Frequency response is simply amplitude (
level) compared to frequency. As already noted, speakers do not output the same
level at all frequencies and when a specs says "A Hz to B Hz (+/- XdB)" it is saying that given a constant input containing all frequencies at the same
level (or a swept
sine wave input of constant
level), then the output of the
speaker varies within the dB limits given over the frequency range noted. +/-3dB is the traditional limit to be used although some specs may use -6dB, +/-6dB, -10dB or not state any limits at all, making it a virtually useless spec.
The
frequency response is usually measured at a single
point on the vertical and horizontal axis of the
speaker. This measurement is typically in the far field (far enough from the
speaker so it behaves as one device rather than being affected by each
driver) and under free field (an environment free of reflections) conditions. While it is definitely useful data, it is also really only valid at that one
point and under those specific conditions.
Mike is absolutely right that most simplified ratings not only do not necessarily tell the whole story, they are unfortunately sometimes also less than accurate. Be it coverage/
pattern, sensitivity,
impedance or response, all of which vary with frequency, charts and graphs usually provide a much better picture of what is really happening. For example, it is not unheard of to find a
subwoofer spec that states a sensitivity that turns out to be the peak sensitivity at a frequency much higher than a
subwoofer would normally operate and that in the frequency range it will actually be used the sensitivity is several dB lower. There are also some
speaker specs that show the
frequency response after applying the recommended processing rather than just the
speaker response itself. So read specs carefully, they are pretty easy to manipulate.