Energy density parameter of the background produced by the coalescence of compact binaries for the different values of the metallicity and for the different models considered in this paper: AZ (black continuous line with triangle symbol), BZ (black continuous line with square symbol), Az (gray continuous line with triangle symbol), Bz (gray continuous line with square symbol), A (light-gray continuous line with triangle symbol), and B (light-gray continuous line with square symbol). Black lines correspond to high (solar) metallicity, gray lines to low metallicity, and light-gray to populations with mixed metallicity. The sensitivity of ET-B, ET-D and advanced LIGO/Virgo are also shown for comparison purposes.