Northern Architecture

Control Of Duct Borne Noise

Duct Borne Calculations

A typical duct borne noise transmission problem is illustrated in Fig. 14.14. A fan is located in a mechanical enclosure and transmits noise down a supply duct and into an occupied space. On the return side the ceiling space acts as a plenum for return air, which enters through a lined elbow. There could well be more paths to analyze, such as breakout from the side of a supply or return elbow before the silencer; however, for purposes of this example, we limit it to these two.

The starting point is the sound power level emitted by the fan, which we calculate from the operating point conditions. In this example the fan has a forward curved blade, producing 5000 cfm at 2" of static pressure.

Figure 14.14 Roof-mounted Built-up Air Handler

Air Handler

Figure 14.14 Roof-mounted Built-up Air Handler

Air Handler

Return

Supply

Return

Supply

Using the fan equations, we can calculate the sound power in octave bands as shown in Table 14.9. We then follow along each path, subtracting the attenuation due to each element and then adding back the sound power that each generates. The computer program used to generate these numbers uses a0dB self-noise sound power level as the default value or when calculated levels are negative. This has a slight effect on the very low levels but is of no practical consequence.

Table 14.9 HVAC System Loss Calculations, dB No. Description

Octave Band Center Frequency, Hz 63 125 250 500 1000 2000 4000 8000

Table 14.9 HVAC System Loss Calculations, dB No. Description

Octave Band Center Frequency, Hz 63 125 250 500 1000 2000 4000 8000