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Chapter 1 - About the Point Seven
The frequency response of Alesis' ProPrietary tweeter is designed to be flat (t 2 dB]
from itJ2kHz crossover frequency to 10kHz. At 10kHz the response drops smoothly
to Z7kJl.lz, at which point it drops by -6d8. This is an optimal response for non-
fatiguing, long term, high levef nearfield mixdowns. This desigp results in a flat,
linear mix when played back on home or car systems from appropriate distances.
Magpetic shielding of both the Point Seven's drivers was not an afterthought.
Rather,
from design inceptioo
the system had to exhibit less than three gauss leakage so that
even extremely sensitive 21" computer monitors would be unaffected by close
proximity
positioning. Shielding of both woofer and tweete! is accomplished by the
use of a second opposing-field-oriented
'bucking" magnet, plus a sixteen gauge steel
cup which encases the entire matnetic structure.
The eleven element crossover features modified algorithm third order Butterworth
filters for both the high and low pass sections along with woofer impedance
compensation. (Compensation is unnecessary on the tweeter. The tweeter's
impedance characteristic is exceptionally flat as desigrred and the resonance peak is
ahlady very effectively damped with the ferrofluid.)
The crossover frequency of the
Point Seven is 2OOOHz. The actual voltage curve, however, shows the woofer
beginning its rolloff at 800H2 @ 6dB / octave to 1600H2. At this point the curve
"knees"
a second time and from 1600H2 to 3200H2 the rate is l2dBloctave.
The tweeter voltage cuwe also
"knees"
twice but the curves are substantially steePer.
From 3400H2 down to 1700H2 the tweeter rolls at l2dBloctave. Below 1700H2 the
acoustic rolloffs approaches 60dB/octave. This radical curve shaping yields a tweeter
with prodigious power handling capability, even at the extrenely
low 2kllz
crossover point. (Note that the slightly non-complementary curve slope spreads of
1.600H2-3200H2 for the woofer versus 3400H2-1700H2 for the tweeter are the
acoustical
"offset"
required to compensate for mechanical offset of the woofer and
tweeter. It is this
"offset"
or frequency spread which brings both drivers into proper
time alignment.) The advantages of these
"altered
algorithm" high and low pass
Butterworth
alignments are:
a) a virtually perfect group delay characteristic (almost a straight tine) yielding
accurate phase.
b) exceptionally linear power response (several averaged, off-axis frequency
response curves) through the rniddle octaves. Along with the 5 1./4" cone size this
linear (Ilat) power response projects an exceptionally large
"sweet
spot".
c) a longer smoother, sound characteristic blend between the non-woven CF woofer
and silk dome tweeter.
The Point Seven's cabinet conskuction employs Alesis'proprietary
non-skid rubber-
vinyl composite material laminated to a .625" MFD (medium density fiberboard)
core. The eight comer protectors used on our Monitor Two have been adapted to the
Point Seven in anticipation of its portable application.
Dual, front baffle mounted, 9" long ports have been carefuIly integrated to yield a
Qtc = 1.2 (a 3dB bump @ 120I{z) which provides a balanced, full range sound when
used alone. For critical mixing/playback,
recessed head, polyethylene
"port
plugs"
are provided. The plugs transform the Point Seven into an acoustic suspension
design giving a more critically damped response (Qtc = .9) and improved transient
characteristics. This versatile,
"convertible"
ported/closed box feature is Possible
because the woofer's EBP (E{ficiency Bandwidth Product has been designed to fall
into the narrow paramete! range wherein both configurations
will work equally well.
Point Seven Refermce Manual
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