A stereo equalizer is a tool or method used to adjust the amplification of sound at a variety of frequencies. Frequencies in this sense refers to the pitch of a sound rather than a radio or television broadcast frequency. The method and complexity of a stereo equalizer vary greatly depending on the type of equipment.
The simplest form of a stereo equalizer is nothing more than a hi-fi which features separate controls for bass and treble. These represent the lower and higher frequencies of sound respectively. More complex home equipment could have five or more adjustable frequencies, while professional recording and playback equipment in a studio could have as many as 30. In professional use, these will generally be labeled by frequency for precision, though in home use the controls are often not labeled in this way.
The level of control which can be applied depends on the equipment. Cheaper home stereo systems may have switches which mean only fixed levels can be used, such as high, medium and low. More sophisticated home equipment uses dials or sliders so that more precise changes can be achieved. In a professional setting, computer equipment could be used so that an exact figure can be input.
Most portable music equipment, such as an mp3 player, has some form of stereo equalizer. In some cases this allows different frequencies to be set to specific levels. More often the function is limited to switching from a variety of pre-set levels corresponding to a particular type of music. Generally this will be done to emphasize the frequencies most commonly used by a style of music. For example, "pop" settings may raise the higher frequencies to give more prominence to vocals, while a "dance" setting might put the emphasis on lower frequencies so that the bass effects are easier to hear.
Many forms of stereo equalizer are also known as a graphic equalizer. This term is usually used to refer to a set-up in which the levels can actually be seen in a row. This can be achieved either by lights, a computer display, or simply by the positioning of vertical slider controls. These slider controls are technically known as potentiometers. In nearly every case, the lowest frequency is displayed to the left and the highest to the right. This effectively means that the display shows a graph of the adjusted sound, plotting the gain level against the frequency.
