Here's a nice chart showing frequency response.
We acoustic engineers like to talk about frequency response and range. Here's a chart that sums up range quite nicely:
And here's another chart showing the response of various instruments:
Frequency Response is a measure of the magnitude of the output relative to frequency. For example, if a piano player hits every key on the piano with the same exact pressure, we might call this a 'flat' frequency response. Meaning, each note is as loud as any other note. But if the piano player bangs the high keys with extra force, we might say the frequency response is 'bright.' Tricky stuff, this Frequency Response measurement. In short, the highest quality speakers have the flattest response.
And here's a link to Win Burhoe's mini-book written a while back. It does a great job of explaining the terms we use in the loudspeaker business.