Distortion is a measure of how well a speaker can do it’s job of playing music, without adding extra music of it’s own!
The missing spec
I’m going to be brief about this subject here because, you simply won’t see any distortion figures from other portable speakers. You won’t even see them with Hi-Fi units. You’ll have to go to high end pro-audio speaker manufacturers to see them quoted. It might be because all speakers distort much more than electronics do (our studio electronics distort about 1000x less than the best speakers), but a distortion plot is a good gauge of the quality of the speaker drivers.
Distortion varies with volume and frequency and also there are different types of distortions, some actually quite nice to listen to, others appalling!
Here’s a SoundBucket THD response:
As you can see distortion rises at lower frequencies, this is because the bass driver has to move further to produce these low notes. Thankfully, human’s are less sensitive to low frequency distortion.
There’s also a dip at 3KHz of the third harmonic – nasty distortion, and a rise in second harmonic – nice distortion, which works in our favour.
As a note, most portable speakers I’ve measured have distortion figures at least 10x worse than the SoundBucket, which manifests itself in a hard brittle sound.
To round off, THD is important in assessing a speakers performance, just don’t expect it to be quoted in portable speakers spec sheets, the figures are often a bit embarrassing.