If a headphone has frequency-dependent impedance there will be voltage-divider modification of its frequency response when it is used with a signal source having finite output impedance. This graph illustrates how large this effect is by plotting attenuation versus frequency when the headphone’s left capsule is fed from four different source impedances covering the typical range of headphone amplifier output impedances: 1 ohm, 3 ohms, 10 ohms and 30 ohms. In some instances the modification of frequency response may be large enough to have a readily perceptible effect on tonal balance.
This graph is obtained by post-processing the impedance data to calculate attenuation in decibels from 20Hz to 20kHz for the four different source impedances. Note that the test parameters below are for the latest impedance measurement method. Some older impedance measurements were performed using a pink noise rather than stepped-sine test signal. If this is the case it will be stated in the graph's information box.
Test signal: stepped sine (1000 steps)
Sampling frequency: 96kHz
FFT length: variable, 64pt to 1,048,576pt
Frequency resolution (measurement): variable, 0.092Hz to 1.5kHz
Frequency resolution (graph): 1/100th octave
Measurement length per step: 100,000 samples minimum