A softer and smaller scaled Diapason. Audsley describes its tone as being midway between a Diapason and a Dulciana. He recommends a maximum wind pressure of 2 1/2", and gives the following scale, ratio 1:2.66, 2/9 mouth or less.
No examples of Echo Principal or Montre Echo are known (contributions welcome). Osiris contains but a single example of Echo Diapason: