They accour due to nonlinearities in a speaker sytem. Speaker system should be linear in its operating range.
Cms is nonlinear element, it ads odd harmocs.
Air compression ads even harmonics.
Without going into too much detail, I consider less than 3% 2nd Harmonic, 1% 3rd Harmonic, 0.3% 4th Harmonic and so on in a monotonic sequence to be essentially inaudible at a SPL of 96db (Patterson, Bekesy). The same holds for any intermodulation products that stem from the same non-linearities. Here it is much harder to apply weighting, but sidebands spaced closely to the main tones will be more audible than those spaced widely, corresponding to more audibility for IMD produced by nonlinearities producing primarily odd order Distortion. (Shorter).
Despite these severe deviation from anything remotely resembling linear frequency response we can easily and clearly identify sounds reproduced or even produced under such conditions. As a result I would contend that modest deviations from a linear Frequency Response for a High Fidelity application are acceptable. My personal definition of “modest” is +/-6db in 1/3 octave smoothed measurements.
A formal study on the requirements for high quality reproduction recommended less than +/-2db, a performance that is absolutely unobtainable unless the room is designed for good acoustics and heavily treated even if the Transducer is perfectly linear. Most of us never get such listening conditions, so I would suggest that under domestic conditions +/-6db are acceptable for the 3m in room response within the 100Hz –10kHz range (the study recommended 40Hz – 15kHz). Note that this for the response at the listening position (hence not necessarily on axis), not just the raw Driver/Speaker anaechonic response.
So as long as our entire replay chain produces distortion lower than that we can consider it as being as “High Fidelity”. It is worthwhile that the study already mentioned suggested 0.25% THD for the entire replay chain to be the limit, I have no indication what spectrum of distortion was used and what the maximum SPL’s used where.
Anyway, with the above we have a good deal of info in order to “define” High Fidelity reproduction.
I personally would propose the following:
Reproduction shall be considered of High Fidelity for Music IF:
I. the Frequency response linearity is better than +/- 6db for the 100Hz –10kHz range with Sound produced outside this range being no more than 10db down on the average SPL in the 100Hz – 10kHz. The total response should cover at least the normal range of fundamental notes and overtones produced by Acoustic Instruments. For reference, the lowest note on the Piano Grand corresponds to 32Hz, the upright bass lowest fundamental is 41.4Hz. There is no specific limit at high frequencies, as many instruments contain overtones reaching beyond 60kHz.
II. the level of the 2nd harmonic overtone is no more than 3% and of the 3rd harmonic overtone is no more than 1% at a SPL Level of 103db/1m for a Mono System or 99db/1m for the individual speaker output of one spaeker in a Stereophonic System for the “critical” range of 100Hz – 10kHz. Outside this range levels of 10% and 3% respectively for 2nd and 3rd harmonics must not be exceeded at the same SPL’s.
III. the degree of compression over the medium term (10 Minutes) for a SPL of 103db (Mono) or 99db (1 half of a Stereo System) is less than or equal to 1db.
Note that I have not included the phase response here and that I have not mentioned time alignment per se, as in a mono-system quite substantial shifts will not be audible. Only very “rapid” shifts of the phase with frequency will become audible. Note that I have also not included the levels of background noise. Without going into too much detail we should have less than 30db background noise at the listening position with less than 20db above 500Hz. Furthermore, the issuse of step response, ringing and resonances and pulse fidelity has not been addressed.
In short we should add to the three above conditions:
IV. the phase of the system is not subject to severe and rapid phase-shifts in the “critical frequency range of 100Hz – 10kHz
V. the level of background noise is lower than 37db/1m overall and lower than 27db above 500Hz, the range between 500Hz and 50 (60) Hz showing rising slope to reach 37db at the lower point. This is for control settings that will produce the maximum operating SPL as discussed as above.
VI. the system shows a simple step response (showing one spike and then a fast and monotonic return of the pressure to normal) and good preservation of the Waveform for a Squarewave in the 100Hz to 10kHz range.
I believe that the above set does indeed describe quite well the requirements for “High Fidelity” Reproduction. It should be noted that all the above the conditions are to be taken within the Room the system is used in and include further environmental factors (for example the back ground noise levels include noise not generated by the replay system).
Note that is quite desirable to improve on the bare minimum conditions given above, however ideally one would avoid to improve one parameter on the expense of another. Furthermore the above makes no claim on comprehensiveness or completeness, there are many other factors present that are not accounted for here but have a strong bearing on the perceived quality of the reproduction.
|elasticity of suspension + persistence + compresion of the air|
Impulse Response. (How fast does it settle to zero? Is there chaotic hash in the decay region or is it a single, smooth resonance? Are there two or more resonances?)
2) The Group Delay vs. Frequency Response. (How ragged is the frequency range above the first breakup? Can it be fixed in the crossover?)
3) The Waterfall Cumulative Decay Spectrum. (Can I accept the resonances that can't be fixed in the crossover? If crossover correction is required, how complex is it going to be?)
The flatness of Frequency Response in the working band. (Can I accept the broad, low-level colorations that may appear here?) Listening and measurements are equally essential. Both give only a partial picture of the actual driver. Even the finest modern audiophile system will have very serious sonic deficits 5 years from now; measurements provide a reality-check on colorations that present-day equipment may not reveal. In turn, the MLSSA system can point out troublesome colorations to listen for; some are much more audible than others. The thoughtful designer is obliged to be as careful as the craftsman (or craftswoman) who lavishes a full measure of care and attention on even the hidden parts of their creation.}