To investigate the mechanisms by which unattended speech impairs short-term memory performance, speech samples were systematically degraded by means of a noise vocoder. For experiment 1, recordings of German and Japanese sentences were passed through a filter bank dividing the spectrum between 50 and 7000 Hz into 20 critical-band channels or combinations of those, yielding 20, 4, 2, or just 1 channel(s) of noise-vocoded speech. Listening tests conducted with native speakers of both languages showed a monotonic decrease in speech intelligibility as the number of frequency channels was reduced. For experiment 2, 40 native German and 40 native Japanese participants were exposed to speech processed in the same manner while trying to memorize visually presented sequences of digits in the correct order. Half of each sample received the German, the other half received the Japanese speech samples. The results show large irrelevant-speech effects increasing in magnitude with the number of frequency channels. The effects are slightly larger when subjects are exposed to their own native language. The results are neither predicted very well by the speech transmission index, nor by psychoacoustical fluctuation strength, most likely, since both metrics fail to disentangle amplitude and frequency modulations in the signals. (C) 2015 Acoustical Society of America.