def get_num_fft(sample_rate, window_len):
    """
    Function get_num_fft calculates optimal number of FFT points based on frame length.
    Less number of FFT points than length of frame
    will lose precision by droppping many of the samples.
    Therefore, we want num_fft as a power of 2, greater than frame length.
    
    @param sample_rate: The sample rate of audio signal we working with.
    @param window_len: Time interval we are taking within frames.
    @returns: Optimal number of FFT points.
    """
    frame_length = sample_rate * window_len
    num_fft = 1
    while num_fft < frame_length:
        num_fft *= 2
    return num_fft