Ricci asks and others wonder why aren't the CEA2010 numbers higher?
Well I don't know off hand, these numbers should be set by different things depending on frequency.
When time permits we will try to establish a test which uses a tone burst like the criteria calls for.
Sitting there for minutes at a time with a high level sine wave getting a measurement is probably a bit to harsh as the dark brown voice coils on the first two drivers, the first time around suggest.
Thinking about it now, the 5-6 cycle tone burst signal would already be about 1/3 octave wide in bandwidth so it may be that the readings on the detection side would be different with such a short excitation. I think we will visit this again as the upper frequency numbers ought to be higher even with a sine wave.
Alternately, I am curious to find out what aspect of it is the limiting factor up high.
Also, while I have no reason to suspect anything in the electronics chain, the test criteria is reached which the sum of the flaws in the system reach the levels the standard calls for.
For example, amp clipping, mic, limiting, mic pre clipping, input clipping, any of these could cause the threshold to be reached prematurely.
Lastly, if a speaker made enough air noise, it could exceed the criteria at the high end where the test is most sensitive.
This is sort of why distortion measurements are questionable when taken in a room as anything buzzing or vibrating can be picked up as well.
Like I said, I think we will visit this rating thing again and I think we will try to establish numbers at 8 and 10Hz for fun.
What can you hear
Well, a numeric THD figure is essentially useless in defining what you can hear.
The reason is due to something called masking.
You hear pitch not in a continuous span but in small segments or critical bandwidths called a Bark. Two pure tones of equal amplitude within the same Bark, sound like one note. Two equal loudness tones in separate Barks are heard separately.
A nice over view of all this can be found here;http://is.rice.edu/~welsh/elec431/psychoAcoustic.html
Take a little bit and read this and follow it to about half way down to where it talks about Auditory Masking.
The little graph has an example of he masking curve for what looks like a 250Hz tone.
The dark curve is the masked threshold that the 250Hz tone produces.
Now, in the presence of the 250Hz signal, ANY tone below the black curve is inaudible, masked by the 250Hz tone.
The reason for the CEA 2010 (it would seem to me) is that it weighs each harmonic more like the shape of the masking curve for the frequency under test.
This is partly why one can have a huge second harmonic and not hear anything, it is the most strongly masked harmonic AND it is a musical second which is part of a chord.
The third harmonic threshold is lower stepping down to the highest (10) harmonics which are at -40dB
For those extraordinarily interested in single digit bass, examine the shape of that masking curve, notice that there is downward masking also. While there are no sub-harmonics generated by loudspeakers like on the high side, that downward masking also means that to hear the single digit sounds, they have to be the loudest thing spectrally or they can be masked by a louder low bass signal higher up.
Remember at low frequencies, keep the shape of the hearing sensitivity curve in mind.
Now, all this has applied to tones (steady state signals).
An old speaker guy acquaintance, Earl Geddes, has concluded and proposed the rather controversial conclusion that with music, the low order distortions most loudspeakers exhibit (as opposed to high order distortion like electronics tend to produce) is (up to a significant level) inaudible and had backed it up with some blind testing.
Anyway I am rambling and have to get ready to go out.
So, at some point we will re-visit this measurement, possibly with a tone burst..
As far as the upper frequencies, above 30Hz particularly, I do not know what the limiting factor in the measurements was here, the levels that were measured certainly are not the maximum output but that is something I will look into.
Have fun all,