I heard the term "resonant frequency" being thrown around on here on the topic of equalizing headphones, as if it were some variation in how we hear sound. What is it exactly, and how is it determined?
What is "resonant frequency" (hearing) and how is it determined?
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Not sure about the usages you are referring to specifically. Usually the term is used to refer to the Thiele-Small Fs value - the free air resonance of the driver itself, or a specific resonance frequency of the driver housing (enclosure or material). That is a specific frequency where the material or space itself is excited and resonates (multiplies and adds harmonics). Think the proverbial wine glass breaking from the opera singer - that's due to her hitting the resonant frequency of lead crystal.
In equalizing, they are probably finding peaks where something in the headphone is resonating and adding volume where there shouldn't be - at a specific frequency... and then working to correct for that.
You could determine it a few ways. Complex mathematic models, frequency sweeps and careful measurements with microphones and analyzing software, sometimes by ear...
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The resonant frequency of a headphone driver is usually where there's a peak in the impedance curve (usually around 100 Hz).
Example: somewhere around 70 Hz for the HD650
The resonances of the ear canal and pinna combined are at around 2700 Hz and 5000 Hz. This is different from person to person however.
And of course this also changes if you insert in-ears.