If ``index`` is contained in a nonspeech interval,
        return a pair ``(interval_begin, interval_end)``
        such that ``interval_begin <= index < interval_end``,
        i.e., ``interval_end`` is assumed not to be included.

        Otherwise, return ``None``.

        :rtype: ``None`` or tuple

def inside_nonspeech(self, index):
        """
        If ``index`` is contained in a nonspeech interval,
        return a pair ``(interval_begin, interval_end)``
        such that ``interval_begin <= index < interval_end``,
        i.e., ``interval_end`` is assumed not to be included.

        Otherwise, return ``None``.

        :rtype: ``None`` or tuple
        """
        self._ensure_mfcc_mask()
        if (index < 0) or (index >= self.all_length) or (self.__mfcc_mask[index]):
            return None
        return self._binary_search_intervals(self.__nonspeech_intervals, index)