Intrinsic flux RMS (vs. wavelength) for SNe after division into four bins according to increasing light curve width (dark to light lines). Spectra were initially normalized to have median flux of unity over the [3300, 6900] Åwavelength region. Panels from top to bottom show the three sample phase regions (pre-peak, peak and post-peak). Vertical (dotted) lines mark boundaries of the four U-band regions. Thered dashed red line shows the RMS for the full sample.