def from_frequency(cls, freq):
        """
        Construct a :class:`Tone` from a frequency specified in `Hz`_ which
        must be a positive floating-point value in the range 0 < freq <= 20000.

        .. _Hz: https://en.wikipedia.org/wiki/Hertz
        """
        if 0 < freq <= 20000:
            return super(Tone, cls).__new__(cls, freq)
        raise ValueError('invalid frequency: %.2f' % freq)