....except that - in order to get those incredibly quick roll offs, one must destroy the waveform in the process!
Steep filters engender large timing errors
we call 'phase shift' which means that higher frequencies are delayed in time, and often inverted as well!
This causes a gross distortion of the waveform that is easy to grasp. Here's a simplified example of how this works.
Contents
Slanted Baffle Aligns
Acoustic Centers
Crossovers, aka Filters,
aka X/Os
Step
Response -
The Truth Be Told
Other Troubles
With Complex Filters
Suppose a waveform has both a high frequency event and a low frequency event like this:
A time coherent speaker will recreate the waveform exactly :
A steep filtered design will delay the high frequency event relative to its associated low frequency event (note that correct amplitudes are maintained) :
Typically, steep filtered speakers also invert part of the signal, thereby arbitrarily converting compression into it's opposite: rarefaction !!!
An interesting aspect of this anomalous behavior is that a time incoherent speaker can destroy the waveform in this way yet have superb frequency response specs, and even excellent harmonic distortion figures. This is because the speaker is, in fact, being faithful to relative amplitudes and does not generate ugly harmonics - - even though it is scrambling the time domain.
Now, measuring frequency response and harmonic distortion is a simple matter that results in data that can be readily reduced to a one or two dimensional spec like: "THD <1%" or "40-20KHz +/- 3dB". Manufacturers and magazines can present these specs as well as frequency response graphs to their potential customers who can readily interpret them as proof of high performance.
But the kind of waveform distortion caused by timing errors is much more elusive. It cannot be easily represented as a one or two dimensional spec that can tell a customer just how much damage has been done to the time domain.
And since it is, indeed, a rare audio customer who is even aware of this aspect of speaker performance - magazines and manufacturers of speakers alike are pretty much free to ignore the whole knotty issue.
