  It is known that haloscopes that search for dark matter axions via the
axion-photon anomaly are also sensitive to gravitational radiation through the
inverse Gertsenshtein effect. Recently this way of searching for high frequency
gravitational waves has gained momentum as it has been shown that the strain
sensitivities of such detectors are of the same order of sensitivity to the
axion-photon theta angle. Thus, after calculating the sensitivity of a
haloscope to an axion signal, we also have calculated the order of magnitude
sensitivity to a gravitational wave signal of the same spectral and temporal
form. However, it is unlikely that a gravitational wave and an axion signal
will be of the same form, since physically the way the signals are generated
are completely different. For gravitational wave detection, the spectral strain
sensitivity is in units strain per square root Hz, is the natural way to
compare the sensitivity of gravitational wave detectors due to its independence
on the gravitational wave signal. In this work, we introduce a systematic way
to calculate the spectral sensitivity of an axion haloscope, so instrument
comparison may be achieved independent of signal assumptions and only depends
on the axion to signal transduction sensitivity and noise in the instrument.
Thus, the calculation of the spectral sensitivity not only allows the
comparison of dissimilar axion detectors independent of signal, but also allows
us to compare the order of magnitude gravitational wave sensitivity in terms of
spectral strain sensitivity, allowing comparisons to standard gravitational
wave detectors based on optical interferometers and resonant-mass technology.
