Abstract
Although the speed of sound in air was measured with reasonable accuracy about the middle of the 17th century, intensity measurements did not properly begin until Rayleigh, in 1882, published his experiments with what came to be called the Rayleigh disc. The accuracy remained relatively low (about 10%) until about 1940. In the meantime the working standard for measurements in acoustics had come to be the condenser microphone which is a reciprocal transducer. By using the reciprocity method its calibration has improved by about two orders of magnitude, being now carried out to a precision of about 10-3. This is quite adequate for most purely physical measurements and more than adequate for many others such as sound absorption coefficients of surfaces or sound transmission loss of partitions. In most measurements of a psychoacoustical nature (threshold of hearing, loudness, etc.), or even where a human subject is involved in only a passive role, the errors of measurement are still very great being of the order of 10-1 or often greater.