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Original Message

RE: Objective Measurement Results for Cables

Posted by Jon Risch on October 1, 2007 at 17:31:26:

Having some familiarity with interpreting multitone spectrum plots, I can shed some light on this.

The signal being measured is the electrical one, both at the amp and at the speaker end. The levels of the fundamentals of the test signal multitone (The ten frequencies that start at 100 Hz, and ending at 10 kHz with 8 more in the middle evenly spaced logarithmically, all of which are the ONLY lines shown that extend all the way to the top of the graph, and possibly beyond) would not change appreciably, energy that is at -40 dB down would only have the potential to change one of the fundamentals by approx. 0.1%, or approx. 1/10 of a dB.

What you noticed is the significant increase in distortion products, of which most are IM products rather than harmonics. If you look at fig.6.7, this shows the residual level of the various harmonics and IM products which are present with this particular test tone and spectrum analyzer, the pattern of which was predicted by modeling of that particular tone in one of the CV AES papers on multitones. The increase in distortion due to the cable and loudspeaker interacting with the amplifier, is the INCREASE above the levels in Fig. 6.7 that is shown in Fig. 6.8 and 6.9

What is significant, is that even right at the power amp, distortion is much higher with a real world loudspeaker load compared to a simple resistive load.

bjh wrote:
"Assuming this is the case (difference plots would help) my only guess for the apparent paradox would be that the additional enegry is Back EMF that is attenuated by the cable (to account for the lesser energy at the source). "

This additional distortion energy is the combination of back EMF and acoustically generated inter-driver and inter-speaker system crosstalk.

What is not clear in the Newell and Holland web article, and no information is given, is if the loudspeaker load was in isolation, that is, whether or not it had a stereo companion, or was being exposed to the test signal by itself, with no other loudspeakers playing nearby.

As I show at my web site, at:
http://www.geocities.com/jonrisch/biwiring3.htm
Fig. A, B and C ,
When another speaker is also playing, then a direct generated voltage is present, and this voltage ALSO gets injected into the amplifier outputs via the speaker cable. Since the other loudspeaker would be somewhere on the order of 6 to 12 feet away, the acoustic output will not reach the other loudspeaker till approx. 5.3 to 12.6 ms later, thus there is a significant time delay for the external stimulus being generated and reaching the power amp.

It is one thing for the distortion to be occurring at the same time as the original signal, this is bad enough, but with masking and various other aspects, it makes it harder to notice the distortion when the original tones are playing too. But when the distortion is a full 5 to 13 ms AFTER the signal, which may have ceased sounding for a moment, the distortion generated from the output of the other speaker will stand naked for that brief moment, making it MUCH more audible than when the original exciting signal was present.

This very aspect is what makes it ludicrous to try and simply look at the "thresholds of audibility" and declare something either hearable or not.

Of course, most objectivists like to limit the situation to load resistors and a silent room, but we don't listen to music in a vacuum with resistors.

In any case, it is highly likely that the measurements showing the multitone distortion represent just ONE loudspeaker being driven at a time; add in a stereo partner, or multiple surround sound and center channel speakers, and you have even more potential sources of intermodulation and increased levels of distortion.

Thus these measurements may not fully represent the complete and full extent of just how much distortion is actually present, it is likely that the actual distortion would be even greater than that shown.

Jon Risch