Within the past decade, the trend in design of air distribution systems for buildings has changed significantly. At the present time, due to the emphasis being placed on energy conservation, the variable air-volume system has become the most frequently encountered design in practice.

The typical variable volume system consists of a supply fan feeding a series of controlled air valves, which in turn feed a network of outlet terminals distributed within the conditioned space. Each air valve and its connected outlet terminals typically represent a zone which is controlled by a thermostat located in the space served. Signals from the thermostat cause the air valve to modulate the flow in response to the demand, independently of changes in the system pressure at the valve inlet.

In normal applications these valves experience fluctuating inlet pressures in the range of 1 in. - 3 in. (25.4-76.2 mm) w.g., or higher, and must be capable of modulating the air volume from full design capacity down to very small quantities under these conditions. Inherent in this process is the generation of noise whose magnitude is a function of both the pressure drop across the valve and the quantity of airflow through it. The shape of the noise spectrum is a function of both the valve design and the extent of the acoustical treatment contained within the box that encloses the mechanism.

The noise produced by these air terminal boxes is of concern in the application from two aspects: First is the magnitude of the noise coupled to the room by the downstream ductwork, which is commonly termed "discharge noise." Second is the noise radiated to the surrounding space through the casing of the box and the inlet/outlet connected ductwork which is typically referred to as "radiated noise."

In the past considerably more attention has been focused on the discharge noise aspect of the problem than with radiated noise. However, a number of field problems with low-frequency "rumble" have been experienced because this latter factor was not adequately taken into account. In a majority of typical installations these air terminal boxes are located in the ceiling plenum above spaces that are acoustically sensitive, and radiated noise is frequently the controlling factor in achieving a satisfactory background sound environment. This aspect is becoming even more significant with the larger box sizes that are now appearing on the market, and which can supply the greater air quantities specified by the system designers who are joining the trend towards fewer "zones" and greater areas of coverage per box.

This paper deals with a recent survey of air terminal box noise characteristics and the present industry practices in the noise rating of these devices. The question of how these ratings are being translated in terms of the acoustical requirements of the application is also addressed.

This study was sponsored by the Air-Conditioning and Refrigeration Institute (ARI) as a part of a current program to develop certified rating and application Standards for variable air volume terminals. The test data upon which this study is based were obtained from experiments designed by the author and conducted under his supervision in two highly qualified industry laboratories.