-A Spherical Experience-
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Design Goals and Driver selection:
This design started out when my good friend
Wayne Wendel offered me a set of his
spheres. These were fairly good size at 17" in
diameter, and since I'd been impressed with his
other spherical speakers, I jumped at the
chance to do a speaker design utilizing them.
The baffle size suggested a 6.5" driver, so I
chose the RS180 to go along with the RS28A. I
wanted a system with good sensitivity, and
minimal padding on the mid, so after some
investigation, a pair of RS225's joined the mix.
Why a sphere?
In spite of the complexity of construction, the
spherical enclosure has a couple of interesting
and potentially useful attributes:
Other than the baffle step, the normal baffle
edge diffraction effects are ameliorated, as the
wave front appears to follow the curve of the
sphere. Just exactly how the wave front
propagates around the sphere may be open
for debate, but I can say that from my empirical
observations and measurements indicate it
provides a smoother off-axis response. The
resultant in room sound is quite spacious,
similar to, but not as manifest as a dipole. The
imaging and power response also hold up
nicely off axis.
Also intrinsic to the nature of a sphere is its inherent structural rigidity compared to a similar flat panel enclosure.
Consequently no bracing is required, and extraneous wave propagation due panel flex is virtually eliminated. While it can
be argued that the construction of a sphere can be an extensive undertaking, if one considers the effort it would take to
construct a comparably rigid flat panel enclosure, the playing field certainly becomes more level.
The detriments of spherical enclosures:
Theory states that enclosures develop standing waves of frequencies dependent on their internal dimensions, and much
has been said about the golden ratio providing the best enclosure for mitigating those effects. Since a sphere has the same
dimension in all directions, conventional wisdom suggests it would exhibit a single standing wave frequency, and represent
the worst possible case for standing waves. Well, I've not found that to be the case. Take for example the sphere of the
Exclamation, which as an internal diameter of roughly 14 inches. This should result in a fundamental standing wave of
approximately 485 Hz. Clearly it should be audible, especially if the sphere is only lightly stuffed. Interestingly enough, this
has not been my experience, and any audible effects of standing waves in the spheres I've auditioned appear to be subtle,
if they exist at all.
Other detriments: -I have found they have an annoying habit of rolling away if you set them on down on a non-level
The woofer enclosure:
I spent some time cogitating on the appropriate woofer enclosure. Setting the sphere on a conventional rectangular box,
while certainly appropriate from a performance stance, just did not do the design justice aesthetically. So, for once I put
form before function. To make the 100 liter enclosure not appear disproportionate with the sphere, I chose a narrow
trapezoidal shape for the front baffle, which necessitated a deep enclosure, but hides its bulk well when viewed from the
front. The large side panels necessitated substantial bracing, so I elected to design the bracing to emulate a TQWT. The
shape of the enclosure required the taper to be non-linear. However, subsequent measurements indicate the enclosure
has at least some the attributes of a TL, in spite of the design compromise due to the enclosure shape. The last section of
the line is actually a 3" port tube 16" long not including flares. Consequently the design blurs the demarcation between a
ported and T line. Let's just call it a 4th order alignment that is tuned very low. -Somewhere around 17 Hz, based on
empirical testing. The relatively deep enclosure allows them to be placed relatively close to the wall, so the real estate they
require in a listening room isn't as expensive as it might seem. Viewed head on, their massiveness is easily forgotten.
An Exclamation! next to the 36" tall TriTrixTL