def bark(self) -> np.ndarray:
        """
        :return: Bark scale for discrete fourier transform sample frequencies.
        """
        if self._bark is None:
            self._bark = 13 * np.arctan(0.00076 * self.fft_frequencies) + 3.5 * np.arctan(
                np.square(self.fft_frequencies / 7500.0)
            )
        return self._bark

        :return: Bark scale for discrete fourier transform sample frequencies.
        
art/attacks/evasion/imperceptible_asr/imperceptible_asr.py