thin, closely spaced spiral grooves that allowed for increased playing time on a  rpm microgroove lp led to a faint pre-echo warning of upcoming loud sounds. the cutting stylus unavoidably transferred some of the subsequent groove wall's impulse signal into the previous groove wall. it was discernible by some listeners throughout certain recordings but a quiet passage followed by a loud sound would allow anyone to hear a faint pre-echo of the loud sound occurring 1.8 seconds ahead of time. this problem could also appear as post-echo, with a ghost of the sound arriving 1.8 seconds after its main impulse. pre- and post-echo can be avoided by the use of direct metal mastering.