THOSE interested in noise measurement will be aware that a new unit of equivalent loudness, the British Standard Phon, has been introduced by the British Standards Institution (Glossary of Acoustical Terms and Definitions B.S. 661—1936). The standard of reference is a plane sound wave of 1,000 cycles per sec, heard alternately with the noise to be measured by an observer facing the source and listening with both ears. When the reference tone is judged by a normal observer to be as loud as the noise, the equivalent loudness in phons is given by the intensity level of the reference tone above a reference pressure of 0·0002 dyne per sq. cm. The experimental realization of the definition requires the resources of a laboratory, so that it becomes of interest to inquire how far the indications of portable secondary meters used for measurements in the field conform to the standard.