  We calculate the signal-to-noise ratio (SNR) of the stochastic
gravitational-wave background in an extreme case that its spectrum has a sharp
falloff with its amplitude close to the detection threshold. Such a spectral
feature is a characteristic imprint of the change in the number of relativistic
degrees of freedom on the stochastic background generated during inflation in
the early Universe. We find that, although SNR is maximal with the correct
template which is proportional to the assumed real spectrum, its sensitivity to
the shape of template is fairly weak indicating that a simple power-law
template is sufficient to detect the signature.
