Ideally, a stereo system would have a "flat" frequency response. That means the graph of the output sound level would be a horizontal line. No frequencies would get special emphasis. The graph shows the frequency response of such an ideal system.

Most stereo systems have a reasonably flat frequency response over midrange frequencies. That's why speech can sound good even on inexpensive radios. But music has a much greater frequency range than speech. At very low or very high frequencies, amplifiers and speakers do not work well. Some organizations test stereo systems and publish the results. The graph below shows the accuracy of a loudspeaker tested by Consumers Union. It is the graph for one of the better speakers they tested. Notice that the frequency scale is logarithmic. We are not sure what is plotted on the vertical axis, because they did not label the axis! They also defined a "limited spectrum accuracy" and a "full-frequency spectrum accuracy" for each loudspeaker they tested, but they did not tell the reader how it was defined. It has something to do with the departure of the frequency response from "flat." A musician may wish to change the frequency response of an amplifier to produce a special effect. The frequency response can be changed with a tone control or equalizer. Boosting the bass means increasing the response for low frequencies. Changing the treble changes the high-frequency response.