In today's world of medium and low velocity duct system design, the use of acoustical treatment within many systems is not necessary, due to natural attenuation of noise by the duct elements themselves. However, this is not always the case and cannot be assumed. The natural attenuation of the duct system must be evaluated and weighed along with other design considerations.

In those systems requiring additional acoustical treatment, the use of acoustically lined duct and fittings has been accepted as an effective and efficient means of noise control. The reasons behind this wide acceptance are logical. In addition to controlling the duct system noise, lined duct and fittings do so with a minimal increase in static pressure requirements over the initial system design, and provide thermal insulation along with the noise control.

Lined duct and fittings can be used throughout a duct system for the control of both primary and secondary noise sources, i.e., fan noise and airflow-generated noise, respectively. The acoustical design of a duct system is neither as complicated or as "mystical" as many design engineers feel, and the use of proper design techniques will provide a system free from noise problems, that is both cost and energy efficient.