you play the keys near A-440 (A-440 is the A key below middle C) you will notice that they are louder than other piano keys higher and lower on the scale.  This is called a change in tone.  I say we have turned the volume up in the region near A-440.  I figure there really aren’t tone controls, only volume controls.

             To properly equalize a room simply divide the room into three parts, front to back with imaginary lines A and B.  Set your best microphone at the first 1/3 or 2/3 point (lines A or B) facing the speakers.  Set the microphone directly in front of a speaker stack.   Set all the slide controls on the equalizer to their center position. 

Room

                                A                                  B

Plug the microphone into your mixer and slowly bring the volume up.  The trick is to bring the volume up very, very slowly until it starts to softly feed back.  I emphasize softly because you don’t want to feedback at full volume because you will probably break something like your speakers or your ears.   If you are doing it correctly, you will notice that the feed back is only one or two notes. Sometimes it’s a high note and sometimes it’s a low note.   If you know how to whistle you could whistle the same note.  If you can find a piano or guitar you can find a note on that instrument that will be in tune with the feedback.  The distance between the room’s walls, ceiling and floor all have a profound effect on the sound.   The shape of the room and the materials used to build the room also have a great effect on the sound.   So does humidity, temperature, altitude and the people in the room.

Once the system is softly feeding back you can start this sequence:  

• Step 1.  Lower a frequency volume slide control.

• Step 2.  If it doesn’t make any difference then put it back where it was previously set and go to the next slide control and repeat step 1.

• Step 3.  If it makes the feedback go away, then set it as low as necessary to keep it  that way.

 After you have found the filter volume control on the equalizer that turns the note off, turn the volume up again very slowly until another note feeds back.  Then go to step 1 again.  Repeat this process until, instead of one or two notes feeding back, it seems as if all the notes are feeding back.   At that point, you have successfully equalize the sound system for that room and its environment.   You don’t have to start at one end of the equalizer and go to the other end every time.  With a little practice you will learn where the general area is and start there.

             Most equalizers have a master volume control.  The reason for the volume control is to adjust the volume so that when the equalizer is in or out you don’t have any volume change, only a change in the “tone”.   Since we only cut the volume of  the “bad” frequencies and never raised any, a small volume gain from the equalizer’s master volume will probably be necessary.  The main reason we only cut the volume on the filter controls is the equalizer adds noise or hiss when the filter volume controls are boosted.  Noise is not our friend and should be avoided when ever possible.

             One other thing, most sound systems can’t efficiently reproduce any sound below 50Hz.   Also, the lower the frequency the more power it takes to produce that frequency.  Therefore, turn off all the frequency slide controls under 40Hz.  It will allow the wasted power to be used in the area of the sound that is audible and that will make your system louder, cleaner and help your amplifiers to run cooler.

             Some musicians don’t want a flat sound and that is ok too.  Just set the room for a flat sound and then readjust those areas of the equalizer that give you the sound you want.  By readjusting the equalizer it may be necessary to turn down a bit so the system won’t feed back.   Any sound system if driven loud enough will feed back. 

Author:  William Mellon