What is the compression ratio for the input-output function at 50 dB SPL?

• A. 2:1; 50 dB SPL
• B. 1:1; 50 dB SPL
• C. 3:1; 50 dB SPL
• D. 4:1; 50 dB SPL

Answer: (A)

The main idea is how much the output level grows for a given rise in input when a device uses compression. The compression ratio is the amount the output changes divided by the input change. When a compressor operates, the output increases more slowly than the input; a 2:1 ratio means for every 2 dB the input increases, the output only increases by 1 dB. At 50 dB SPL, that slope corresponds to the 2:1 compression, so the input-output function has an output increase of 1 dB for every 2 dB of input. The other options would indicate no compression (1:1) or stronger compression (3:1 or 4:1), which aren’t indicated for that level.