A graphic equalizer operates like a typical volume control, except it changes the volume level at different frequencies. For example, a string bass and a piccolo produce very low- and very high-pitched sounds, respectively. A graphic equalizer can be used to increase the volume of the low-frequency sounds and decrease the volume of the high-frequency sounds to make the string bass louder and the piccolo softer.
Graphic equalizers can be used to stop "humming" noises, or feedback, from speakers and microphones. If an audio source is humming, you can get rid of the hum by lowering the volume of the frequencies that are causing the feedback. To identify the feedback-causing frequencies, try increasing the volume on different frequencies one at a time to see which ones cause the humming to get louder. Decrease the volume on the the identified frequencies to cut out feedback.
A graphic equalizer can improve how a speaker or a microphone interacts with sounds. Different hardware will likely require different equalizer settings because different speakers and microphones emphasize different sonic frequencies. Having identified what frequencies different instruments produce, you can choose to emphasize some sounds over others as well as specific ranges of the sound over others. The ideal sound quality depends on what the listener wants to hear.
If you want to emphasize a low instrumental part like a bass part over other instruments, try increasing the dials closest to the 31 to 262 Hz range.
If you are trying to de-emphasize a high-pitched singer's vocals, try decreasing the dials within the soprano vocal range between 262 and 1,047 Hz. However, the vocalist is probably not singing across the entire range, so start with decreasing the dial closest to 1 KHz and try lowering one or two consecutive dials at time until you find the singer's range.
While an instrument's sounds may operate on a low frequency, the instrument may produce additional sounds at higher frequencies, called "overtones," that give the instrument a fuller sound. If you were to target a bass guitar part, you would turn up the dials closest to the 31–262 Hz range, kick up the dials around the top of the instrument's overtone range, around 15 KHz, and lower the volume on the other dials.