  Speckle metrology is a powerful tool in the measurement of wavelength and
spectra. Recently, speckle produced by multiple reflections inside an
integrating sphere has been proposed and showed high performance. However, to
our knowledge, a complete characterisation of speckle sensitivity to wavelength
in that geometry has not been performed to date. In this work, we derive a
general model predicting the variation in a speckle pattern as a result of a
generic transformation. Applying this to a shift in the incident wavelength, we
show that the speckle sensitivity is mainly governed by the radius and surface
reflectivity of the sphere. We show that integrating spheres offer sensitivity
three orders of magnitude above that of multimode fibres of a similar size, and
discuss analogies with the transmission line of a Fabry-P\'erot interferometer.
