History: What does the threshold of a compressor do?

What does the threshold of a compressor do?

• We will reduce any amplitude over the threshold.
  That is correct.
• The threshold determines how much we will reduce the amplitudes over the ratio.
  The words are reversed.
• Threshold impacts how quickly the compressor will respond to changes in amplitude.
  That is attack and release.
• A threshold is a collection of individual compressors for specific frequency ranges.
  That is a multi-pressor.