Public domain photos

The image shows a black and white photo of a waveform with a line graph in the center. The waveform is composed of a series of concentric circles, each with a different color, and the line graph is labeled with numbers at the bottom.

The image shows a black and white photo of a waveform with a line graph in the center. The waveform is composed of a series of concentric circles, each with a different color, and the line graph is labeled with numbers at the bottom.Open in library