ABSTRACT
In the nineteenth century, Rayleigh devised a suspended disc system of which the deflection is proportional to the square of particle speed. This may be said to indicate intensity, but, of course, it only does so in simple travelling wave fields: it is not, however, a practical measuring device, because it is also disturbed by air movement. The recorded history of attempts to measure the flow of sound energy in complex sound fields goes back over 60 years, to 1931 when Harry Olson of the Radio Corporation of America submitted an application for a patent for a 'System responsive to the energy flow of sound waves'. The patent was granted in the following year (Figs 2.1 and 2.2). As explained by Wolffand Massa,l the 'Field Wattmeter' system was designed to process the signals from a pressure microphone and a particle velocity microphone using the 'quarter square' multiplication principle by which the product of the two signals was obtained from the difference between the squares of the sum and difference of the signals. In an article published many years later,2 Olson describes a suitable microphone (Fig. 2.3), and a development ofthe basic wattmeter to incorporate band pass filters (Fig. 2.4). Curiously, the literature appears to contain no evidence that this device achieved significant practical utilisation.
