7. An echo suppressor as claimed in claim 1 wherein said fast Fourier transformation means produces frequency components x.sub.k of said system input request and y.sub.k of said input speech signal, with k=1 . . . N, and wherein said band reduction means comprises an allocation table i=i(k) and means for producing said first set of sub-band signals x.sub.i according to ##EQU4## and means for producing said second set of sub-band signals Y.sub.i according to ##EQU5## wherein i=1 . . . M, M&lt;N.
8. An echo suppressor as claimed in claim 7 wherein said outputs of said adaptive filters comprise Y.sub.i, with i=1 . . . M, and wherein said band restoration means comprises means for respectively multiplying said outputs Y.sub.i by respective coefficients ##EQU6## wherein w.sub.i is a width of sub-band i equal to a number of spectra combined in said sub-band i, by conducting a reverse allocation relative to an allocation performed by said allocation table in said band reduction means.