code: def test_substitute(self):
      data = AudioLoader.load_mel_spectrogram(
          self.sample_wav_file, n_mels=128
      )
      aug = nas.FrequencyMaskingAug(stateless=False)
  
      aug_data = aug.augment(data)
      aug_audio = aug_data[0]
  
      self.assertEqual(
          len(data[aug.f0]), np.count_nonzero(data[aug.f0])
      )
      self.assertEqual(
          0,
          np.count_nonzero(
              aug_audio[aug.f0][aug.time_start : aug.time_end]
          ),
      )
      self.assertEqual(
          0, len(np.where(aug_audio[aug.f0][: aug.time_start] == 0)[0])
      )
      self.assertEqual(
          0, len(np.where(aug_audio[aug.f0][aug.time_end :] == 0)[0])
      )
  
file_path: nlpaug/test/augmenter/spectrogram/test_frequency_masking.py
imports: import unittest
  import os
  import numpy
  from dotenv import load_dotenv
  from nlpaug.util import AudioLoader
  import nlpaug.augmenter.spectrogram
label_desc: Use image node_id 4 for calling the TestFrequencyMasking obj's underlying member method code with example usage: obj.test_substitute() without return types