This option determines the spectrum bar's peak decay rate, in decibels per second. It's only relevant when peaks are being shown, i.e. when Show Peaks isn't "Off". A peak begins decaying when the peak hold time expires, and the bigger the decay rate, the more rapidly it decays. If the decay rate is zero, peaks don't decay, in which case they accumulate and persist until the plot is reset. To reset the plot, use the Reset command in the Spectrum bar's context menu, or switch to a different document.