Spectral decay
Any sudden change (especially if high-Q) in frequency response always causes ringing (sometimes also referred as ‘stored energy’), and that’s why a smoothly sloping FR down to as close to direct current (0 Hz) as possible causes the least ringing and results in fast decay at all frequencies. Also same applies to the upper end of the frequency response. A steep low pass filter causes a rise in group delay and ringing near cut-off/crossover frequency. Spectral decay graph shows the frequency response after the input signal has stopped. I used a 200 ms gate time and a 40 ms slice spacing up to 200 ms. The spectral decay is evaluated down to -24 dB level from the highest amplitude point. The measurement itself was taken at around 90 dB level, which allowed a good S/N ratio but didn’t push subwoofers into compression. The same info is often presented with a ‘waterfall graph’.
Spectral decay should be as fast as possible over as wide frequency range as possible. Ideally the graph should only have one frequency response slice. Ringing at low frequencies isn’t as harmful or audible as ringing at higher frequencies.
