Amplification
The early devices had to face the challenge of being heard in a potentially loud room such as the saloon in which Glass installed his first device. It solved the problem by making the listener use a listening tube, one of four on the machine, in order to hear their selection. Later devices used horns set atop the machine which were effective for those immediately adjacent to the device, but was not audible across a crowded room. These issues with audibility ultimately led to a decline in the popularity of these devices when radio became widespread.
Friction Amplification
One of the first attempts to solve the issue of audibility was friction amplification, which made use of mechanisms inside the machine to give the needle extra pressure against the wax cylinders, which produced a slightly louder sound. An amber wheel connected to the feed screw of the device rotated, which rubs a friction shoe, the tension of which presses the needle more firmly against the record. This method was rather crude, as it placed increased wear on the records and needles as well as offering a relatively small amount of amplification.
Pneumatic Amplification
Pneumatic amplification attempted to solve the audibility issue by compressing the air passing into the diaphragm of the device, producing more amplification than with the friction method. These so called Compressed Air Gramophones used a set of sound combs, placed next to one another. The vibrations of the needle would move one comb, and the compressed air would move the other, resulting a much greater amplification of sound.
Electronic Amplification
With the invention of the vacuum tube, it was now possible to create a coin operated phonograph with sufficient sound levels to be heard by more than just a few people. This revolutionized the industry and ultimately gave us the jukeboxes we are familiar with today. The nostalgic image of a group of teenagers gathered in a soda shop listening to the latest hits on a Wurlitzer jukebox was now possible.