Ashly Electronic Amplifier none - Manual (Page 31)

makes it sound harsh. One thing that is clear, though, is that a steeper
slope will afford more protection to high frequency drivers than a gentle
rolloff. For the majority of audio applications, a 12dB per octave crossover
will be found to be perfectly adequate. Where extra protection is deemed
necessary, choose the 18dB per octave type.
SUNMED PHASE RESPONSE
Nearly all crossovers introduce phase shifts that vary as a function of
frequency, but this is not considered to be a problem since our ears don't
seem to be very sensitive to phase shift. Just to set up a suitable A/B
comparison to test the audibility of phase shift would be quite a feat, since
it would require near-perfect speakers and source material. After all, phase
shifts
are introduced virtually everywhere in an audio system; microphones,
transformers, equalizers of all kinds, effects units, crossovers and speakers
all contribute significant amounts of phase error. In short, crossover phase
shift effects are not considered to be an important factor in the accuracy of
a sound system.
SETTING UP YOUR SYSTEM FOR ACCURATE REPRODUCTION
In order to realize the best possible performance from your sound system, it
is
desirable to strive for the best possible phase relationships between
separate drivers. Since it is impossible to have all of your speakers radiate
their energy from the same point in space, your only option is try to keep
their acoustical centers coherent in at least one plane, i.e., try to stack
the speakers with the drivers centered in one vertical line. This will
minimize phase cancellations and improve
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projection pattern of the systi
Notice we aren't saying you'll achieve perfect phase correlation in your
system, because you won't. There are simply too many variables in a multi­
speaker system.
For example, we've recommended that you stack your speakers in one vertical
plane,
but what if you've got 2, 4, or 6 bass bottoms per side, all
reproducing the same frequency? In that case, you've got wavefronts radiating
from physically separate points in both the vertical and horizontal plane,
which means that the system's polar pattern and amplitude response will be
subject to the way those wavefronts add and subtract as they recombine.
Then, there's the knotty problem of determining where the wavefront actually
emanates from in a real speaker. Does it radiate from the diaphram in the
driver,
and should that be considered the acoustical source for alignment
purposes? Or, if that driver is connected to an exponential horn, should we
consider the mouth of the horn the correct alignment point? What's the phase
relationship between the mouth of the horn and the diaphram? What about a 15
inch woofer flush-mounted to a ported enclosure? Or a dual 15 inch folded "W"
cabinet—where is its "acoustical center"? As you can see, there's room for
error in a multi-speaker system. Generally, the more complex the system, the
more difficult the phasing question.
All of this is not to say that contemporary sound systems don't work or sound
good—they can and do. Just keep in mind that there are physical limitations
in a real sound system that make perfect phase alignment nearly impossible.
Again, if you'11 try to keep the system al igned in at least one piane, you'l 1
probably be doing the best you can.
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