Faintness, noise, and tone were the then three available ways to improve the listening experience. This segues nicely into an explanation of human hearing sensitivity. It's a simple fact that human hearing is most sensitive between 500 and 5,000 cycles. This explains the "smiley face" EQ we see preset into stereo amplifiers today. To compensate for the shortcomings of the human ear, response above and below that range proportionately.
It refers to stereo sound as experimental. Instead it breaks down the categories of Brilliance, Bright, and Mellow. These concepts never went away. In the 1980s Sony Walkman's some series offered a button called "Mega-Bass." It was just another circuit that modified the EQ. Here is the same idea presented in 1940.
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