Second, the warbling sound of PSK31 may not stand out as clearly in the noise as a continuous tone. Finally, the transceiver is typically adjusted for an offset of 800 Hz or higher, which is well above the optimum pitch for aural detection of weak signals in noise. In other words, if you try hard enough not to hear a signal, you won't hear it, but that doesn't make it a good weak signal mode. Having said all that, PSK31 performs quite well, and produced good copy when the signal amplitude was reduced by 12 dB from the standard reference level. Partial copy is still possible at -15 dB, as shown in the DigiPan screenshot below.
Opinions differ as to what is the optimum speed for weak-signal reception.
Anyway, my test used 12 WPM at a frequency of 400 Hz. In the brief wave file below, the CW signal has been attenuated by 18 dB prior to mixing with the noise. It consists of a number of carriers of different intensity, superimposed on the usual LF background of static crashes. The second WAVE file below is the same signal and noise, passed through a filter bandwidth of approximately 50 Hz in Cool Edit. The main purpose, however, is to provide improved copy under noisy conditions.