code: def mel_to_hertz(
      mels: Union[float, np.ndarray], mel_scale: str = "htk"
  ) -> Union[float, np.ndarray]:
      """
      Convert frequency from mels to hertz.
  
      Args:
          mels (`float` or `np.ndarray`):
              The frequency, or multiple frequencies, in mels.
          mel_scale (`str`, *optional*, `"htk"`):
              The mel frequency scale to use, `"htk"`, `"kaldi"` or `"slaney"`.
  
      Returns:
          `float` or `np.ndarray`: The frequencies in hertz.
      """
  
      if mel_scale not in ["slaney", "htk", "kaldi"]:
          raise ValueError(
              'mel_scale should be one of "htk", "slaney" or "kaldi".'
          )
  
      if mel_scale == "htk":
          return 700.0 * (np.power(10, mels / 2595.0) - 1.0)
      elif mel_scale == "kaldi":
          return 700.0 * (np.exp(mels / 1127.0) - 1.0)
  
      min_log_hertz = 1000.0
      min_log_mel = 15.0
      logstep = np.log(6.4) / 27.0
      freq = 200.0 * mels / 3.0
  
      if isinstance(mels, np.ndarray):
          log_region = mels >= min_log_mel
          freq[log_region] = min_log_hertz * np.exp(
              logstep * (mels[log_region] - min_log_mel)
          )
      elif mels >= min_log_mel:
          freq = min_log_hertz * np.exp(logstep * (mels - min_log_mel))
  
      return freq
  
file_path: transformers/src/transformers/audio_utils.py
imports: import warnings
  from typing import Optional, Union
  import numpy
label_desc: Use image node_id 2 for calling a global function with example usage: mel_to_hertz(mels, mel_scale) and returns: freq, unknown, unknown