def fft_frequencies(self) -> np.ndarray:
        """
        :return: Discrete fourier transform sample frequencies.
        """
        if self._fft_frequencies is None:
            self._fft_frequencies = np.linspace(0, self.sample_rate / 2, self.window_size // 2 + 1)
        return self._fft_frequencies

        :return: Discrete fourier transform sample frequencies.
        
art/attacks/evasion/imperceptible_asr/imperceptible_asr.py