The figures of distortion by manufacturers are measured at a certain point, certain frequency range and certain power (RMS). Now the point is why Audiophiles pursue less than audible figures.
The distortion increases when the power goes up and the frequency spectrum they have to reproduce widens up. Manufactures sometime include a chart or birth sheet with their amps and if you can see the graph you will digest the above part easily.
There are two types of distortion:
Linear & Non Linear
Linear:
Any divergence from flat in the frequency response would be considered a form of linear distortion. Though, typically below the beaming point, drivers are fairly flat. Outside of beaming is where breakup occurs and is typically the area where linearity in response is compromised. It also does not change with volume.
This is typically used to determine low-pass values as cone break-up is the worst offender after the beaming point.
Non Linear:
It changes with volume and also referred to as harmonic distortion, THD, etc. This is typically used to determine your high-pass values.
The response type of drivers (tweeters) shape their sound. The response types are following.
Frequency Response
Power Response
Polar & Bipolar Response
Power Response
&
Beaming
Acoustic Roll Off
It makes me mad when people complain about harshness of tweeters and they dont give attention to the above factors and placement. Crossovers and EQ, reflections off the windshield, i mean plenty of factors there but the tweeter is too blame :(.
The crossovers and placement goes a long way into the output of tweeters. The dispersion pattern of tweeters can only help you decide how it should be aimed .i.e. On axis or Off axis