If you stimulate something it responds back in kind. Strike a bell and it rings. Pass a transient through an audio circuit and it too rings giving you more than you bargained for in the process. Loudspeakers ring, microphones ring, just about everything in nature responds back when you change its state.
We don't see this added energy in typical audio measurements such as THD and IM because these distortion measurements are made with steady state audio - meaning they do not stop and start - rather they are recorded with steady tones which do not change states. So when we see an amplifier with a THD reading that's 0.001%, way below anyone's ability to hear if a few of those zeros are missing, we haven't any clue how it responds to transients - and transient response and distortion play a HUGE role in how our systems sound.
Transients provide essential clues to our ear/brains on the nature of what's being presented to us in our music systems; information that is absolutely critical for us to identify what's going on and whether a system is believable or not.
The sound of live unamplified music is full of transients, each with their own set of "ring tones" attached to the original impulse. The audio equipment's ability to faithfully reproduce these transients, without adding more ringing, is critical to our system's success if we're trying to build a credible high end system in our music rooms.
Designers and reviewers alike sometimes use what's called impulse responses to see how their systems respond. Here's a picture of an impulse response:
Note the big spike in the middle, flanked by multiple smaller spikes on each side of the big one? The big one is the impulse and the smaller versions of it are the ringing that it produced. Just to be clear, the big one is all we want to see - and all the little ones are added and inaccurate distortions.
This particular impulse shows ringing before and after the impulse - a strange feat indeed - and this is typical of many digital circuits called "pre-ringing". Pre-ringing pretty much never happens in analog circuits.
We can also see this ringing when we place another type of impulse through our system called a square wave.
This should just look like a perfectly flat line but instead note the ripples.
What's all this mean? It means that whenever a musical transient is playing through your system you are getting far moreinaccurateinformation than you ever wanted.
These are the areas that we look at very closely when we design a product because too much ringing sounds hard and bright - a characteristic none of us are happy with.
So, the next time someone says their amp or loudspeakers are perfect because "all the measurements show no distortion" you'll know better than to believe them and trust your ears a bit more instead.
