’’Effective quiet,’’ the highest SPL of a noise that will neither produce a significant temporary threshold shift (TTS) nor retard recovery from a TTS produced by a prior exposure to a higher level, is shown to be about 76 dB for octave bands of noise centered at 250 and 500 Hz, and around 68 dB for those centered at 1000, 2000, or 4000 Hz. On the other hand, a mean TTS2(TTS 2 min after exposure) of no greater than 10 dB at all frequencies from 500 to 5600 Hz is produced by a broad‐band noise whose octave‐band spectrum falls off at −5 dB per octave (’’magenta’’ noise) and which has anA‐weighted level of 90 dBA. When the group mean TTS2is 10 dB, less than 10% of normal ears will show a TTS2of 20 dB. Therefore if a 20‐dB TTS2is tolerable day after day with no adverse effects, the present industrial noise exposure limit of 8 h at 90 dBAwould adequately protect more than 90% of exposed workers in noises with similarly falling spectra. Correction factors for unusual spectra should, however, be developed.Subject Classification: [43]65.66, [43]65.64; [43]50.70.