Experiment:
JBL LSR 308
"Normal" room noise
Simultaneous measurement, single speaker active
About 82db SPL at listening position
Mic 1 at Listening Position
Mic 2 hanging over the front of the speaker, so the capsule is in the plane of the front of the cabinet - more extreme than the Klippel measurement position, but I don't have a stand to hold the mic where I might like to put it.
Using dBc setting - graph sets tone level received to 0dB, to allow comparison of levels of noise and distortion relative to the tone
View attachment 92262
Tone and harmonics are much higher relative to room noise in the close measure
Third harmonic just peeking out of the noise floor at the listening position
Disclaimer: quick test, not trying to make it look bad or good, there will be variations with frequency and the phase of the harmonics (levels will change) at the listening position. Take the measure with a little suspicion, but useful to illuminate the basic idea.
I set it up, frequency selection - random, at least, nothing specific and no tuning - saw harmonics at the speaker, raised amplitude until one of the harmonics peeked out of the noise floor at the listening position.