Top: NW relic of Abell 3667 from de Gasperin et al. (2017). Colors indicate the spectral indices, and different filaments show different characteristic indices. Bottom: spectral indices along five of the filaments, showing how the variations along them are significantly smaller than the differences between them. Numbers on the right indicate the equivalent Mach numbers (and their scatter) if these indices were due to DSA. Errors per measured point are just slightly smaller than the variations, so the intrinsic variations are significantly smaller than observed.