https://diyaudioheaven.wordpress.com/tutorials/how-to-interpret-graphs/csdwaterfall-spectrum/
"In short: a good headphone does NOT exhibit resonances that ‘ring’ for a relatively long time period (above 500Hz they should remain below 2ms before they have dropped 30dB in amplitude). So the better headphones should have a ‘wall’ of a waterfall drop as far in the back as possible. Ridges present at or near the front of the plot (even when already low in amplitude) are NOT desirable. Long resonances usually indicate an issue in that specific part of the frequency range and may mean that the headphone doesn’t resolve very well in that part of the frequency range"