filter or LTI system, where the coefficients
    are ordered from highest to lowest degree. Or, the roots  of the
    transfer function numerator and denominator (i.e., zeroes and poles).
N : int
    The length of the array to be computed.
kind : str {'ba', 'zp'}, optional
    Specifies whether the numerator and denominator are specified by their
    polynomial coefficients ('ba'), or their roots ('zp').

Returns
-------
w : (N,) ndarray
    A 1-D array of frequencies, logarithmically spaced.

Examples
--------
Find a set of nine frequencies that span the "interesting part" of the
frequency response for the filter with the transfer function

    H(s) = s / (s^2 + 8s + 25)

>>> from scipy import signal
>>> signal.findfreqs([1, 0], [1, 8, 25], N=9)
array([  1.00000000e-02,   3.16227766e-02,   1.00000000e-01,
         3.16227766e-01,   1.00000000e+00,   3.16227766e+00,
         1.00000000e+01,   3.16227766e+01,   1.00000000e+02])
