Phon

In a clangorous world in which municipal antinoise campaigns have attracted wide and favorable attention, most laymen know that a decibel has something to do with the measurement of din, although few could define the term. Meanwhile acousticians have taken up a newer and less well-known unit, the phon, which may well become familiar to laymen because it is even more closely related to the sensibilities of the human ear than the decibel.

The decibel is an arbitrary unit such that, starting from the zero level or threshold of hearing, each increase of one decibel represents an increase of 25% in the physical intensity of the sound. The human ear has an enormous range. It is not pained by loudness until the sound is about ten trillion times as intense as a whisper at the threshold of hearing. Thus it is not very sensitive to small intensity changes. The decibel is intended to represent roughly the smallest intensity change which the ear can detect.

Sounds of the same intensity, however, sometimes seem unequally loud to the hearer, especially when they differ in pitch, so the phon was chosen as a unit of loudness. In the British journal Nature last week Dr. George William Clarkson Kaye of the National Physical Laboratory described the phon scale as "a loudness scale which is based on the accepted ability of the average individual to compare and match loudness." Thus, while the decibel is an objective measure of a sound's physical intensity, the phon is a subjective measure of its apparent loudness to the ear. There has been some difficulty with phons in international exchanges of research results because in the U. S. the decibel was generally used as a loudness unit as well as an intensity unit and in England and Germany different zero levels, different frequencies for the reference tone and different listening techniques were in use. At a recent international conference in Paris these differences were ironed out and a standard phon scale agreed on involving a uniform listening technique, reference tone, and zero level. Some loudnesses measured on this scale are: ticking of watch at 3 ft., 30 phons; tearing of paper at 3 ft., 40; quiet conversation, 60; noisy conversation, 70; noisy truck, 90; proximity of airplane engine, 120; near threshold of pain, 130.