Ringing audio is output response of audio device or DSP algorithm of software (firmware) to input impulse signal. This measurement intended for testing of audio gears to quality of playback of signal with sharp attack (piano, guitar, drums, etc.).
For sample rate converter AuI ConverteR 48x44 we see graph "Comparing of SRC ringing":
Graph 1 (click to enlarge)
Input test signal (1 sample with maximal amplitude) we can see here:
Ideal output signal must same to input signal (Graph 2).
It is possible when audio device or algorithm have flat amplitude-frequency response from 0 to Infinity Hz.
But many algorihms (as example: sample rate conversion) use some filters.
For qualitative rejection of conversion artefacts and minimal loss of width of pass band filter must have more sharpen transition from pass band to reject band.
But filter with more sharpen transition have more amplitude (energy) of oscillations out of time (1 sample only) input impulse. The oscilations is ringing.
Resume: We must use optimal filter for decreasing of ringing (maximal save pass band and maximal sharpness of transition from pass to reject bands).