Editing Johnson–Nyquist noise (section)

=== Power spectral density ===
The mean square voltage per [[hertz]] of [[Bandwidth (signal processing)|bandwidth]] is <math>4 k_\text{B} T R</math> and may be called the [[power spectral density]] (Figure 2).{{NoteTag|This article is using [[Spectral density#One-sided vs two-sided|"one-sided" (positive-only frequency)]] not "two-sided" frequency.}} Its square root at room temperature (around 300&nbsp;K) approximates to 0.13 <math>\sqrt{R}</math> in units of {{Sfrac|nanovolts|{{sqrt|hertz}}}}. A 10&nbsp;kΩ resistor, for example, would have approximately 13&nbsp;{{Sfrac|nanovolts|{{sqrt|hertz}}}} at room temperature.