Noise Control

Design Criteria
Duct System Costs
Duct Construction
Leakage Calculations
A New Problem for Rectangular Duct System
Spiral and Flat Oval Duct
Duct Heat Gain
Noise Control

Low frequency noises or "rumble" in duct systems come from the fan and from fitting turbulence. Low frequency noises create the most serious problems as they are the hardest to attenuate and the easiest to break out through flat duct surfaces.

TRANSMISSION LOSS

Transmission loss is a measurement of the capability of a duct to contain noise within the duct. The higher the transmission loss, the less the noise will radiate from the duct.

This noise will pass into the occupied spaces through acoustic ceilings, as they are "sound transparent". A comparison of the transmission losses for round spiral, flat oval spiral and rectangular sheet metal ducts is shown in the chart below.


Although not shown below 63 Hz, the transmission loss for round spiral duct increases as the frequencies lower. This is important as typical HVAC motors and fans generate noise in the low 15 to 30 Hz range.

Although rectangular ducts have a higher transmission loss at easier to attenuate higher frequencies, most higher frequency noises are generated by terminal devices such as registers and diffusers.

NATURAL SOUND ATTENUATION

Rectangular ducts have less natural attenuation, but are more easily lined with sound absorbing materials. However, the sound lining attenuation is limited, especially at frequencies 250 Hz and lower where "rumble" occurs.

The natural attenuation of unlined ducts is shown in Table 2 for rectangular ducts and in Table 3 for round spiral ducts.

Rectangular fibrous glass ducts have natural attenuation, hut have no transmission loss as the duct material, like acoustic ceilings, is sound transparent.

Table 2 Natural Sound Attenuation in Unlined Rectangular Sheet Metal Ducts (ASHRAE)
Octave-Band Center Frequency, Hz


63

125

250 and Over
P/A Ratio (a)
in in/in2
Attenuation, dB/ft (b)

Over 3.1
3.1 to 0.13
Under 0.13

0
0.3
0.1

0.3
0.1
0.1

0.1
0.1
0.1

(a) Perimeter divided by area
(b) Double these values if the duct is externally insulated
Table 3 Natural Sound Attenuation in Unlined Straight Round Ducts (ASHRAE)
 
Approximate Attenuation, dB/ft.
Octave-Band Center Frequency, Hz

Diam.
In.

63

125

250

500

1000

2000

4000

6
12
24
48

0.2
0.15
0.1
0.04

0.5
0.3
0.2
0.1

1.0
0.7
0.5
0.3

1.8
1.5
1.0
0.6

2.2
2.2
1.7
0.8

2.2
2.2
0.9
0.8

2.0
1.5
0.5
0.5