An equalizer, or EQ, allows for manipulation of ranges of frequencies in audio. This allows customization of the sound to match a certain song, speaker or pair of headphones. Equalizers typically have adjustments for frequencies ranging from 32Hz to 16kHz, which is the approximate range of what can be heard by the human ear.
An EQ is used to adjust the sound to compensate for poor sound, which may be caused by environmental factors or a low-quality pair of headphones or speakers. For example, an EQ could compensate for headphones or speakers with too much bass by lowering the bass or increasing the treble. The majority of professional sound engineers recommend decreasing the level of a frequency before increasing it. This is because having too many frequencies at high levels causes the audio to sound muddled.
EQs are mostly seen in the form of a digital graphic EQ, which is a row of sliders on a digital device such as a computer or smartphone. Each slider corresponds to a different range of frequencies, and moving the slider up or down changes the volume, or gain, of that frequency range. The graphic EQ is modeled after the physical EQ, which has gone out of style in the past decade in favor of the graphic EQ.