Echo Frequency

The relative power of an Echo is depicted as a measure of the magical reverberations it produces, also called its 'Frequency'. The more powerful an Echo becomes, the greater its Frequency. It's an abstract concept, really, given there's nothing on or in the item that explicitly says "herr derr I'm level 6" (unless you've got one of those muttering items, maybe), but it's an easy way to interpret and measure it.
All Echoes begin with a Frequency of 1, and can have their Frequency increased up to 16 through Tuning. As the Echo’s frequency increases, its ‘rarity’ steps up as well. Keep in mind this is really just a way to help keep track of the item’s relative strength and power, as canonically Echoes are unique to their creators and would otherwise be considered legendary items for that reason alone.
As your Echo’s rarity increases, the augment bonus it grants to your chosen modifier (weapon, spell, or AC) increases as noted in the frequency table above. Assuming your Echo was created using a magic item as its base item, its initial Frequency is adjusted to account for the additional item as if through Tuning as explained in the next section.