@java.lang.Override
  public double getVolumeGainDb() {
    return volumeGainDb_;
  }
<pre>
Optional. Input only. Volume gain (in dB) of the normal native volume
supported by the specific voice, in the range [-96.0, 16.0]. If unset, or
set to a value of 0.0 (dB), will play at normal native signal amplitude. A
value of -6.0 (dB) will play at approximately half the amplitude of the
normal native signal amplitude. A value of +6.0 (dB) will play at
approximately twice the amplitude of the normal native signal amplitude.
Strongly recommend not to exceed +10 (dB) as there's usually no effective
increase in loudness for any value greater than that.
</pre>

<code>
double volume_gain_db = 4 [(.google.api.field_behavior) = INPUT_ONLY, (.google.api.field_behavior) = OPTIONAL];
</code>

@return The volumeGainDb.
java-texttospeech/proto-google-cloud-texttospeech-v1/src/main/java/com/google/cloud/texttospeech/v1/AudioConfig.java