scope:

INTRODUCTION

A study of the classrooms in a university building was undertaken, with the goal of investigating the acceptability of, and the relationship between, airflow rates and noise levels from the heating, ventilation and air-conditioning (HVAC) system, and how the noise can be controlled. The HVAC noise was evaluated according to three different criteria for rating background-noise levels. Measured volume airflow rates were compared to their specifications, and related to the background- noise levels. This paper reports the tests done and the results. The objective was not to perform a detailed, exhaustive investigation of system performance. It was to provide direct evidence of the relationship between ventilation performance and noise, and to discuss how HVAC noise can be controlled. This paper is directed at ventilation engineers who may not always be aware of the acoustical consequences of their work, not to acoustical engineers.

There is no one way to rate the acceptability of classroom noise. Existing and proposed standards use a variety of background- noise rating methods to quantify suitable classroomnoise levels. The classroom noise in the study building was evaluated according to three methods. An ANSI standard uses an A-weighted sound-pressure-level rating (dBA), and limits classroom noise to below 35 dBA for classrooms with volumes less than or equal to 20,000 ft³ (566 m-³), and 40 dBA for those of greater volumes (ANSI 2002). A second standard under consideration uses the Noise Criteria (NC) method of evaluating noise, and recommends a limit of NC 30 (Lilly 2000). The Room Criteria (RC) rating is recommended by ASHRAE when assessing HVAC noise, and limits classroom noise to RC 35 (N), the "N" indicating that the noise spectrum must be of neutral quality (broadband noise) (ASHRAE 2007).Of these criteria, the Room Criteria method is the only one which evaluates sound quality as well as level.

The building investigated in this study was chosen because previous work had shown it to contain classrooms with high HVAC noise [Hodgson 2002]. It had five classrooms (here called Rooms A-E) on the second and fourth floors. It was built in 1962. The three unrenovated classrooms and their ventilation systems had negligible sound absorption. However, classrooms A and C on the second floor were renovated in 1999 and 2001, respectively. Renovations included upgrading wall and ceiling sound absorption, and upgrading the HVAC systems by increasing the number of branch ducts supplying air to the classrooms and adding duct liner inside them, to provide sound absorption and control internal noise. All five classrooms were located on the west side of the building's main hallways - three on the second floor and two on the fourth floor. A fan located in the first-floor mechanical room supplied the west side of the building. A second fan, positioned in the penthouse, supplied all other rooms. Plan layout drawings of the classrooms and HVAC systems of the second and fourth floors