The one-eighth rule tells you whether a source will colour your headphones. It does not tell you how. This does: it takes the shape of a headphone's impedance curve, puts your source's output impedance in series with it, and draws the frequency response you actually end up with.
Then it does the same for all 53 sources in the database and ranks them, so you can see at a glance which ones leave your headphones alone.
Your source's output impedance sits in series with your headphones, and the two form a voltage divider. Because a headphone's impedance changes with frequency, the divider changes with frequency too — so the source stops being neutral and starts drawing its own tone control over your music:
attenuation(f) = 20 · log10 ( Z(f) / (Z(f) + Zsource) )
Everything on the chart is plotted relative to 1 kHz. That matters, because it is the reason two correct calculators can print different numbers for the same pair — some reference the lowest impedance, some the nominal figure. Referencing 1 kHz matches what you would hear after matching levels by ear, which is the comparison people actually make.
Where the headphone's impedance is high, less voltage is lost in the source and that region comes out louder. That is why a dynamic driver's bass resonance turns into a bass boost on a high-impedance output, and why a multi-driver armature IEM — whose impedance can swing by a factor of five across the band — can come out sounding like a different headphone entirely.
A planar is the opposite case and worth trying in the dropdown: its impedance is almost perfectly flat, so no source impedance can tilt it. The response comes out level no matter what you plug it into. This is the part of the one-eighth rule that gets lost — the rule is about the spread between a headphone's lowest and highest impedance, not about its nominal figure.
First, these are archetypes, not measurements of specific models. The shapes are typical of each driver type and the numbers are yours to adjust; nothing here claims that your particular headphone measures this way. If you have a real impedance plot in front of you, read the lowest point, the highest point and where the peak sits, type those in, and the answer becomes specific to your pair.
Second, impedance has phase as well as magnitude, and this calculates with magnitude alone because phase is almost never published. Where a headphone's impedance is strongly reactive the true attenuation is slightly less than shown — for a 60Ω load at 45 degrees fed from a 90Ω source the difference is about 0.66 dB. It moves the numbers a little; it does not change which sources are safe.
For the simple yes-or-no version, the one-eighth rule checker is quicker. For whether the source can also make enough power, the matcher does all three checks at once.