WE owe it largely to the public interest in the abatement of noise that two hitherto unfamiliar, but now international, units-the decibel and the phon- have come into common use. The decibel, which arrived from America via the telephone engineer, represents approximately a 5/4 geometrical increase in acoustical energy or intensity. This forms the basis of a logarithmic scale of energy levels which advance by increments of one decibel, starting from a 'zero' which is arbitrarily fixed near the threshold of hearing. The phon, which was imported from Germany, is the unit of loudness or, more precisely, of 'equivalent loudness'. It is derived through the medium of a pure tone of 1000 cycles per second which is set up as a standard of reference. Where the reference tone is stimulated by an energy level equal to n decibels above the zero, its loudness level is declared to be n phons. If, further, the reference tone has been regulated so that, as heard by an average ear under specified conditions, its loudness is assessed as equalling that of some other sound or noise, then the equivalent loudness of the latter is also declared to be n phons.