Specs & measurements
Frequency response
Also called FR, frequency response graph, FR curve
Frequency response is how loudly a product reproduces each frequency from bass to treble, shown as a line on a graph. It is the measurement that best predicts how a headphone or speaker will sound, though it does not capture everything.
A spec sheet figure such as "20 Hz to 20 kHz" says almost nothing, because it omits how much the level varies across that range. The graph is what matters: a bump at 100 Hz means extra Bass; a dip at 3 kHz means distant vocals; a peak at 8 kHz means possible Sibilance.
Headphone graphs are measured on an ear simulator and compared against a target such as the Harman target or Diffuse-field curve. Different rigs disagree above about 8 kHz, so two graphs of the same headphone can look different in the treble. Speaker graphs are measured in an anechoic chamber or quasi-anechoically and are more comparable.
audiophila. shows frequency response measurements attributed to their source on product pages where they exist. The response describes Tonality; it says little about Soundstage, Dynamics or distortion.