A proposito delle piccolissime perturbazioni della risposta in frequenza:
Brevi estratti da questo articolo:

“…The evidence

In the late 1950s, H D Harwood at the BBC made a discovery whose importance is still not fully recognised. In investigating the performance of loudspeakers, he discovered that low-level delayed resonances severely coloured the reproduced sound even if these resonances were 40dB below the main speaker response. At first sight there’s nothing very world-shaking about that. But consider what the effect of such a delayed resonance is on the amplitude and phase response; 40dB down means a signal whose amplitude is only 1% of the main signal. This means that the amplitude response must vary only between 99% and 101% of flat, ie within ±0.1 dB. The effect on phase response must similarly be within 1/100 rad, ie within ±0.6o. In other words even in the late 1950s Harwood showed that variations in phase response of around only 1o and in amplitude response of ±0.1dB produced audible colouration…”

“…In the mid 1980s, a second piece of evidence – that the ears could easily pick out tiny deviations in amplitude and phase response – emerged, in connection with digital filters. In an early attempt at digital noise suppression, Roger Lagadec, then at Studer, investigated a multiband digital noise gate that split the audio signal into 512 bands, noise-gated the bands separately and then put them back together again. Although this was very effective in reducing noise, it was discovered that there was a disturbing audible colouration, even if the noise-gating action was switched off. It was discovered that this colouration was due to the amplitude response of the filtered bands added together again not being quite flat – there was a ±0.1dB ripple in the frequency response. It was found that to remove the upsetting audible colouration required this ripple to be reduced to around ±0.001dB. In this case, all the digital filters had linear phase responses – so only the amplitude response could be blamed for the colouration…”