I have a pair of old speakers that I want the EQ calibrated for, and I found their manual with its SPL graph. However, I'm wondering if the graph is even accurate now (the speakers are around 15 years old). I'm also wondering how the read the graph, since there's more than one line and I don't know which one to use when calibrating my EQ.
Well, calibrating EQ for home speakers is not a common practice. However it seems that the easiest and cheapest way to do it is just play some music through them and record the output. Then you could use one of the free audio software (e.g. Audacity) to import the recorded audio and get the frequency spectrum graph. Using this graph you might get a general idea of salient frequencies.
And now regarding the graph you see.. You're right, usually the frequency response is represented with one line in dB/Hz units. However in your case there are actually 6 different graphs in one picture:
- Regular frequency response (one of the 3 bottom lines with no comments near it)
- Frequency response of 2nd and 3d harmonics distortion (the ones wit 2nd HD and 3rd HD comments near them)
- What seems to be a phase response (the 3 graphs on top). This one is pretty unusual, especially for home speakers.
Frequency response is going to vary at different levels of signal being sent to the speaker. The graph is useful for assessing general speaker quality, but you will probably get the best adjustments by doing it by ear.
If you really wanted to do a high quality job, you would need a pink noise generator, a real time analyzer and a 31 band equalizer. The way that a professional PA is "calibrated" is to play pink noise (a broad spectrum noise with a known frequency distribution), and then use the Real Time Analyzer to examine the SPL(sound pressure level or "volume") of various frequencies in the room from the primary listening location at the typical overall SPL and adjust the 31 band EQ to make a curve that fits the desired sound when the noise is subtracted out.