The ideal headphone would have no resonances within the audible frequency range, but few models even approach this ideal. This test identifies the presence of resonances – typically caused by 'breakup' modes within the drive unit diaphragm or modal behaviour within other structures or enclosed air spaces of the headphone – using a plot of cumulative spectral decay (CSD).
CSD is the oldest method of displaying wideband resonance behaviour in audio transducers, originally developed for loudspeakers by D E L Shorter of the BBC in the 1940s. It comprises a pseudo-three-dimensional graph with frequency, amplitude and time axes, in which a succession of frequency response traces is overlaid, each derived from successive time steps through the device's measured impulse response. The ideal CSD result (see below) shows rapid decay in output at all frequencies; more normally there are ridges present where the decay is lengthened due to resonance. Because of its appearance the CSD plot is commonly termed a 'waterfall'.
Test signal: pink-spectrum periodic noise
Sampling frequency: 96kHz
FFT length: 8192pt
Time window: 980 samples (10ms)
Measurement frequency resolution: 24Hz
Measurement averages: 10
Trace smoothing: 1/12th octave
CSD lines: 61
CSD line spacing: 8 samples (83μs)