THE HARTLEY POLYMER BACK
LOUDSPEAKERS
In the past, very special people have known about HARTLEY speakers. Audiophiles, engineers, musicians,
recording technicians, scientists and other professionals who use sound or consider sound to be a very important
part of their lives or careers. No matter what the price, HARTLEY speakers are considered among the best available
anywhere in the world. Meticulous hand-crafting and unique problem solving of the physics of loudspeakers are
the reasons for our long standing reputation. We don't sell a lot of speakers that way, but we don't have to
compromise our standards either. In fact some of our drivers require over fifty hand operations to build.
THE CONE
Only in recent times do we find companies using cone materials other than paper: polypropylene, kevlar, aluminum,
synthetic polymers and other plastic formulations to name a few. Their more recent revelations was our understanding
over FIFTY years ago! Starting with polmerization to paper cones, in 1956 Dr. Harold Luth, chief engineer and Master
Chemist developed the World's First Synthetic Loudspeaker Cone. Until that time the only non-paper cones were that of
bakelite paging speakers used by the Navy. To accomplish this truly revolutionary feat Dr. Luth had to invent a new
synthetic to which the world had never seen. A series of monomers cross-linked became the solution which led to other
problems and solutions. Platos' loudspeaker cone is one that has: no mass, infinite stiffness, perfect geometry and ultra-fast
sound transmission time. Dr. Luth choose a molding process with a special fabric carrier and internally molded ribs.
With his physicist hat on (and pipe) he designed a superior geometry never before seen. The cones were shallow,
incredibly light and stiff and could withstand high molecular pressure without edge noding and bending.
THE SURROUND
Most speaker driver manufacturers use foam, treated cloth or butyl rubber as their compliant surround. Foam, untreated
will deteriorate inside-out from humidity and moisture in the air. Treated cloth is much less compliant, and exhibits more
resonances. Butyl rubber a much better choice will break-down due to ozone in the air. The solution Dr. Luth determined
due to his extensive background in chemistry was silicone rubber. Totally inert, extremely low resonance, proper
durometer, impervious to moisture, as well as UV rays and a lifetime of greater than 99 yrs.
Every other manufacturers surrounds are glued to the cone. This can lead to spurious resonances and even separation at
the joint. Dr. Luth's solution was to change the chemical mix during molding from polymer to silicone rubber thus
eliminating a joint completely. A seemless molding solution!
THE VOICE COIL
220 MSG---207 MSG
The dual voice coil in the HARTLEY speakers (220 MSG & 207 MSG) is based on a principle first developed by the
British physicist, A.C. Barker, in 1938. Barker's "duode" coil consisted of two coaxial windings, isolated by a plastic film.
By transformer action, high frequency signals imparted to the copper windings were induced on the aluminum shorted turn.
The aluminum tube could move independently of the copper windings to vibrate the speaker cone to which it was connected.
Barker's voice coil, although sound in principle, was never fully successful because he was never able to fully isolate the two
sets of windings with the materials available at that time. However, developments in synthetic chemistry have enabled us to
produce a highly compliant silicone compound, only 4 mils thin, which isolates the windings effectively and permits the
inner aluminum tube to move independently at high frequencies. The HARTLEY aluminum tube is a complete circuit not
slotted. It should be noted that in the HARTLEY speaker, the aluminum and copper windings are each connected to
respective sections of a dual cone. In the Barker voice coil, the aluminum tube was the only part of the assembly fastened
to the cone.
Electrically, the dual voice coil may be considered an air-cored transformer, modified somewhat by the presence of a small
amount of iron on the pole piece of the speaker magnet structure. The copper windings and aluminum tube comprise a
voltage step-down transformer, in which the copper turns are the primary and the aluminum tube the secondary. Signals
fed to the copper windings of the voice coil are induced by transformer action on the aluminum shorted turn. Because it is a
step-down transformer, the induced voltages are lower and the current higher than the primary voltages and currents.
Air-cored transformers are quite inefficient at low frequencies. Consequently, the current induced in the aluminum tube
begins to drop sharply at 2000cps and virtually disappears at 1000cps. An elegant way of eliminating a crossover with its
losses and distortions.
220 HS---218 HS---224 HS
All HARTLEY HS woofers employ a unique "heat sink" which is actually the aluminum tube the voice coil is wound on.
The tube is cut so that a significant portion protrudes through the apex of the cone allowing the greatest concentration of
heat (around the coil windings) to be dissipated through the tube outside the cone and into the air. During the coil winding
a special high temperature epoxy, manufactured by HARTLEY, is applied to all layers of the windings and baked in with a
temperature of over 450 degrees F. This extreme temperature far exceeds the rating of the copper wire itself!
THE SPIDER
In the first HARTLEY speakers in the 1920's our spiders were made out of bakelite and had four legs that were bolted to
the magnet structure. The shape defined the name "spider". Today we still employ the same design but with materials to
meet 21st Century standards. Spiders for the polymer series are made from tri-laminate fiberglass which is impervious to
adverse weather conditions and strong enough to withstand parallel and perpendicular forces.
THE FRAME
Most driver manufacturers use stamped steel frames. These stampings are inexpensive but they exhibit unwanted
resonances. Ringing is often used when referring to this steel frame. All HARTLEY polymers drivers use a sand-cast
aluminum frame, light and strong, inert and then polished. Yes, an expensive way to make a speaker frame.......also the
best!