{SPOILER WARNING}
In the Doc Savage story "The Roar Devil" (June 1935) the villain uses a machine that completely suppresses human hearing. He does this to cover the noise of massive TNT explosions that a gang is using to uncover a secret cache in the mountains. Phil Farmer in the 1970s considered this to be a "silly" story. I have reread it recently and I don't agree. The story is interesting and the sound dampening device is not that far fetched.
In the 1930s, some airlines had developed an anti-noise machine that was used in the passenger compartments of aircraft to dampen the noise of commuter plane engines. These were big bulky devices the size of a man that were place inside the cabin and they did not work too well. More modern versions of this have been miniaturized so that they will work inside of a set of personal earphones.
The principle behind it is that sound is a series of pressure waves travelling through the air. If you can create an "anti-sound" in which the peaks of the sound are matched by the troughs of the anti-sound, the noise will be cancelled out.
As you can see in the above diagram, the cancelling is not perfect but it is very effective.
So this technology was available circa 1933 and could have been used by the 'Roar Devil' as part of his scheme.
But we are told that the device not only dampened noise but also suppressed human hearing. This actually to is also possible.
There have been several patents since the 1970s of devices that could induce temporary deafness using a combination of high-frequency sound and microwaves. This sounds very similar to the weapon that Doc had in the Super Saga "Fortress of Solitude" (October, 1938) which could induce temporary blindness:
"The blackness was caused by a combination of short electrical waves, and high-frequency sonic vibrations, , which paralyze the functions of the rod-and-cone mechanism of the optic nerves in the eye."
It may be that Doc had used the device from The Roar Devil and applied it to vision instead of hearing.
In any case, the device described in The Roar Devil is not far fetched at all. I thin we must assume that it used a combination of anti-noise with a device that caused temporary deafness.
