Some manufacturers of registers and grilles provide information on the sound characteristics of their devices. Others suggest minimum velocities to avoid noise problems.
Field installations substantially differ from the conditions of the sound tests, and can seriously alter the actual sound levels generated. Sound measurements in the laboratory are made with a uniform air velocity profile through the outlet. Thus, in illustration ¿A¿ the nonuniform flow through the outlet could increase the noise generated. Often, equalizing grids are specified to control noise as well as mixing performance. Severely offsetting the outlet from the duct connection, as in illustration ¿B¿ can also increase noise above ratings. Increases as much as 10 to 20 decibels are possible depending upon the severity of the offset.