It looks like Bats are being catered for in the frequencies being presented and being a outcome of a Theorem presented.
@pindac The theory behind wide bandwidth in a zero feedback system has to do with filter theory. In a zero feedback system like a cartridge driving an SUT all the elements are passive so there is no correction.
To prevent phase shift (per filter theory) wide bandwidth is required.
Filter theory as it applies here:
Typically when a single frequency pole is present, it will induce a rolloff at 6dB per octave. Phase shift components of a 6dB slope will be induced down to 1/10th the cutoff frequency (the latter being defined as the -3dB point). That is why you see such wide bandwidths promoted in transformers, which typically have no feedback correction.
(As you may have surmised, if there is feedback correction the need for such wide bandwidth is reduced.)
So an SUT that cuts off at 50KHz will have some phase shift down to about 5KHz. The ear perceives phase shift over a range of frequencies although it cannot perceive it at any one frequency. Over a range of 5KHz to 20KHz where phase shift is induced by a 6dB/ocatve rolloff at 50KHz, the ear will perceive this as a ’darkness’ or ’slowness’; IOW a tonality is assigned. Conversely if there is an zero introduced at 50KHz rather than a pole, the ear will perceive this as a ’brightness’. So to maintain a neutral presentation, the bandwidth is important.