From the Beginning of phonography until the mid twenties, horns were used for the concentration (sometimes, for convenience, called amplification) of the acoustically-produced sound waves from the phono soundbox. At first, these horns were cone-shaped, then had curved yet symmetrical sides - but none were really "scientifically" designed. When the internal-horn type machines (Victrolas, et al) came out, sound quality actually declined a little, as symmetry was sacrificed for space considerations. Then the Bell Labs (the phone company research unit) began doing serious acoustic research, and developed the Exponential Horn - which had walls that widened the sound passageway in a predictable, regular fashion and therefore introduced less distortion and enjoyed higher "amplification" efficiency.
The response of the public was overwhelming. They bought them like mad, until the crash of '29 and Victor's purchase and domination by a holding company and eventually RCA. They sound GREAT - even now - in a sort of indescribable way. One could call it an "organic" sound - mellow yet clear, and amazingly realistic. Some enthusiasts will listen to records of this vintage on no other type of phonograph.
So we hobbyists have about four years (1925 - 1929) worth of these special phonographs upon which to draw, many of which got trashed during the depression in favor of radios, which of course gave one new music for free. As the crash deepened, record production plummeted, and phonos became surplus. So Orthophonic Victrolas are pretty rare these days. As many of them were top line machines, they can be found with optional factory-installed electric motors (number plate will read "VE-" rather than "VV-") and therefore lose the romantic marketing appeal to some of the crank-up spring motor. But for those of us that listen to them, the electric motor is the way to go!
See Baumbeck's "Look for the Dog" - the definitive history of the Victor company - for model-specific information, pictures, production and sales figures, and many more details.