As a design team, we work hard at extending our amplification product’s bandwidth beyond that which humans can hear. We do that because it sounds better: more open, extended, transparent.
Yet, how could that be true? If we cannot hear above 20kHz (and most of us not above 12kHz), then why should it matter that our preamplifier extends to beyond 60kHz?
Spoiler alert. The answer is phase shift, something we humans are quite sensitive to. In an analog circuit with a maximum bandwidth of 20kHz, phase shift occurs well into the audible band. Take, for example, this simple graph below. It’s a simulation of phase shift from a 1kHz signal from both a high pass (bass roll off) and a low pass (high frequency roll off) function. Note how much phase lag and lead occur away from the center frequency.
To get phase shift out of the audible band we need to extend the roll off point well beyond that which we can hear.
The point of all this is simple. Sometimes we’re looking so hard in one place we ignore what’s actually happening in quite another.