The signals can be heard from a few hertz to hundreds of megahertz. Our AM radio hears these bursts as the snap and crackle of static. If you were to line up several receivers tuned to different frequencies, it is likely that you would pick up the same static bursts at the same time. Most of lightning's effective radio energy is concentrated in the 1 to 30 kiloHertz region. This region of the radio spectrum is considered the VLF, or Very Low Frequency, radio range.
Spectrograms are a visual analysis of sound.
They are specifically useful for viewing VLF recordings. When viewing spectrograms, you will notice dark lines, which represent sounds. The time scale is represented by the x-axis. When listening to VLF radio, you will often hear static, like pops and crackles. These cracks and pops are produced by lightning.